1993 jeep grand cherokee laredo won't crank

[greg92jeepxj]

Is there a way to read the code that may be faulty while the jeep isn't running? I know it will show quite a few of them but maybe I could write them all down and start marking stuff off the list. I have a obd 1 and 2 reader but I have never done it without the engine running.

A code reader will pull trouble codes with key on engine off ( KOEO) however since the PCM was swapped after the fact , the troubles codes will be stored in the old PCM. A PCM can only store trouble codes when running. BTW , still working on your latest post of no power to fuses #2 & #3 . So , these are the only fuses that are dead ?

 

[landonsdad2511]

Yes, fuses 2 and 3 are the only ones and only the asd relay and the fuse pump relay as I can tell. I do appreciate everything and for not giving up as well.

 

[greg92jeepxj]

Yes, fuses 2 and 3 are the only ones and only the asd relay and the fuse pump relay as I can tell. I do appreciate everything and for not giving up as well.

Give up ? Did we give up when the Germans bombed Pearl Harbor ? Lol . ( John Belishi , animal house ) . Times like this have me put down a wrench and make a joke rather than throw it.
Wiring diagrams can sometimes have you chase your tail which mine is and has me wondering , shouldn't this wiring map be consistent with a ZJ since both are 4.0 ? Even a '93 ?
One thing I've found is that the ASD and fuel pump relay's terminal 30 are both fed by a wire which originates from PCM cavity # 3 marked ASD . It is a 14 gauge red wire that splices as follows ; cavity # 3 14 ga. red to splice , from there , 14 ga. red to PDC fuse 15 , 14 ga. red to TCU , 14 ga. Red/ black tracer to fuel,pump relay terminal 30 , and finally , 14 ga. Red to fuse # 2 terminal 30 of the ASD . Rather than dig through the wiring harness for this splice ( I can look up splice location if we need ) , deploy use of the computer safe test light and go after the wire destinations to see what's live since we must see where the current is and where it stops since it dosen't make it up to the relays . Strange , fuse 15 is live and connected to bus bar in PDC but yet dosen't energize the splice ? To me , this would mean a short . This splice is above the alternator bracket where the alternator field branches in to the harness . Please be careful with test light on PCM wiring , hope you've got a computer safe test light ? It's truly not cool using a 12 V test light on these circuits . Ask me how I know. Hope this helps and wondering at this point if the TCU can cause a draw ? Fuse #2 is fed from fuse #15 and fuse #3 is fed from bus bar in PDC as fuse #15 is fed from bus bar as well . But if bus bar is live , obviously , then circuit is open at opposite ends , creating a short ? Confusing . Does this point to the TCU ? Hmmm !

 

[greg92jeepxj]

I completely forgot to mention that fuse #15 also feeds fuse #4 , which in turn goes to terminal 30 of the auxiliary condenser cooling fan . I think it would be helpful to verify , if you have not already done so that this relay is active . Also , I found something very interesting while tracing wiring from the fuel pump and ASD relays ; the TCU has a wire , an 18 gauge yellow that goes to fuse #3 in the PDC . This is marked fused ignition . I believe I already posted , but will advise that another wire , a 14 gauge red also from the TCU goes to the splice we spoke of and feeds fuse #2 to terminal 30 of the ASD . This is all very interesting but obviously related . I mentioned that I thought there must be a short , but it is likely that there may just be something that is causing a break in connectivity or continuity . It may be possible that if the TCU is bad and has no bypass to allow voltage to fuel pump and ignition , which it seems to be , it feeds the ASD and may be causing the stop in current , preventing electrical flow to the terminal 30's of the fuel and ASD relays. I'm hoping that the PCM is doing its thing and working right to exclude it from this equation but to verify its working , try probing the TPS connection . The computer safe test light is great for this . You can check the ground ( will light green ) and power ( 5V red ) , signal will vary upon accel/decel . Hoping this further research helps . Sorry for the long time out if service. A week is a long time to be without your jeep .

 

[landonsdad2511]

This may be a stupid question but..
everytime I look up TCU i get TCM. Are they they same thing? And if I bypass the TCU The power in the relays 30 terminals come back does that necessarily mean that it's my TCU that's causing everything?

 

[landonsdad2511]

And of you think bypassing the TCU is the right thing to do, how do I bypass it?

 

[landonsdad2511]

THANK YOU FOR EVERYTHING!!!
the jeep is running now. It was the TCM. Changed it and power came back to everything. Still got some work for me cause it sounds like a vacuum leak. Maybe something I unplugged and didn't hook it back but it's dark so I will figure that out hopefully tomorrow. Your info has been a blessing to me and my jeep. I really can't thank you enough...

 

[greg92jeepxj]

Hey , that is great news ! It was the last link in the broken chain , that last switch to be turned on , so to speak , I felt that it was the only place where power was being lost . I'm really happy for you , this is no time for a man to be without his jeep . Hey , sorry to confuse you about the bypas , the bypass I was referring to would be internal in the TCU ( transmission controll unit ) and what that means is if the unit fails , it will still allow the jeep to run. But since it runs through the ASD , so that the jeep cannot be driven with a bad trans controller and cause trans overheating.
The ASD is meant to save our jeep from its own worst enemy , US ! We may try to drive it and burn up a trans since the solinoid won't shift and overheat internally . Sorry I could not address your posts earlier , had to get back to work , but I just had the feeling you would get it running by the time I got home ! You are a way better diagnostic guy than you thought you were !
Brother , you should be darn proud ! Your progress and prognosis will help many should anyone ever encounters this problem . Guys like you make this forum what we are ! Best of all , you did it yourself and owe nothing to the shop !!!! It's a great feeling when you get your own vehicle running yourself ! Go ahead and slam a couple of brewskies and claim your bragging rights ! You earned it ! I say it again , WELCOME TO JEEPZ.com !!!!!! Good job brother !!!! Greg

 

[landonsdad2511]

Hey Greg. Now my jeep is throwing DTC 24. TPS input is higher than maximum voltage. Looked at the wires and they don't look bad. The TPS Was replaced maybe 2 months ago. What causes this? It's just coincidental that this is happening right after I've done everything I have to it. When I pull out of my drive way the road goes up a slight hill. My jeep had no power. With my pedal to the floor it goes maybe 10 mph if not alittle less.

 

[greg92jeepxj]

Wow , sorry to hear there are more problems . On e side of me says to try the old PCM but before we do , let's see how far we go with the currently installed one . No power sounds to me that first we need to verify the usual stuff first . Check fuel pressure just to be certain that's it's ok and to eliminate any possibilities . We'll have to verify spark advance , that is , that your ignition is receiving increased voltage upon acceleration . But first , before anything else is checked , we need to know what's going on with the TPS . Hope you got a trusty ( computer friendly ) digital multimeter . With the key on engine off (KOEO) , we need to check ground , signal and reference . If ground is bad , we got to find out why . That will necessitate a continuity test with battery disconnected and PCM harness too . But first the TPS. I can only quote from my shop manual , a '92 and it's for YJ,MJ & XJ , so I hope the color codes aren't too far off. But we're looking for good ground from a black/lt.blue wire , 5 volts from a violet/white wire (reference) and variable voltage from a orange/dark blue (signal) wire. KOEO , the meter should read 5 volts reference and voltage should vary from signal wire as throttle linkage is either accelerated or decelerated accordingly. Post back what you get when you can . Right now , it seems your pc dosen't seem to know that your trying to accelerate and won't increase voltage to ignition coil or richen the injector pulse.

 

[greg92jeepxj]

One more thing we can check as well , the voltage reference wire also feeds the MAP sensor voltage reference wire too , you can check its reference voltage to see if its normal or not. If voltage is ok at MAP , then I sounds like the TPS possibly bad as far as it not receiving ground very well .

 

[landonsdad2511]

I forgot to tell you. I had to put the old PCM back in cause with the one I bought didn't seen any good. When I finally got power back. The engine crank but shut off immediately. Kept doing it so I swapped pcms back and it cranked and stayed running.

 

[greg92jeepxj]

Sounds like the swap PCM wasn't a match ? If the service numbers do not match , there's conflict with application , too much to explain but we always have to match these numbers . If they did match , maybe a bad PCM ? Ok , so the PCM in the jeep is the replacement , but not to sound repetitive , the service numbers match ? Also , when you first got it running and at this moment , will the jeep start and idle ok ? Just a driveability problem ? Just the DTC code 24 ?
Wait , confusing myself , the PCM presently in the jeep is the original one ?

 

[landonsdad2511]

I don't have the pcms with me but if I'm not mistaking the numbers do match. If not than that's def my fault. Still learning the electrical side of vehicles. But with the PCM I recently purchased the vehicle crank and fired up but instead of ideling it just shut off. I've tried cranking it multiple times and same thing. When I swapped the pcms back it fired up and ran perfectly. Now it's just a driving problem.

 

[landonsdad2511]

So the original PCM is back in the jeep

 

[greg92jeepxj]

Don't worry brother , totally forgiven. We all have to learn and we still do . Ok , so we are working with the original and correct PCM which will start the jeep and idle it normal , which is great . The PCM is picking up a DTC code 24 which we are now dealing with but all else seems ok.
Have you had a chance to take readings from the replacement TPS ? Also , the MAP sensor voltage reading ? In my mind I am trying to figure if the voltage output to the TPS is correct and the TPS is just not working right. If the MAP is reading a 5 volt reference , then I'm blaming the TPS . Heck , you may want to swap the old TPS back in just for testing purposes unless it's totally wasted ?
i don't know if you do any work on desktop computers or laptops , but Microsoft windows reacts this way every time there is a change in hardware or software , like cd/dvd drives , update drivers , software , etc. and this is not a whole lot different . Only we don't have a screen to see it , unless we had a scan tool . Hope this analogy helps a little . Hoping I'm not driving you crazy with my explanations . Sorry if I am . As soon as the jeep engine warms up to operating temperature ( coolant temp sensor will tell it ) the PCM goes from open loop ( cold warm up mode ) to closed loop ( normal operating mode ) and receive all input from sensors and run the engine accordingly . An engine is blind and it's eyes and ears are the PCM . The PCM is blind and the eyes and ears for it are the sensors. Crazy , but we must deal with it . Sometimes I just want my old '66 Chevy back ! No computers !

 

[landonsdad2511]

I had to go in to work and can't do anything to it till after 6 tonight but I will let you know. And the analogies don't bother me. They help me understand how it works so keep me coming. I do have a obd 1 and 2 scan tool. But other than that I'm as blind as the pcm. As soon as I get home I will check the TPS and the MAP sensors

 

[greg92jeepxj]

Alright brother , take it at your pace , we'll get it . We didn't come this far to quit. Glad you got a real nice scan tool , I once meant to get one like yours , OBD 1 & 2 . Wishing I did . I know it will be Sunday nite , so trying to get a lot of info will have to be another day. If all there is as far as dtc's is code 24 , I would say real quick to verify volt reference 5 volts or so on the MAP and TPS.
I ask for MAP because the wiring diagram shows the same wire feeds the MAP as it does the TPS and your post said you were getting more than 5 volts ? Well , the scanner will be real helpful that when your looking at TPS readings , you can accelerate and see if the TPS is responding correctly . It's hard to gently roll the accelerator linkage and try for a gradual increase in voltage but you'll pretty much get the idea . More analogy ; the TPS works just like a wall switch dimmer , you click it on and gradually increase or decrease the knob to brighten or dim the light .
This is how the PCM knows if you are accelerating or decelerating. A MAP sensor tells the PCM whether the engine vacuum is raising or lowering in accordance with rpm rise or fall. The PCM has to know these things so that it can increase or decrease spark advance and fuel trim ( rich or lean) .
Without this info , it will go,on a default mode with check engine light on lean spark and lean fuel trim. This causes power loss. Your power loss is extreme , but no stalling ? No MIL ( malfunction indicator lamp ) ? Just a huge flat spot and no go . Hope fuel pressure is ok but I would think since it's starting ok that it is ok . Alright bro , good day at work and we will do what we can today and we will keep at it . Sorry it will be week two . But we're almost there .

 

[landonsdad2511]

Well I'm def sure it's the TPS. the plug if the MAP sensor and TPS both are 5.0 volts exactly. The TPS is approximately 4.4 with the throttle open and closed

 

[landonsdad2511]

It's like it's reading open all the time