Help no spark wiring nightmare! - Jeep Cherokee Forum

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Help no spark wiring nightmare!

Old 10-18-2018, 11:03 AM
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Default Help no spark wiring nightmare!

Just bought an 87 Comanche the wiring is a thing of beauty! (Not!) Sheís a non runner. anyway Iím tracking down my issues one by one and at this point I get a crank but no start and no spark. the smaller yellow wire to the icu is only getting .64 volts (supŰs-se to be 5v) also the TPS is only getting .2 volts Iím thinking I have a short to ground on one of the 5v reference wires. If I put my meter to ohms and connect to the 5v wire and touch to ground I get continuity but unplugging either plug ab or cd from the Ecu I lose continuity... is there a list of which sensors use 5v? I havenít been able to check the cps according to cruiser54s test but it looks like the Po put a brand new on. I get power and grounds to the Ecu except for the latch relay but if I jump 12v to the ecu from the relay socket I have no change in readings.. any help would be appreciated
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Old 10-18-2018, 08:13 PM
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You might try unplugging sensors one at a time and see if you get your 5v back. Sometimes a sensor going bad can short it all out.
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Old 10-18-2018, 11:51 PM
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Thanks Garrett! I unplugged the TPS, MAP, and the ICU with no change is there anything else that takes 5v?
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Old 10-19-2018, 08:13 AM
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I think the crank sensor gets a 5v reference too. if that doesn't work you have to start checking the wiring for damage.
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Old 10-19-2018, 10:39 AM
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The Renix only has 2 wires for the crank so I donít think it gets 5v ref. I took my better multimeter home last night and checked the CPS voltage and it was low (around .25v) this morning I went out and unplugged the dizzy thinking maybe it had 5v to it and magically my 5v came back to my tps so i checked the icu and had 5v there... so I plugged the dizzy back in to see if it would drop again and it didnít 🤷*♂️. So I decided to pull the top bolt from the cps and get it closer to the flexplate for temporary testing I now get .45 which I hear is good and checked it all the way at the ECU and I still get .45 so it should spark right? well itís still not sparking anyone know any other good tests to isolate the problem?
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Old 10-19-2018, 04:50 PM
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I think I found a problem! Cracked open the computer and found these
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Old 10-19-2018, 04:55 PM
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Originally Posted by RustyRug View Post
The Renix only has 2 wires for the crank so I donít think it gets 5v ref. I took my better multimeter home last night and checked the CPS voltage and it was low (around .25v) this morning I went out and unplugged the dizzy thinking maybe it had 5v to it and magically my 5v came back to my tps so i checked the icu and had 5v there... so I plugged the dizzy back in to see if it would drop again and it didnít 🤷*♂️. So I decided to pull the top bolt from the cps and get it closer to the flexplate for temporary testing I now get .45 which I hear is good and checked it all the way at the ECU and I still get .45 so it should spark right? well itís still not sparking anyone know any other good tests to isolate the problem?

As you found, the CPS generates an AC signal.

Your photo didn't show up.

With your 87, you need to go to my website and do Tips 1 through 5 very soon.

If you need an ECU, check this out: https://www.youtube.com/watch?v=52CG...ature=youtu.be
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Old 10-19-2018, 11:28 PM
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Well I opened the ECU and found part of the sensor ground burnt up I soldered it back but no improvement was shown also the computer was commanding the fuel pump on whenever keyed on so I ordered an 89 ECU should get it in the morning... gonna need a fuel pump for sure and the tank needs cleaned out... hasnít been driven since 2011 I think. Thanks for your help Cruiser, I plan on performing many of your tips, I am only waiting to get it functioning to some point so I can tell when I do something right and when Iíve screwed something up
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Old 10-20-2018, 12:09 AM
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Originally Posted by RustyRug View Post
Well I opened the ECU and found part of the sensor ground burnt up I soldered it back but no improvement was shown also the computer was commanding the fuel pump on whenever keyed on so I ordered an 89 ECU should get it in the morning... gonna need a fuel pump for sure and the tank needs cleaned out... hasnít been driven since 2011 I think. Thanks for your help Cruiser, I plan on performing many of your tips, I am only waiting to get it functioning to some point so I can tell when I do something right and when Iíve screwed something up

You're doing just fine.

Re-do the grounds as in Tip 1 before you plug the new ECU in.
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Old 10-20-2018, 07:29 PM
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Got it! new Ecu and fuel pump along with many other misc items and itís running! Thanks for the help! I will be performing the tips soon! But it needs a lot more work before driving daily
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