QuestionQUESTION: My jeep stalled while driving down a mountain pass. It restarted 2 minutes later. Went 1 mile and stalled again (instant shut off), it started again an went another mile and quit. Stayed still this time after starting and it quit after 3 minuets never to start again. Cranks over hard and fast, but no start.
Classic Crankshaft Position Sensor issue I assumed. Tested CPS for ohms and got nothing, ( tester never went off 1, like the leads never touched anything) replaced with CPS from AutoZone which showed 4.8 on 20k ohm setting. Engine still did not start until I removed battery cable to clear error code (51, lean mixture)
It ran for 10 minutes in the garage, Wahoo! but did not start the next morning.Boohoo. I rechicked connections and set motor to TDC and checked the rotor alignment, and it looked good. Tried starting again and it started right up. 6 times in a row for up to 10 minutes.
Went out for a test drive and it died (instant cutoff again). Now it hasn't started since. so..... I have replaced the CPS (CKP), tested coil and plugs for spark (yes) tested fuel pressure (45psi) checked dist cap and rotor (fine) checked resistance on CAMshaft sensor, checked for 5vt at sensors, voltage at injectors, (all fine) swapped ASD relay (no difference) checked vacuum (12psi), cussed, kicked tires... What am I missing?
Is there supposed to be about 5 resistance between pins B @ C on the CPS? (at 20k setting... is that 5K Ohms?)
Is there any tests to check the PCM , like voltage and ground to specific pins?
It's the intermittentness that is killing me here.
ANSWER: Since you have spark and fuel that tells me that the PCM is operational, so there is no need at this point to check power and grounds. The problem appears to be in the fuel injection circuit. One quick test would be to check for power at the injectors. While cranking the engine probe the blue with yellow tracer wire at any of the injectors with a test light. If it lights up the ASD circuit is OK, if not you'll have to trace the wiring back to the fuse box to find the broken wire. If the power circuit is good you'll need to check the injector signal. Since there is no other way to check the injector output from the PCM you'll need a special tool. Go down the the local parts store and buy a noid tester to check the injector pulse signal. That's the only thing I can think of at this point that you may have missed. It might also be a good idea to have the codes read again. And that you read 12Hg of vacuum on an engine that is not running has me a bit mystified.
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QUESTION: The Vacuum reading was during a time when I got it running.
I had said that I tested for power at the injectors.
Tonight I unplugged all the sensors and checked for voltage again, (and the condition of the connections). After a battery recharge and tons of cranking, it sputtered and finally turned over again. Tapping the gas makes it sputter, but it revs up OK. It never died totally tonight, but after I turned it off it was hard to start again (4-5 times).
I'm thinking maybe I got a bad replacement CPS from AutoZone. (it's been mentioned as happening a few times on the forums)
So my question more specifically is:
How do you test a CPS on a 1993 4.0 motor (out of a 93 Grand Cherokee)
The wires to the CPS are:
pin 1 = K25 20wt/bk
pin 2 = K4 18bk/lb
pin 3 = K27 20rd/lg
I think this year was a split year for wiring, there are 93 schematics showing various wires as different colors.
I have read there should be open (infinite) resistance on bin B-C (2-3?) and some say 200 ohms.
My meter has settings for 200, 2000, 20K, 200K 2000k ohms.
My Old CPS shows open between all pins at all settings.
The New CPS shows open on all pins and all settings EXCEPT between pin 1 and 2 (A and B) at 20k, 200k and 2000k it shows 4.8 ohms.
So what are the true and proper readings?
Again....It's the intermittentness that is killing me.
AnswerTo check the CPS you'll need to verify 5v at terminals 1 and 3 with the ignition on, which you have already done. Verify the ground circuit on terminal 2, which you have also done. Measuring the resistance between pins 1 and 2 doesn't tell you much, if its has some its good. You are only measuring the resistance of the coil in the sensor, which has to have some resistance for internal diagnostic purposes. What you need to do is test the output of the sensor itself. This can be done with the engine running or not. Set your oscilloscope to read a 12v DC square wave with a time reference of 40ms. Back probe terminal 3 of the sensor. Crank or run the engine and observe the pattern. The pattern should show clean square waves broken up at regular intervals of 0v. If the pattern is good then back probe the circuit again at the PCM terminal 24. You should observe the same pattern. This test is one that we do with some regularity. It will tell you that there is a valid CPS signal getting to the PCM but it can't tell you if the PCM sees it. For that you will need access to a DRBIII diagnostic tool.
Thank you for clarifying the vacuum concern. As a note, 12hg of vacuum is to low and would indicate an engine mechanical problem, such as late valve timing due to a stretched or skipped timing chain, as an example. It could also indicate a restricted exhaust system.
