CPS

I've done the search thing, but I don't think I've ever seen the answer. Will the engine still crank if you have a bad CPS? My '89 4.0 5 spd. died in traffic last night while sitting at a light. No sputtering or nothing. Fuel at the rail was about all I looked for on site. Towed it home and confirmed no spark at plugs. Too late to look at anything else. Did some searching and this was the only question I had. Thanx.

Engine will definitely still crank if the CPS is bad. There is no tie-in between the CPS and the starting circuit. However, if the CPS is bad, the engine won't start. (you won't have spark or fuel to the injectors) Testing of the CPS is included for your reference....Good Luck!

CPS TESTING PROCEDURE 1991 – 2001 4.0L H.O. engines

1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (A-B-C looking into connector left to right with the part with the notch in the middle on the right) Ohmmeter should be set to 1K-to-1OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

CPS 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.

I had the same thing happen. but if i let my jeep cool down a little bit it would fire right up. but its really not that hard/expensive to replace (40 bucks or so at schucks) so ya might try that...possibly coil...check it with volt meter.

-PJ-

Thanx guys. Replaced it 2night, and all seems well. Vortex, I tried the cool down test too. Weird how it starts after cooling down. Does the wires get hot being next to the exhaust causing problems?

Ya know i never really figured that one out i always thought it was a spark issue but tried it without touching it and it would always fire right up. But im glad it worked out for you so i hope all goes well in the future.

-PJ-

One of the failure "modes" of the CPS is thermal failure. If you are lucky enough to have that happen to your sensor, it will generally work fine when the engine is cold and when the sensor warms up enough, it will fail. Cool down again and it works. If you have these symptoms, the problem is strictly within the sensor itself. I've seen this happen more than a few times but most of the time, the CPS goes completely out and doesn't work at all. Happy to hear that replacement took care of yours....

Retep Draw said:

Thanx guys. Replaced it 2night, and all seems well. Vortex, I tried the cool down test too. Weird how it starts after cooling down. Does the wires get hot being next to the exhaust causing problems?

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electrical resistance increases with temperature. A marginal CPS can function at cooler temps and fail at higher temps if it's signal strength is only strong enough to travel through wires with little resistance. It may be exacerbated by the exhaust heating the wires, but the CPS is still failing at that point.

A heat sleeve on the CPS harness wouldn't hurt to reduce future problems along with a shield to reflect heat away from the CPS, but when faced with the same concern, I was too lazy to create anything. Maybe I'll look into it if this one fails again.

if the CPS gets debris on it it will seem like its bad too something to look out for incase you have a bad rear main or a bad front seal on your tranny