No spark: how to tell if crank position sensor is bad?

[TheLoreAxe]

Morning Yall, I have a wrangler 97 4.0 manual.

The last few days my jeep has failed to start when the weather is cold in the morning. (EDIT: Cranks but does not start). If I wait till after work, it starts up (15 degrees warmer). I have just started having this issue now that the weather has started cooling down. While yes, it could be totally unrelated, it definitely is suspicious.

Last time it did not start, I did a few tests:
1. Took the battery to Autozone and they said it was good (but they didn't do a load test???).
2. Manually checked battery, 12.6v cold and 11.5 starting.
3. Fuel pressure good.
4. Pulled out ignition switch and tested all wires.
5. Pulled a spark plug and didn't have spark.
6. Checked fuses 9, 10, 11. After wiggling them, it started up. (could be related could be random coincidence.)

Three warm days later it got cool and it did not start again, checked fuses and nothing. Hit CPS and it started.
Today, cool morning, it did not start, hit CPS and it did not start. Had to drive a different car.

A month ago I replaced the Crank Pos Sensor and the fuel pump. (Sadly did not keep the original CPS). Replaced due to no start but that turned out to be the PCM. Side note FYI for those out there: IF GUAGES ONLY HALF WORK, IT IS THE PCM. Too many mechanics told me it was the fuel pump or CPS and every thread I see relating to gauges not working they blame something else. Only one thread was correct about it being the PCM.

My assumptions from reading other threads are it is either the Crank PS, Cam PS, frayed wire to starter not giving full amps, or distributor cap.

I replaced the distributor cap about two years ago.

Anyways, any suggestion is appreciated!

 

[GregBelleville]

Where did you get the replacement PCM? If it isn't a wranglerfix, I wouldn't trust it.

 

[TheLoreAxe]

Where did you get the replacement PCM? If it isn't a wranglerfix, I wouldn't trust it.

Flagship One. PCM failures result in the gauges not working from my experience. The only thing that fails on a pcm is the capacitor leaks out or blows. The rest of it is built bullet proof basically. I have taken apart one in the past to fix them. But I could be wrong that they messed it up and the soldering was a fail in some other aspect.

 

[JKP]

This sounds like the caps in the PCM if it's like the YJ PCM. If you turn the key to ON for a few minutes and let the PCM warm up, will it start?

You can also check for thermal issues by using a hair dryer or heat gun to warm the PCM up to verify it's a temp issue. Could be a bad solder joint, etc.

The Sirius XM antenna on my Honda only works when it's cold...

 

[tworley]

Flagship One is notorious for providing faulty pcms.

The crankshaft position sensor and the camshaft position sensor need 5V. The PCM reads these during start up, if one is out of whack, then it will not sync with the other and cause starting issues. That is where I'd start.

 

[TheLoreAxe]

Flagship One is notorious for providing faulty pcms.

The crankshaft position sensor and the camshaft position sensor need 5V. The PCM reads these during start up, if one is out of whack, then it will not sync with the other and cause starting issues. That is where I'd start.

Where do you suggest I should get a new one if that is the issue? I plan to start with testing the cam and crank sensors when I get home.

 

[TheLoreAxe]

This sounds like the caps in the PCM if it's like the YJ PCM. If you turn the key to ON for a few minutes and let the PCM warm up, will it start?

You can also check for thermal issues by using a hair dryer or heat gun to warm the PCM up to verify it's a temp issue. Could be a bad solder joint, etc.

The Sirius XM antenna on my Honda only works when it's cold...

Ill have to try the suggested bits when I get home. Really hoping I did not get a faulty PCM.

 

[tworley]

Where do you suggest I should get a new one if that is the issue? I plan to start with testing the cam and crank sensors when I get home.

@Wranglerfix has a solid reputation here for PCMs.

IIRC, the 5v to the CPS and CAM are provided from the ASD relay. So check that too.

 

[Pascal25]

Make sure to check your grounds too.

 

[TheLoreAxe]

Make sure to check your grounds too.

I have gone through the grounds but it never hurts to check them again.

 

[TheLoreAxe]

@Wranglerfix has a solid reputation here for PCMs.

IIRC, the 5v to the CPS and CAM are provided from the ASD relay. So check that too.

I have switched it with the horn to test that.

 

[TheBoogieman]

Where do you suggest I should get a new one if that is the issue?

Call Mark Phone: 866-888-7710.