1. 99 Jeep Cherokee blowing #21 fuse

    I have a 99 jeep cherokee sport 4.0. In approx. 2011 I had to install a used engine from a 98 Jeep Cherokee ( don't think that matters, but want everything known on it) . Back a little before 10-15-21 , a couple times I would go to crank engine and it would try to turn over but wouldn't, try again and crank fine. Battery was only a few months old and was showing around 12.5 volts and around 13.5 while running. I decided on 10-15-21 to go to O'reillys and buy a starter relay, because several years ago I had problems cranking and it was a bad relay. When I went to leave O'reillys , would not crank. Tried jumping it and not crank. One of the employees came out with some little device and checked battery, starter and alternator. They told me the alternator was bad. So I bought one , had a wrecker bring me home and replaced it. Still no crank. Decided to remove starter and took it and the old alternator back to them to test. Now starter bad and old alternator good. So bought a new starter and gave them old alternator for core fee. Put starter on and cranked fine. Yea, problem fixed, or so I thought. A few days later, drove to town and on way back, approx. 15 miles total and at about 45mph, engine dies. Have towed back to house. My neighbor , who actually works on cars, found no fire from coil. Decided to go ahead and get one. Installed and still no crank, I then discovered the F21, 20 amp fuse blown. Replaced and cranked, drove around neighborhood a few miles and seemed fine. Cranked and let run several minutes a day ( I dont drive it but a couple times a week usually ) , anyway seemed fine. A few days later drove to town and on way back about 1/4 mile from house it dies at about 30mph. I found fuse blown and put a 20 amp fuse in it and it went about 10 feet and died. Installed 25 amp and made it home. That fuse , F21, shows to operate the coil, fuel injectors and pcm. Tried checking wires but didn't see anything easily visible. I put 20 amp fuse back in and let run for over 1 hour , idling, revving it up , etc. ran fine. I decided to take to a childhood friends shop and one of the largest in town to see if he could find problem. It drove there no problem , about 3 miles and while I was there they looked at wiring , etc, while engine ran for about 1 1/2 hours. I left it with them and they kept for 3 days. Said fuse blew when they went to get on hwy after I left , installed a 25 amp and drove back. Never could find problem and said they drove several times those next days. Told me to pick up and try. ( I think he didn't want to spend anymore time on it ) . Anyways drove home and drove it for an addition 8 miles around neighborhood. The next day drove about 8 miles to put gas in it and was fine. A friend of a friend said he had had a problem with that fuse blowing and it was the TPS ( throttle position sensor) , so i went and bought one and a IAC ( Idol air control) I had been having problems with it for awhile. I took it out and drove for 20 miles on hwy and drove fine. I have driven several more times the last couple weeks and no problem until today. On way back from store it dies at about 25 mph. Install a 30 amp fuse and drove about 1/4 mile and dies again . Get pulled into a store and this time that fuse was not blown but now the F19 fuse is blown. It's a 25 amp and goes to the ASD Auto shut down. I replaced and it cranked but didnt want to attempt driving it back on the busy 2 lane hwy, So Wrecker hauls me home. It seems to be running fine idling, but no idea whats causing it. My dad and I keep thinking something with that new alternator, because didn't have this issue until I replaced it and the starter, but don't think it's the starter. Everything I read keeps saying wiring, but after it being in shop and they said they checked the wires ( but know they didn't without taking about a mile of electrical tape and all that wiring harness apart) . Anyway, I know this is long, but has anyone had this issue and how did u fix it. I'm no engine mechanic, but have worked on hydraulics for over 40 years. Hope someone can help. Thanks Hawke

  2. #2
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    99 Jeep Cherokee blowing #21 fuse

    Thanks for the full description. It is useful.
    Was the wiring harness, fuse box and PCM swapped with the engine? If so be sure you are looking at the correct wiring diagram for the fusebox you have.
    Batteries are typically 12.6V with engine off and 14.4V engine running.
    The crank position sensor is known to fail without symptoms and sometimes with random no start or shutdown. But it does not blow fuses.
    20A fuse is a sizable fuse to blow. So id do two things. One is trace the wires to components the fuse protects. Make sure there are no pinches, cuts or splices. You dont have to unwrap the wiring harness just note the wire color(s) and verify the color at the wire color at the components. And assume no cuts or splices unless the harness looks messed up or cut.
    Second is to pull the fuse that is blowing and replace with a multi meter (20A capible ) and monitor the current to determine if it is constant 20A or a spike. Electric motors draw a spike of current when they kick on.


    Never look down on anyone unless you are helping them up - Jesse Jackson
    Last edited by JPNinPA; 11-28-2021 at 08:40 AM.
    "Never look down on anybody unless you're helping him up" - Jesse Jackson

  3. #3

    Re: 99 Jeep Cherokee blowing #21 fuse

    Quote Originally Posted by JPNinPA View Post
    Thanks for the full description. It is useful.
    Was the wiring harness, fuse box and PCM swapped with the engine? If so be sure you are looking at the correct wiring diagram for the fusebox you have.
    Batteries are typically 12.6V with engine off and 14.4V engine running.
    The crank position sensor is known to fail without symptoms and sometimes with random no start or shutdown. But it does not blow fuses.
    20A fuse is a sizable fuse to blow. So id do two things. One is trace the wires to components the fuse protects. Make sure there are no pinches, cuts or splices. You dont have to unwrap the wiring harness just note the wire color(s) and verify the color at the wire color at the components. And assume no cuts or splices unless the harness looks messed up or cut.
    Second is to pull the fuse that is blowing and replace with a multi meter (20A capible ) and monitor the current to determine if it is constant 20A or a spike. Electric motors draw a spike of current when they kick on.


    Never look down on anyone unless you are helping them up - Jesse Jackson




    Thanks for the reply. No just the engine was swapped. They said they checked all the wires while in shop and it's possible they just moved something and it temporarily worked. I assume when I put the 30 amp fuse in there to try to get home it caused the 25 amp fuse to the ASD to blow. Is it anyway possible that the new alternator is somehow putting to much current to that fuse at random times and causes it to blow. This started right after I changed the starter and alternator. I am fixing to go try checking wires again and will try multimeter thing also. Thanks for help

  4. #4
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    Re: 99 Jeep Cherokee blowing #21 fuse

    I would check the part number of what they put in or sold you to a listing or replacement parts. Most if not all TJs have the rectifying diodes in the PCM and not on the alternator like earlier years and other vehicles.
    The alternator should not be putting out more voltage or current then what the load calls for. The circuit blowing the fuse has an issue.


    Never look down on anyone unless you are helping them up - Jesse Jackson
    "Never look down on anybody unless you're helping him up" - Jesse Jackson

  5. #5
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    Re: 99 Jeep Cherokee blowing #21 fuse

    Check and clean all major ground and power connections.


    Never look down on anyone unless you are helping them up - Jesse Jackson
    "Never look down on anybody unless you're helping him up" - Jesse Jackson

  6. #6

    Re: 99 Jeep Cherokee blowing #21 fuse

    They gave me a Ultima 117 amp Part #:R111073A . Would that diode in PCM cause the F21 fuse to blow at random times ? I put multi meter on battery , red to hot, black to ground and battery at 12.5. Crank and idling 13.8-13.9, Rev engine up to 2000 or 2500 rpm and most it goes is 14.2-14.3. Which from watching some videos is good. I crawled all under it and checked all the wires to O2 sensors again, also the wires at fuel injectors and ignition coil. Can something be wrong in the power distribution center that would cause it to blow randomly ?

  7. #7
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    Re: 99 Jeep Cherokee blowing #21 fuse

    That is an acceptable replacement.
    If the diode/voltage regulator was bad you would read 19V or greater at the battery.
    Yes a bad PCM can cause issues however i think Id rather checking and changing less expensive items first.
    Di you have a check engine light come on? Does the engine run snd idle smooth?
    One easy test is to disconnect the O2s connector and tie them up so they do not get damaged.
    You will get a CEL but you could rule out the O2.
    I would also be sure the ign coil is properly grounded. Id disconnect and polish both sides just to be safe.

    I havent come across anything like this.


    Never look down on anyone unless you are helping them up - Jesse Jackson
    "Never look down on anybody unless you're helping him up" - Jesse Jackson

  8. #8

    Re: 99 Jeep Cherokee blowing #21 fuse

    No codes and engine runs smooth. Question, If the actual O2 sensor is bad can it blow that fuse. Everything I read and mechanics tell me is a bad O2 would throw up a code if bad and would be running rough. I will do that with the coil. This must be a pretty rare problem . With this doing this so randomly it's almost impossible to repair. If I do something , the only way to know if that fixed it is to drive it for no telling how many miles. THis last time it had been driven almost 70 miles total . I've spent 300.00 in wrecker fees the last few weeks. And I'm worried it's eventually gonna die in the middle of a intersection or somewhere I can't get off the road and smash 'O' , ole Hawke is pushin up daisies ! I appreciate your help.

  9. #9
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    Re: 99 Jeep Cherokee blowing #21 fuse

    You are correct. O2 sensor usually fail open and cause rough idle. However if each time the fuse pops of the engine dies the codes get cleared, then the engine runs off of a lookup table open loop until the engine comes ip to temp and other conditions are met. Quick check would be to check the trip odometer. If it resets when the fuse pops then any code is being cleared. And there is no time to run rough.

    Yes this is odd and rare.

    So clean grounds and power cables an connection points. Both sides. Battery and other engine, chassis, alternator and starter
    The coil ground and power.

    Then replace the fuse with the ammeter to read current with key off, key on engine off and key on engine running.



    Never look down on anyone unless you are helping them up - Jesse Jackson
    "Never look down on anybody unless you're helping him up" - Jesse Jackson

  10. #10

    Re: 99 Jeep Cherokee blowing #21 fuse

    Thank you . I will try all this and let you know back

  11. #11

    Re: 99 Jeep Cherokee blowing #21 fuse

    I removed the coil and cleaned and sanded the bracket it bolts to and the grounds below it. Used multimeter and got a reading of 1.2 primary and 10.35 on secondary ( which I found online should be .8-1.5 primary and 9.5- 17.5 secondary ) . Cleaned alternator connections, ground by battery, took back cover off PDC to inspect wires, cleaned the hot terminal and the connections on it. When I got everything all back , cranked and let idle for maybe 15 minutes while i put away tools . Got in and idled up to about 1800 rpm and it shut off. #21 fuse blown. Installed another one , idled up again to about 1800 rpm and blows again. So this is now different. That was my last 20amp fuses , so have to get more today. Not sure if me moving things around has now made it start blowing the fuse quicker , but probably a good thing. Should make it easier to find I hope.

  12. #12
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    99 Jeep Cherokee blowing #21 fuse

    1800 rpm. Sounds like a vacuum leak.
    Did you sand the ground to body and ground to engine?
    Read the resistance from the neg battery terminal to body. Like a door hinge body side. The battery ground to engine.
    Should be less than 2 ohms
    Still would like to know the current draw on the fuse blowing circuit. Does it start at 20A and slow blow or is it a 30A spike..?


    Never look down on anyone unless you are helping them up - Jesse Jackson
    Last edited by JPNinPA; 12-02-2021 at 01:55 PM.
    "Never look down on anybody unless you're helping him up" - Jesse Jackson

  13. #13

    Re: 99 Jeep Cherokee blowing #21 fuse

    Ok, gonna tell what I did yesterday. I decided to take the new alternator off and have it tested at Oreillys where I bought it, because I wasn't having this problem until I changed it because they had told me my old one was bad and wasn't, it was the starter. I also decided to go ahead and just buy another new ignition coil too. I went in store and manager said no way was alternator bad or cause a fuse to blow, but I got them to test it and guess what it was bad. Then he kinda mumbled that maybe the internal ground was bad in alternator, but then started telling me that whatever was blowing fuse had ruined the alternator. But went ahead and replaced it. I came back and put it on and the coil I had bought. Checked , cleaned , sanded all the grounds I could find again. One by battery, two post under coil, a small couple wires close to dip stick, a strap from firewall to back of engine valve cover, a couple ground connection by PCM. I cranked and let warm up , ran it up to 2000 rpm for approx 5 minutes, tried from idle to 2000 back and forth many times and fuse didn't blow. Haven't drove it yet. I did notice that one of the small wire that connects to the plug that plugs into alternator is dark green and orange stripe , the same that comes from that fuse to coil, pcm, injectors, O2's and I don't know what else. I had found nothing showing that the alternator was on that same circuit, but I guess it is. Not sure what it goes to , my neighbor thats a mechanic says it goes to pcm. There is also a kinda light green in same plug to alternator. Is anyway possible that the alternator having a bad "internal ground" could cause that F21 fuse to blow? When I told my neighbor what the guy at Oreilly's said about whatever was causing the fuse to blow had ruined the alternator, he just laugh and said no way. I'm hoping that it was the problem, but it's gonna **** me off if it was that and has cost me 300.00 in wrecker charges and other parts and countless hours of my time. Thanks for all your help.

  14. #14
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    99 Jeep Cherokee blowing #21 fuse

    Well i hope you have it fixed.




    Never look down on anyone unless you are helping them up - Jesse Jackson
    "Never look down on anybody unless you're helping him up" - Jesse Jackson

  15. #15

    Re: 99 Jeep Cherokee blowing #21 fuse

    I went in store and manager said no way was alternator bad or cause a fuse to blow, but I got them to test it and guess what it was bad


    My wife used to have a merc that ate starters, 5 in 180k [7yrs] I only had to pay for one due to lifetime warranty. I brought one in and they put it on the testing machine and it passed. He told me that starter is still good, I said then why is smoke curling out of it - they gave me a new starter.

    I also hope you got it fixed and don't have any more issues for a long time.

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