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Spark problem, no spark when letting off the key in the run position?

Tiny
ROBERT BAUGHMAN
  • MEMBER
  • 1986 JEEP COMANCHE
  • 2.5L
  • 4 CYL
  • 2WD
  • MANUAL
  • 200,000 MILES
Only getting spark in the cranking position, no spark when letting off the key in the run position. Replaced the crank sensor. Also replaced the coil unit off a known good one.
Wednesday, October 18th, 2023 AT 5:54 AM

13 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,867 POSTS
Hi,

That is a tough one. It sounds like power is lost when the key returns to the run position. So, it could be at the ignition switch. On the other hand, the ignition control module may have failed as well.

Here is what I need you to do. When you lose spark, with the key in the run position, check to see if there is power to the coil.

Next, I need to know if this has a carburetor or is fuel injected. One last thought. Follow the directions in this link and let me know if you are able to retrieve any diagnostic trouble codes:

https://www.2carpros.com/articles/jeep-diagnostic-trouble-codes-for-car-built-on-or-before-1995-odb1

Let me know what you find.

Joe

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Wednesday, October 18th, 2023 AT 5:55 PM
Tiny
ROBERT BAUGHMAN
  • MEMBER
  • 8 POSTS
Its throttle body, I will try what you suggested. I changed the ignition switch on the column with a known good one. Thank goodness I have several of these Comanches to get parts off of, all ran when parked with no problems. This problem started when I drove it 2 blocks and went across the intersection and it stalled then this problem occurred.
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Thursday, October 19th, 2023 AT 10:55 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Thanks for the update. I had one of these back in the 1980's. 4.0L 5 speed 4wd. I absolutely loved the truck. It was fun to drive. I don't know where you are located, but here (PA), there are none left. Rust got them all.

Take care and let me know what you find.

Joe
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Thursday, October 19th, 2023 AT 5:39 PM
Tiny
ROBERT BAUGHMAN
  • MEMBER
  • 8 POSTS
No codes at all when turning key as suggested. Check engine light has never come on in all the 15 years I have owned it. Fuel pump runs all the time. Has pressure and volume. Still same problem.
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Tuesday, October 24th, 2023 AT 12:41 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,867 POSTS
Hi,

Okay, we need to check if power to the ignition control module (ICM) is present when the key is in the run position. I attached the schematic below. I need you to locate the ICM on the right side of the engine compartment. The coil will be mounted to it.

Once you locate the module, there will be a yellow wire going to it. With the key in the start or run position, that wire should have power.

Check this and let me know what you find.

Take care,

Joe

See pic below.
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Tuesday, October 24th, 2023 AT 5:25 PM
Tiny
ROBERT BAUGHMAN
  • MEMBER
  • 8 POSTS
There is power there at the yellow wire when the key is in the run position.
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Tuesday, November 7th, 2023 AT 7:19 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,867 POSTS
Hi,

Was power present if the key was in the start position? Let me know. Another tech reminded me of the old-style ballast resister. What you experience is textbook when one would fail. I went through two different manuals hoping to find one on this vehicle with no success. Do you physically see one on the firewall or engine compartment?

I suspect that we have a faulty ground for the ICM, a faulty ICM, or a faulty ECM. I attached further diagnostics below for testing the ICM. Let me know if you are comfortable performing them. Note the last step requires a very old-style scan tool. Chances are you won't find one, but let's get to that point.

Take care,

Joe

See pic below.
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Tuesday, November 7th, 2023 AT 6:24 PM
Tiny
ROBERT BAUGHMAN
  • MEMBER
  • 8 POSTS
Yes, voltage in start and run positions. I took an ICM off another truck I have that is known to be good and the same situation. No ballast resistor on this vehicle. I do have two 1989 Comanche trucks with the same engine and throttle body engines both ran when parked. Will the computer off these trucks work?
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Wednesday, November 8th, 2023 AT 7:02 AM
Tiny
ROBERT BAUGHMAN
  • MEMBER
  • 8 POSTS
Terminal B to ground has continuity.
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Wednesday, November 8th, 2023 AT 10:33 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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If we have power and ground, it sounds like the ICM isn't getting an ignition signal from the ECM.

See if the ECMs from the donor vehicles have the same part number, and connectors, or if anything appears to be physically different. If it isn't compatible, chances are it simply won't start.

Sorry, the wiring schematics I have due to the model year are really difficult to follow and extremely vague.

Let me know.

Joe
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Wednesday, November 8th, 2023 AT 5:45 PM
Tiny
ROBERT BAUGHMAN
  • MEMBER
  • 8 POSTS
Your schematics are just like the factory schematics. I have the original schematic book from the dealer. I bought the last vehicle from the Jeep dealer I used to work at when they closed, and it happened to be a Comanche. When they had their auction, I bought all the Comanche books they had. I changed the computer, and it starts and runs as it should. So, if anyone else runs across a no spark when the key is in the run position, the crank sensor is good, the fuel pump runs all the time, the coil is good, and the ignition control module is good then the computer is bad. Do you know anyone who rebuilds these Remix computers? In the past when I had the same problem, I just changed the crank sensor and it started. I thank you for all the help you have given me.
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Tuesday, November 14th, 2023 AT 12:58 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Thanks for the update. I'm glad you got it going again. As far as places that rebuild them is a hard one to answer. Have you checked online? I know there are several different places that rebuild them.

Let me know.

Joe
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Tuesday, November 14th, 2023 AT 2:59 PM
Tiny
ROBERT BAUGHMAN
  • MEMBER
  • 8 POSTS
SIA Electronics will rebuild the Jeep Renix computer for around $129.00 lifetime warranty.
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Monday, November 20th, 2023 AT 12:24 PM

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