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I think my CPS is bad, but would like second opinion

bigalpha

bigalpha

Moderator
Location
Tucson, AZ
88 MJ, 6cyl, 150,xxx miles

Symptoms:
1. Truck was running then bogged out and died; won't restart
2. No spark
3. Tested the resistance of teh CPS and got 295 ohms, which is just outside the range that is listed on lunghd website
4. It has gas in the tank
5. Battery shows 12.xx V with a multimeter
6. Battery drops to 9-10V when cranking, then immediately returns to 12V
7. Fuel pump primes
8. I tested for voltage at the CPS connector while the engine was briefly turned over and got 0.00V
9. I have reverse lights

So I'm thinking that the CPS is bad but I wanted someone who isn't as bad of a mechanic as I am to pitch in their opinion.
 
CPS needs to be getting 5V. Whether yours is good or bad (your resistance check seems to say it is good), it still won't work unless it is getting 5V. The power supply comes from the ECM, but can also be taken down by a short or other bad sensor on that same 5V supply circuit.
 
I see, so I check the resistance on the sensor side of the wiring and the voltage on the Jeep side of the wiring?
 
Sorry, but I may have given you bad advice. Reading the RENIX manual some more, it looks like the OTHER sensors get 5V, but the CPS actually just induces voltage to the circuit using the Hall Effect as the flywheel teeth pass the magnet in the sensor. So there would be no voltage in the harness.
 
winterbeater said:
Sorry, but I may have given you bad advice. Reading the RENIX manual some more, it looks like the OTHER sensors get 5V, but the CPS actually just induces voltage to the circuit using the Hall Effect as the flywheel teeth pass the magnet in the sensor. So there would be no voltage in the harness.
Click to expand...

No worries.

Still 0V coming from the CPS as the engine is briefly cranked over. Seems like the CPS is bad, then.
 
Have you checked the CPS itself for producing voltage (test 2). I found this in something I had saved:

TESTING PROCECURE for 1987 – 1990 4.0 L engines

Test # 1

· Get a volt/ohm meter and set it to read 0 - 500 ohms. Unplug the CPS and measure across the CPS connector's A & B leads. Your meter should show a CPS resistance of between 125 - 275 ohms. If the CPS is out of that range by much, replace it.


Test # 2

· You'll need a helper for this one. Set the volt/ohm meter to read 0 - 5 AC volts or the closest AC Volts scale your meter has to this range. Measure across the CPS leads for voltage generated as your helper cranks the engine. (The engine can't fire up without the CPS connected but watch for moving parts just the same!) The meter should show .5 - .8 VAC when cranking. (That's between 1/2 and 1 volt AC.) If it's below .5vac, replace it.
 
The Renix CPS\CKP crank sensor needs to read 200 ohms plus or minus 75 ohms--295 ohms is out of spec and you need to replace it.
 
The fact that it's reading a little too high in the resistance department, and the fact that it's generating no voltage says that the CPS is *for sure* bad and needs to be replaced?
 
I just wonder whether a digital meter would even register 0.5 intermittant volts. Another way to check is whether your injectors are firing. A noid light or maybe the smell of fuel would tell you that. If the injectors are firing, the CPS is signalling.
 
Would it be best to test the CPS connector with an analog meter instead of a digital meter? I would sure hate to buy and replace the CPS of it's actually good.
 
Didn't get a noid light, but I did get an analog meter from the store.

Cranked the engine with the meter hooked up to the CPS side of the wiring and the needle moved ... nowhere. Not even on it's lowest setting of 1.5V did it even tick.

I guess I'll be replacing the CPS soon.
 
bigalpha said:
6. Battery drops to 9-10V when cranking, then immediately returns to 12V
Click to expand...

If the voltage drops to 9 volts, the ignition module won't fire. Check your battery cables and the battery connections as a poor connection will rusult in poor cranking. I would also recommend checking that voltage with a meter while you crank as the dash guage isn't terribly accurate.
 
The connections are good n tight. I have a new 5-90 neg cable installed.
I got these numbers by using a DMM. I also tested that on my XJ and it also dropped to 9-10V briefly before it started up.

What's weird is that i test the battery and it will show 12V but the dash gauge shows it being much lower.
 
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