'89 Cherokee Won't Run - Help!!

I can't get my Jeep to run, so I figured I'd ask the experts for some help. It's an '89 4.0/5 speed that I've owned since new, currently has 250k miles on it. It had been running kind of rough and stalling so I performed the following maintenance:
* removed & cleaned throttle body & IAC pintle.
* noticed a leaky injector o-ring, so I replaced all o-rings,
* checked all vacuum lines (all lines were replaced with OEM harness not too long ago, O2 sensor replaced 6,000 miles ago)
* installed new cap & rotor.

After getting it all back together, it cranks but won't start. I disconnected the CPS to get the fuel rail off, so I think that might be part of the problem. On a cold engine, resistance between the two CPS leads at the connector reads just under 200 ohms. My Chilton book says to check for resistance at the ignition control module between the A and B pins. Is this check supposed to be performed with the ignition on or off? I was afraid to check this with the ignition on because I thought I might fry something. With the ignition off, I measure 0 ohms between the pins at the ICM connector whether the CPS is plugged into the harness or not.

What is the proper way to verify CPS functionality? Would the simple act of unplugging it be enough to kill it? This is the original CPS, so I'll replace it if I have to - I just don't want to replace it if it isn't bad.

Any help would be appreciated.

Thanks,
Jim

CPS on these is always suspect, but first I would check if you have spark? and fuel pressure at the fuel rail? This is where you should start. If no spark, did you forget to reconnect something, or did the parts guys give you a bad/wrong part?
I'm a little suspicious that it ran, allbeit poorly, before you did the work, but now not at all.
Check those things and get back to us. Maybe someone else has ideas?

Everything is hooked up. The only pieces changed were the injector o-rings and cap/rotor. There appears to be plenty of fuel pressure at the relief valve on the fuel rail. What would be the best way to check for spark? Also, in regards to how I'm checking for CPS resistance at the ICM - is that the right approach?

Thanks

I suspect something came unplugged. The wires running off of the front of the injector rail and down (dangerously close to the exhaust header) are the 02 senor, knock sensor and temp sensor (for the computer) never have unplugged them and tried to run the motor, but all are required for proper ignition and/or timing. A loose MAP plug will stop you dead in your tracks.
If I remeber my wires correctly, at the coil, the large yellow wire is power, the small yellow wire is trigger (pulse), the green tach and the black ground.
I´ve checked the CPS for pulse, by hooking my voltmeter (lowest volt setting) to the leads coming out of the CPS and cranking the motor with the coil unhooked. Voltage is really low but the pulse can be seen.
Hold the coil wire a short distance away from a good ground, crank the motor, not much or it may damage something. Might try it at a spark plug boot, but much more likely to shock the stuffing out of yourself.

Beagles said:

This is the original CPS, so I'll replace it if I have to - I just don't want to replace it if it isn't bad.

Click to expand...

You're joking, right?

At 250,000 miles you still have the ORIGINAL CPS and you don't want to replace it "if it's not bad"? Most of them don't last 100,000 miles, fer cryin' out loud.

Pull the plug on the CPS and test resistance right at the connector. Then crank the engine and see if the CPS generates any voltage.

Eagle is correct (250K on OEM CPS is somewhere beyond amazing!) but he forgot to mention that you should use an analogue voltmeter, rather than a digital voltmeter, as the analogue version will respond to the small AC voltage signal from the CPS more visibly. Replacement? Expect to pay about $40 at the dealer, and the ones at Q-tec won't work (they're for the HO electronics, and are Hall-effect switches rather than voltage generators.)

Also check to see if the CPS harness has rested on the exhaust too long and melted thru - I just went thru that myself about Thursday (I think it's the OEM sensor, but it was grounding itself out on the exhaust header intermittently.)

Also, find and buy a 3/8x36" ratchet extension if you can find one! I picked one up at a "Tool Exchange" type store a couple years ago, and it cut the job from 2-1/2 hours to about 20 minutes. Also get yourself either an 11mm or 7/16" socket with the universal joint built in, as it will save lots of trouble with the job as well (I've got changing these things down to a science now - I think I can swap a CPS in about 10 minutes...)

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Appreciate the comments on the life of my CPS, but it appears to be the root of the problem. I checked for a voltage spike with my meter while cranking the motor and didn't see any deviation from the base reading (which was about 3.75 V).

Are any of the aftermarket sensors any good or is the dealer the only way to go? I usually can't get to the dealership when the parts counter is open.

Thanks again!
Jim

The aftermarket sensors are a different sort - they will only work with HO electronics. They are actually a switch, where the 87-90 (Renix) sensors generate a small varying voltage.

If you want to try the boneyard crawl, 89-90 are the same, 87-88 are the same (per OEMR parts catalogue,) and I do not know if they will actually exchange between the two, as I haven't tried yet.

For new, you are looking at dealer only. There are no aftermarket sources for Renix CPS that I have found yet (but I would be pleased to be proven wrong!)

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