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Q: No crank when hot

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When hot no crank, no starter signal from start relay, relay test 83 tohms, power and ground at terminals in fuse box, Ecm shows crank request-yes in pid, purple/white from 26 in ecm connector c1 shows a voltage drop to .034 when the no crank occurs this voltage gradually increases to .470 at that voltage the car starts again (5-10 minutes) peak voltage is about 1 v during start up. Pretty sure this is a Ecm failure but the flow chart I have access to has no info on what the spec is for this wire.

My car has 125000 miles.
My car has an automatic transmission.

A: The PCM supplies power to the purple/white ...

The PCM supplies power to the purple/white wire which activates the starter relay. I would disconnect that wire from the PCM, mostly likely it would need to be cut somewhere it can be easily repaired or you will need to remove the wire from the relay connection. Once this is accomplished, I would use a jumper wire to supply power to the relay. This will test the entire system except the power coming from the PCM. Keep in mind, you want to do this when the car is hot and won’t crank. If the starter turns the motor over normally, I would definitely suspect the PCM.

In addition, I would check the voltage at the starter relay coming from the PCM when the car is cold. I would check voltage and not do a voltage drop. A voltage drop is not the appropriate test for this particular problem. I would then test the voltage at the relay coming from the PCM when the car is hot and won’t crank. If the PCM is indeed failing when hot, which is a common type of failure for a module, I would compare the difference in voltages. I would expect to see a difference of at least three or four volts. But most likely more.

Whenever I am potentially diagnosing a computer module, I do every test I can think of to be sure of my diagnosis. Modules do not fail as often as most believe and can be very expensive. I personally have only replaced a hand full in twenty years, with the exception of PCM’s in Chrysler minivans. If you need some help with these checks, have a certified technician, like one from YourMechanic, perform these tests and properly diagnose your starting issue.

Good luck!

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