1. Exclamation Brakes lights activate when engine on. Please Help

    Hello Jeep brothers,

    I own a 2003 Jeep Wrangler Sport / 4.0 liter / Automatic

    I made the mistake of using my brake light wires to add a custom light to the back of my jeep. The light failed and I snipped it using
    my wire cutters. I should have removed the darn thing.

    Later, it caused a short. The connector that attaches to the brake switch was slightly melted on one side. I replaced the connector and the brake switch (and the fuse).

    I then noticed my brake lights activate when I turn the engine on. If the engine is off, it doesn't matter where the key switch is, they work just fine. Brake lights come on, turn off when you release the pedal, turn signals, parking brakes, everything works.

    It is only when the engine is running that my brake lights are full on. If the engine is on, I get no reaction from the brake switch whatsoever. When I press the brakes or remove the brake switch and use my fingers to activate brake light switch, they still stay full on Red.

    Can someone please help. I am good with voltage meters and have some tech experience on amps and radios, I am ok with car wiring but in no way am I an expert.

    Please help. I am at the end of the trail and will need to use the winch on this one. Below, (my son, I know, had to post this one. He loves jeeps and I love him). The second pic is the connector I purchased at the salvage yard. The connector on my jeep had one extra wire, both going to one pin. I simply spliced them together to the same pin as the original.

    By the way, just one more comment. This is a spot on website/forum. I tried several others and they were hellacomplicated . Thank you!
    Attached Thumbnails Attached Thumbnails Brakes lights activate when engine on. Please Help-wp_000866.jpg  

    Brakes lights activate when engine on. Please Help-20150226_132257.jpg  

    Brakes lights activate when engine on. Please Help-20150217_091700.jpg  


  2. #2
    Join Date
    Apr 2013
    Location
    Staten Island n.y.
    Posts
    2,000

    Re: Brakes lights activate when engine on. Please Help

    Welcome to JEEPZ.com . I don't have the wiring diagram , but from your picture of the harness plugs , it looks as though the 12 gauge white which I believe goes to your brake light is shorting with the pink wire. Try a continuity check across the two to see if they are . The plug looks over heated and may have lost some of its ability to separate the two and the terminals are shorting. That would explain why the brake lights are live when ignition is on. If the plug is compromised and the pink wire is live ignition on , it may be feeding the brake light wire without the brake pedal depressed.
    p.s.- check with the little guy , maybe he has a wiring diagram you can borrow ?
    Never overlook the obvious . It's usually right in front of us

  3. #3

    Re: Brakes lights activate when engine on. Please Help

    That's where I will start today. Boss just gave me the day off (the little guy has an earache).

    I'll check continuity. I replaced the connector that was damaged (burned) with the one on the right. Finding a connector was extremely difficult.

    Good place to start though. Thanks for the help!

    Arizona weather will be lower 80's next four days! Blacksjeep and kid need to go for a ride!
    Hopefully I can figure it out.

    Quote Originally Posted by greg92jeepxj View Post
    Welcome to JEEPZ.com . I don't have the wiring diagram , but from your picture of the harness plugs , it looks as though the 12 gauge white which I believe goes to your brake light is shorting with the pink wire. Try a continuity check across the two to see if they are . The plug looks over heated and may have lost some of its ability to separate the two and the terminals are shorting. That would explain why the brake lights are live when ignition is on. If the plug is compromised and the pink wire is live ignition on , it may be feeding the brake light wire without the brake pedal depressed.
    p.s.- check with the little guy , maybe he has a wiring diagram you can borrow ?

  4. #4
    Join Date
    Apr 2013
    Location
    Staten Island n.y.
    Posts
    2,000

    Re: Brakes lights activate when engine on. Please Help

    Feel better little guy . Ok , this may wind up being the brake light switch itself. The continuity check will show something. You'll probably try disconnecting the switch to see if the short disappears. Will look for your post. Thanks . Greg
    Never overlook the obvious . It's usually right in front of us

  5. #5
    Join Date
    Apr 2013
    Location
    Staten Island n.y.
    Posts
    2,000

    Re: Brakes lights activate when engine on. Please Help

    In my last post to you , I was in agreement with the continuity check. Being advanced in electrical , I'm certain you know introducing current to a loaded curcuit is not advisable , please proceed with caution . Your brake light curcuit is obviously working therefore shouldn't need to be checked for continuity other than a possible short . What I'm thinking is to find what is charging the brake light curcuit while the brake pedal is not depressed. Since the fault is found while the engine is running , the circuit will have to be tested to find the fault with engine running. From your post , I noticed the replacement harness plug has one less wire . That has my interest. How were the splices made ? If I am reading your post correctly , the problem was found AFTER the plug was spliced in ? I believe this is where to start. With the plug disconnected , check which wire supplies power . Turn off ignition before reconnecting and certainly before disconnecting. I do not have the proper wiring diagram for your vehicle and cannot seem to find the proper diagram on the autozone site so I am unable to find if the body control module comes into,play on this vehicle for this curcuit. We want to be extra careful NOT to damage this component. My question ( and yours ) is what is causing the brake light curcuit to be loaded when brake is not depressed. I mean , what exactly and where is current flowing to brake light curcuit without switch closed from brake pedal being depressed. That I believe is the course of action to first be taken . Please do proceed cautiously since I do not know I'd the BCM ( body control module) is involved. My thoughts are , yes , it most probably is in line with this curcuit. Without a proper diagram , I cannot confirm . I'm certain your voltmeter is compliant as far as automotive / computer safe of the 10 mega ohm Impedience and all that jazz so using the proper range on your volt scale should be safe. You may find yourself back probing the brake light switch harness to find what is live when this fault is active and I think it is best to proceed this way. That will at least point you in the right direction . I'm sorry if I hastily replied and did not caution you before hand. Hope this helps and looking for your success . Greg
    Last edited by greg92jeepxj; 03-07-2015 at 12:02 PM.
    Never overlook the obvious . It's usually right in front of us

  6. #6
    Join Date
    Feb 2009
    Location
    Dayton Ohio Area
    Posts
    6,609
    Somewhere in the system you have ign power comming through the ground of the brake lights. Look at both sockets and verify the grounds of the lights.
    "Never look down on anybody unless you're helping him up" - Jesse Jackson

  7. #7

    Re: Brakes lights activate when engine on. Please Help

    This may be the problem. When I was removing old wires from the kc lights I replaced with LED's, I cut a black wire that was attached to some ground coming from the headlight. The wire came from somewhere on the firewall.

    This is when I began to have problems. I'll have to follow whats left of the wires to see if I cut a ground.

    I'll double check the grounds on the brake lights again. I pulled them apart and checked all the wires, I did not check the grounds.

    Thanks for the advice!

    I'm about ready to give up and pay the big bucks for a repair. Hopefully a couple more attempts will solve the problem.





    Quote Originally Posted by JPNinPA View Post
    Somewhere in the system you have ign power comming through the ground of the brake lights. Look at both sockets and verify the grounds of the lights.

  8. #8
    Join Date
    Apr 2013
    Location
    Staten Island n.y.
    Posts
    2,000

    Re: Brakes lights activate when engine on. Please Help

    As stated by JPNinPA , because there is not a proper ground , you have ignition power coming through a group energizing the circuit . Honestly , you would be the best expert to correct this problem. Retrace your work and restore what the prior condition the wiring harness was in . Since you had performed the job , you know what was done . Sending the jeep off to an electrician/mechanic would mean may$$ in labor. I believe you can get it back to where it was .
    Never overlook the obvious . It's usually right in front of us

  9. #9
    Join Date
    Aug 2012
    Location
    Milwaukee WI next to the lake
    Posts
    289

    Re: Brakes lights activate when engine on. Please Help

    In the engineering arena there is an old adage that goes "Keep It Simple Stupid" (The KISS PRINCIPLE). This applies doubly for trouble shooting. Have you unplugged your brake light and started the engine yet? If so what happens? If you just disconnected the lights you installed, and induced a short, something else shorted. the obvious was the switch which you replaced. How many of us have been lead down the trail by bad parts that were new in the box. IF you have an OHM meter, check that switch first! The kiss principle is directly related to Murphy's Law. Next remember that electricity is like water, It is looking for somewhere to flow to. and it does Not have to be to a ground, just to something that leads to a ground. Your brake light is grounded so the brake light switch sends electricity to it when depressed. however, if you still have the lights illuminating when the connector is unplugged, when the engine is on ONLY, then you have a short, and it may or may not be in the connector. Again use the kiss principle. draw 2 diagrams (as simple as you want) of the offending connector, and check it for voltage twice, once ignition on once off draw an X for no voltage and an O for voltage in the positions of the connector, this will tell you what is happening at the connector, Now kill the ignition, IF you find voltage on two lines next to each other ignition on, and one of them is not to the brake light switch, and you are finding voltage on BOTH lines to the switch ignition on, carefully try to remove that third wire. They are essentially simple crimp connectors, but usually have a tab that is bent back and hooks on a lip in the plastic, If you are careful, you can usually depress this tab so it will slip under the lip and you can pull it out. now try again, do the lights still light up with the ignition on? if not the problem is the connector, if they do there are burned wires elsewhere, reinsert the wire you removed. and try to find a diagram that will help you trace out the rest of the circuit, this is as far as I can walk you not being there, but the procedure is the same, very simple and basic, one step at a time. it eliminates confusion and saves you headaches, and is why electrical trouble shooting is so expensive. its the time it takes to do it correctly.

  10. #10

    Looks like Unfortunately it's going to be the ECM.

    Finally had a tech check it out.

    I must have inadvertently burned a component in the ECM.

    I'm going to have to bite the bullet and pay 399.00 for a re-manufactured, flashed ECM
    or pay 600.00 to the local jeep dealership for a new one.

    That's what I get for monkey rigging wires. Lesson learned.

    Anything I should look out for?

  11. #11
    Join Date
    Aug 2012
    Location
    Milwaukee WI next to the lake
    Posts
    289

    Re: Brakes lights activate when engine on. Please Help

    most of the USED ecm dealers give guarantees on the units when you send in the old one. make sure you send it in or yu can be screwed in short order. and here is why:
    There really is no such thing as a remanufactured ECM. the main components are potted in a thermal epoxy that cant be readily removed with out destroying the electronics themselves. The only part that is repairable is the surge suppressor board that the connectors are on this board regulates the voltage that operates the ecm and prevents voltage spikes from destroying it. If you have ever viewed an auto motive electrical system on an oscilloscope, you would see all kinds of spikes and garbage on it, caused by the voltage regulator operation, electric fans turning on and off, horn and high beam relays, and every mechanical switch and coil including rear windshield defrosters being energized and de-energized. Most "catastrophic" ecm failures are in this board, and not in the "computer" portion itself. so they simply pull these boards check for damaged components repair and send on the way. The problem is the failure of the power conditioning board (or what ever you wish to call it) may have stressed the rest of the ECM and caused it to weaken to the point of failure. it may never fail, it may go in 5 days or in 5 weeks, or it may not have caused any damage at all. And shop for a "reman", the prices vary by the volume on hand, these companies call bone yards across the nation buying them up. One company will have glut of them and lower prices, and as inventor drops, prices rise, while another company will be at the opposite side of the supply cycle, I have a family of four, all of us drive jeeps, and my wife and I have a couple "spares". None of us are keen on anything made since 06, so we've been through most of the games on buying older Jeeps. The ECM game is one that is high profit for suppliers and a PITA for us.

  12. #12

    Re: Brakes lights activate when engine on. Please Help

    You perfectly described the experience I've had the past couple of days trying to buy a re-manufactured computer! Good thing I checked the Better Business Bureau on the two biggest companies out of Florida. One holds onto your money for a month after they fail to tell you they no longer carry the part. The other had so many complaints and warnings about buying from them I cancelled. So here is the million dollar question:

    I found a computer with the exact serial number like the one in my jeep (P56044 096AH). The ebay specifications state the computer is for my exact model but they also say its for a "MANUAL TRANSMISSION".

    Here is my question: WILL THIS COMPUTER WORK ON A JEEP WITH AN AUTOMATIC TRANSMISSION? DOES THE ECM CONTROL THE TRANSMISSION?
    Brakes lights activate when engine on. Please Help-untitled.jpg

  13. #13
    Join Date
    Aug 2012
    Location
    Milwaukee WI next to the lake
    Posts
    289

    Re: Brakes lights activate when engine on. Please Help

    the first thing you need to do is start calling the local new jeep dealers and ask how much to reprogram an ECM. Then check to see if they have any compatibility issues they know of, by supplying your VIN ( they can Identify options and factory settings from it) and then tell them the ECM you have/are considering to be sure. MOST of the settings and options are programmed very few other than year and engine are hardwired since having separate parts for every option would cost jeep more than basic programmable units. But there are always those rare exceptions, and if your luck is like mine, you'll have one. As an example last fall I rebuilt the 249 transfer case from in my 95 GC ltd. A super simple job as they are very simple and reliable xfer cases, but when I went to put it back in the confounded catalytic converter was in the way, so I took it off the transmission jack and lifted it in place and had the spouse try to get one nut started on a stud. long story short, short job still isn't done due to weather issues... I ended up dropping the xfer case on m chest and fracturing my
    sternum. fingers crossed it wil be finished Saturday, I'm just cutting the downpipe to the cat. and then going to weld a stainless sleeve over the cut to splice it back together.

    BTW, yes the ECM does work with the transmission, some control it directly, others talk to the transmission control module, supplying speed throttle position, and so on.

  14. #14

    Re: Brakes lights activate when engine on. Please Help

    I sincerely appreciate your advice. Thanks for taking the time to answer my questions.

    Repairing, trouble shooting electronic problems is worse than going to the dentist. I decided to go with a brand new ecm from the dealer. I think spending a couple extra hundred dollars for not having to deal with an ecm which needs to be programmed or may be unreliable is worth the money.

    I remember helping my brother R&R the transfer case and transmission on his older jeep (I think it was a CJ with an automatic transmission or xfer case someone took off a truck). I was bench pressing 400 lbs at the time and explained to him it would be no problem matching up the bolts. Famous last words! After countless wasted hours of strenuous labor and lost time using the 4x6, brick and muscle method, we finally gave up and called my uncle who had a transmission jack!! Even with the transmission jack we still had to fight the metal beast until the last bolt was in place. That jeep gave us a royal whooping.

    The next time he called for help on his transmission, I informed him I was busy for the next 5 years! Ha!

    Thanks again for your time and assistance. God bless

  15. #15
    Join Date
    Aug 2012
    Location
    Milwaukee WI next to the lake
    Posts
    289

    Re: Brakes lights activate when engine on. Please Help

    If you have the cash, new IS the best bet, however, they still have to program it for your Jeep. its how they are made, just like your smart phone, it has a program that it runs to make things work. That's not a big thing, its the "how long will it last" that makes you wonder. over the years Ive had good and bad luck with them. I have one sitting here that MIGHT be a spare, wont know til I finish reassembling the 98 GC with a stroker 4.0 (now 4.. Originally the Voltage regulator went bad (its in the ecm) so I replaced the ECM. This build isn't using the Chrysler alternator system, its using a GM dual output with internal regulators so I may be able to swap them back and have a spare... I'll have to wait and see...
    Anyway,,, There are a few people about who think this is a game, and have to "know more" than the next guy. it isn't like that. This is real life, If we don't HELP each other, what good is it?

    You are more than welcome my friend. PTL and God Bless.

  16. #16

    Re: Brakes lights activate when engine on. Please Help

    It's been 4 months, 1000.00 dollars later and finally today my jeep will be repaired.
    I've been through two back yard mechanics and 500.00 deep with a certified mechanic. Despite the money I've invested into this repair, I still do not have my jeep. ECM companies selling junk online is a real problem that I do not want anyone to have to experience. People in this thread have been extremely knowledgeable and provided great advice. Now that I am an ECM/Wiring expert, I look back on their advice and see they were right. After removing the harness from the wheel wells, we found it wasn't the computer. Jeep uses wires the thickness of headphones. The harness looked like burned spaghetti. This was the problem. I did not know how to get to these wires. Had I known, I would have saved myself from many headaches and mass frustration, including driving 700 miles through Mexico with brakelights full on.

  17. #17
    Join Date
    Apr 2013
    Location
    Staten Island n.y.
    Posts
    2,000

    Re: Brakes lights activate when engine on. Please Help

    Thank you for your reply . Real sorry you had to part with a grand $$ and untold mental anguish . At this point we're hoping to hear good news that all has been soundly repaired for your sake. It's a shame , but it seems that the only person who you can count on in a difficult repair is you. It's rare that any mechanic/technician will devote the time needed to do the repair right as dragon 5126 said. I've learned that and had to take on the task myself more than once for the right results. I totally share your pain . Had a similar incident on my jeep back in 2004 with Chrysler dodge jeep involving close to a grand in the dead of winter when my multimeter display froze and I was left with no choice . I did all I can but old man winter won. I self inflicted a short to the PCM using a 12 volt test light when I should have used a computer safe test light diagnosing the a/c electrical the summer before. Since then I've been advocating the use of computer safe test equipment to all . Family and friends as well. Blacks jeep , we both graduated from the school of hard Knox and earned our badge of honor the hard way. Wish I had dragon 5126 back then to guide me. I learned to respect delicate connections and sensitive components PCM related and otherwise since but there's no substitute for someone who is ready to say " stop what your doing and think it through ". Although this experience has been painful , try to remember if we fall off our horse , we must get right back on . All I'm trying to do is get better with age and leave a good looking corpse. Just thought I'd give you something to laugh at , but I do take this repair thing dead serious . We know the consequences . Thank you once again brother . Greg .
    Never overlook the obvious . It's usually right in front of us

  18. #18
    Join Date
    Aug 2012
    Location
    Milwaukee WI next to the lake
    Posts
    289

    Re: Brakes lights activate when engine on. Please Help

    Greg, Thank you for the compliment. I really do appreciate it. However at the same time I cant help but stress to everyone to share responsible information. Take Blacksjeep's situation for example. everything said ECM, and it probably Was originally caused by the ecm going bad when the brake light circuit went south, and this took out the harness, there are many cases of these situations happening in the electronics arena. And if you didn't smell the burning insulation you wouldn't look for it because it normally doesn't make sense to. An example is a 5.1 surround sound system I have sitting here to repair that I received from a friend. he was running it without the rear channel speakers connected and blew the power amp. he replaced the amp but it still didn't work, I went over it and he did replace it correctly, but found a burned trace on the circuit board of the main power supply. Some times the simple things take out the hidden things.

    On the frozen display on your DMM that you ran into when working on your Jeep, Not all that uncommon up here in Cold Country. There are also situations where a digital meter wont work, such as when testing the crank sensor on a 4.0, you need to look for a sudden pulse and a digital meter wont react fast enough but an analog needle swing will. So having both types of meters in your tool kit is a major plus, especially for us 'northern boys".

    Once again thanks for the compliment, but the info you and Blacksjeep have shared is just as if not more important, as it brings out the little things that can complicate the obvious issues, and these are often the most important details that save others from making mistakes that are similar, and none of us want to go through that.

  19. #19
    Join Date
    Apr 2013
    Location
    Staten Island n.y.
    Posts
    2,000

    Re: Brakes lights activate when engine on. Please Help

    Your most welcome Dragon 5126. Special thanks to Blacksjeep as well . I think I can speak for all when I say we appreciate when anyone shares their experience , good or bad for the good of everyone to further their knowledge and learn how to safeguard against causing damage to our electrical systems and sensitive components . I believe this is what jeepz.com is all about , helping and sharing. I appreciate that in everyone's feedback . Having heart for another member or even non-member's assistance goes a long way . Every time a new member or current asked for help , it's because we have folks like you both , Blacksjeep and dragon5126 who are willing to share and educate the next person in need and equally caution them from making an error , even if they did not know they were but what the consequences are if you do this and the example from their experience. No one should ever look down on someone for their mistake but thank them for sharing so they may learn how not to commit the same error. If we teach someone the right way and advise them what is the wrong way , they will know for their next move how not to cause harm or prompt them to ask first . I'm proud of the fact that jeepz.com has members that care . Special thanks to you guys and to all who contribute for the good of the forum and to the good of those they assist. Keep,up the good work and let's keep 'em Rollin' ! I agree with your statement dragon 5126 that it's most important to caution someone how to not do what I just did so this may not happen to you. It's a bummer to learn in such an expensive way but at least we care enough to help educate from our mistake(s). Thankful for all contributions made to our forum for the purpose of helpfully educating someone's future repair or project. Thanks everyone .
    Last edited by greg92jeepxj; 05-17-2015 at 10:06 AM.
    Never overlook the obvious . It's usually right in front of us

  20. #20
    Join Date
    Aug 2012
    Location
    Milwaukee WI next to the lake
    Posts
    289

    Re: Brakes lights activate when engine on. Please Help

    Quote Originally Posted by greg92jeepxj View Post
    Your most welcome Dragon 5126. Special thanks to Blacksjeep as well . I think I can speak for all when I say we appreciate when anyone shares their experience , good or bad for the good of everyone to further their knowledge and learn how to safeguard against causing damage to our electrical systems and sensitive components . I believe this is what jeepz.com is all about , helping and sharing. I appreciate that in everyone's feedback . Having heart for another member or even non-member's assistance goes a long way . Every time a new member or current asked for help , it's because we have folks like you both , Blacksjeep and dragon5126 who are willing to share and educate the next person in need and equally caution them from making an error , even if they did not know they were but what the consequences are if you do this and the example from their experience. No one should ever look down on someone for their mistake but thank them for sharing so they may learn how not to commit the same error. If we teach someone the right way and advise them what is the wrong way , they will know for their next move how not to cause harm or prompt them to ask first . I'm proud of the fact that jeepz.com has members that care . Special thanks to you guys and to all who contribute for the good of the forum and to the good of those they assist. Keep,up the good work and let's keep 'em Rollin' ! I agree with your statement dragon 5126 that it's most important to caution someone how to not do what I just did so this may not happen to you. It's a bummer to learn in such an expensive way but at least we care enough to help educate from our mistake(s). Thankful for all contributions made to our forum for the purpose of helpfully educating someone's future repair or project. Thanks everyone .
    Right now is one of those time I curse the system software for not having a like or thumbs up button to hit. Our Jeeps are just like our Lives. We learn and carry it with us... And we are all in it together!

Similar Threads

  1. Brake trouble with 87 jeep wagoneer...please help
    By bigtex in forum Cherokee, Commander, Wagoneer and Liberty
    Replies: 6
    Last Post: 02-06-2012, 01:32 AM
  2. Brake trouble with 87 jeep wagoneer...please help
    By bigtex in forum Wrangler and CJ
    Replies: 3
    Last Post: 09-25-2011, 11:13 AM
  3. Cj idles rough when warms up. Please help!
    By kc jones in forum Wrangler and CJ
    Replies: 6
    Last Post: 01-14-2010, 08:25 AM
  4. engine swap ?? please help !!
    By mxkid in forum Wrangler and CJ
    Replies: 7
    Last Post: 09-16-2009, 11:41 AM
  5. '96 GC brakes have no pressure when engine on
    By jodik in forum Cherokee, Commander, Wagoneer and Liberty
    Replies: 0
    Last Post: 06-28-2009, 05:08 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Log in

Log in