Have Jeep Questions? Ask a Jeep Repair Expert for Answers
Hi there. Welcome to our site. Just logged in and saw your question.
Do you still need help? If so, are you working on a Renix (no vacuum advance on distributor) or Motorcraft ignition?
I see. On a Renix system, the signal from the crank position sensor (CPS) goes to the MCU, which then sends it to the ICM; the ICM acts as a relay that fires the coil.
In order for us to know where the problem is, we would have to check the signal as it travels from CPS to MCU to ICM to coil. We would need an oscilloscope to do this. Do you have access to one? - FYI: They're less than $100 online. Click here to see sample.
sorry for the delay...
We were never allowed at the dealer to open-up an MCU, so I wouldn't know what part "diode 27" plays in the spark creation scheme.
MCU is what AMC called the engine control module or ECM. There is no cam sensor in a Renix system; both spark and injector pulse are controlled by the MCU based on engine speed signal. Once the engine is running, MCU will look at MAP and ECT signals to control ignition timing through the ICM.
The only way I know to check these signals is either with a scope, or a high-end multimeter capable of reading frequency (Hz).
Just occurred to me we could possibly eliminate the CPS signal as being the culprit by checking injector pulse. Do you have a Noid light or LED test light you can use to do that?
I will do my best to assist you, but please keep in mind that while I can provide you with technical guidance, working on this type of issue requires you to have not only access to several different automotive tools, but the skills to use them as well.
At this point, I don't think I can offer any other suggestion that could help you solve your issue. I'll opt out of the question and let other experts chime in. Please do not reply until another expert responds or the site will revert the question back to me.
Hello. Different expert here. In our shop, we have observed this problem many, many times. The problem (according to the Jeep tech service people) is a loss of the crank angle sensor voltage. They recommended that we cut the wire connector out of the harness and off of the end of the crank sensor, then solder and heat shrink the wires back together. They claimed that the wire connector caused enough reduction (we are talking millivolts here) of voltage to not fire the system. I would recommend you try this if you have a soldering gun and the experience to use it. Every time we had one towed in for this, and we cut and soldered the crank sensor, the problem was fixed. Hopefully, this will work for you.
If you have any questions, please feel free to reply.
Thanks for choosing Just Answer. It was a pleasure to assist you today. I trust my input to your request is satisfactory to you and meets your expectations. A positive rating would be greatly appreciated for my assistance on your inquiry.
Thanks in advance and thank you again for choosing Just Answer