CPS Questions

Okay, I know what a CPS is, I replaced the normal one with a high altitude one on my 90. I have a good idea how they work.
I don't understand the behavior of my current sensor.
About 6 months ago, the Jeep quit at the first stop light on the way to work. The idle on it is probably 500 rpm or so, lower than spec. It wouldn't start up until I unplugged the CPS and plugged it back in. Classic CPS symptoms. Drove it to the mechanic a block from the house, told him the symptoms, he agreed on the CPS, and supposedly replaced it.
So a few months later I'm having CPS symptoms again. Driving at 75, momentary cut out, keeps running. A couple more stalling at idle times that were cured by the momentary unplug routine.
I don't understand:
1. Why this new sensor seems bad?
2. If having the lead near the header pipe is causing the effect?
3. What unplugging and plugging the connector does to make it work? The connector contacts are clean.
4. I know I've seen somewhere a procedure for moving the cps closer to the flywheel to make it work better. Anyone know anything about this?
5. Just swap it on general principles, and hope that the last one was marginal to start with?

Thanks,
Fred

Hey Fred (long time!)

Could it be on your engine harness side? how's that side of stuff (not so much the connector, that you've inspected and "passed" but the harness itself...) - the connect/disconnect on teh connector could be offering you a little wiggle to the harness itself reconnecting some intermittant failure there ...

For my dime, I'd wanna set the damned thing on a scope and see what that CPS is doing - the output from that sensor is isn't much to begin with so it won't take much to lose it (confuse it) altogether -

I slotted my mounting holes and moved my CPS a bit closer to the tone-ring on the flywheel, but really can't say that that's done much - (it'd do more for the late model rings I'd imagine) - I'm still paranoid about having the thing crash into the ring, so I carry a spare!

Can't imagine that heat there on the CPS's harness would have too much of an impact (you're smart enough to look for and recognize melting and "real" damage and junk like that...)

When you die, is it really a CPS death (like you're rolling and the Tach drops dead to nothing; not really indicating the residual momentum turning the engine...?)? (I'm wondering if your connector exercise and CPS changing is a placebo effect for a bad ground/pickup on your distributor or something else)

I don't know when (or if) they made the change in the vehicle wiring harness, but for my '88 there is a CPS wiring upgrade available. Apparently the OEM wiring from the CPS connector through the harness to the ECU is too long, or too skinny, or something that causes it to not transmit a good signal. The replacement comes as a set that includes a new CPS and a replacement harness that goes directly to the ECU. In typical Mopar fashion, you can buy this kit for less than the cost of a CPS alone.

I do not know if this applies to the '90 model year. Follow the wires from your CPS connector. If they remain separate and go directly through the firewall, you probably don't need this. If they go into the larger wire bundle along the top of the firewall, this kit might have your name on it.

And now it's dead in the driveway

Hey Stan, Hey Eagle,

All good suggestions. Stan, the tach drops to 0, though I haven't had a dash for a couple of months. Eagle, I'm not clear on this kit. Are you sure this isn't only for the 87's-? that have the big connector on the firewall. My 90 doesn't have that connector, and the CPS wires do go into the main wire bundle over the engine.
Here's some more info from my drive home today.

I've been having intermittent problems with the Jeep dying while driving, usually soon after starting, but occasionally while going 70. I thought it was the CPS. Today while coming home, it died while accelerating through a left turn. I tried popping the clutch a few times but no restart. I aimed it toward the grass off of Fountain, had a little Mo so I popped the clutch again and it started. Died shortly thereafter but restarted while moving. Had another brief blip of dying, and made it home, where I stopped at Butch's to discuss it with him, since he had replaced the CPS this year. He didn't think it was the CPS, more like coil or loose wire. We looked at the coil, and my battery wires. One of the battery wires seemed loose and the positive lead to the coil was loose in the connector(Note, I have an MSD Blaster mounted in an adapter so that it plugs into the stock location on the ignition module). Went home, checked the battery cable, it was fine. Pulled out the coil, put in the stock coil. No Start. Put back in the fixed MSD coil and adapter. No start.

Before that I had tried wiggling everything in the general vacinities while the engine was running and got no reaction.
So now I have a dead jeep in the driveway and not really a clue. The only guess I have left is that the contacts on top of the ignition module for the coil to plug into are bent and not making contact to the coil.

Anyway of testing with a DMM?

Fred

I am positive that it isn't just for the '87s. You're referring to the C101 connector, which was removed from my '88 many years ago. This kit is for any Renix model where the wires from the CPS go into the main wire harness.

If you are having intermittant CPS problems and the wires go into the main harness, I'd encourage you to install this upgrade.

Here is some info I have posted in the past.

If on a crank but no start the unplug and replug of the cps wire harness connector gets you going you may just have a wire harness connector going bad and may not need to replace the cps sensor. Here is how to test: hook volt meter to both wires from cps sensor w. harness unplugged. Set the range on meter to less than one volt. Crank engine and if cps generates .5 to .8 or more volts it is ok and your connectors need tightening or replacing. I got my parts for a couple of $ at Pepboy but advanceauto.com also has them. They were made by Dorman under the Motormite Conductite tech grad name # 85303, 85304 and 85305. There are about 5 pieces in each box. You can also get a small tool set that helps on the install number 85360. I fixed my 89 this way a couple of months ago when I got tired of unplugging and re-plugging the connector every few weeks. My cps was only 1 yr old so the engine side harness connector part was worn out. I replaced both the female and male side internals of the connectors and it not only starts every time but the idle was also improved. Greg P.S. it has now been 1 yr since the rebuild of the cps and it has never run better from a cps standpoint.

In addition check your ground strap at the back of the head that goes up to the bulkhead. When mine broke (due to engine dropping from broken motor mount) the engine would occasionally die or not start and then later start. The dealer diagnosed a bad cps and replaced it but it did it again and that is when I noticed the broken strap was hanging down on the intake mainfold. Hit a bump or engine shake and lose contact = engine dies. I fixed the loose strap by sticking it under a fuel rail to intake manifold bolt -- still there after 5 yrs. Note the cps wire harness is nearby so messing with this cause my dealer to bump the strap and lead to the misdiagnosis. Greg

Fred,

Been way too long since we chatted. Looking forward to seeing you and your wife again.
Isn't there a crank sensor inside the distributor that will cause these same symptoms? Check in the FSM and verify this. I know on the newer XJs this module is there and would cause the problem you are describing.

Greg Smith said:

In addition check your ground strap at the back of the head that goes up to the bulkhead.... Greg

Click to expand...

That strap can corrode practically into powder and still look good from a distance. Definitely worth a check.

Thanks

I'll do some more checking.
At this rate though, I'm not going to make Moab. I still have a wiring harness to finish up for installing an electric speedo that was required for the 241 that I installed, and a whole bunch of vibes, noise, and engagement issues with the 241.
Whoops, that's modified tech.

Fred

Hey Eagle

If you have the part number of the kit, I'd appreciate it. Neither of the dealers in town has heard of such a thing, but then again they didn't know how to order speedometer gears either.

Fred

Where is this conecter at?

I am having simular problems with my 90' xj, and it sounds like it could be the cps. thanks for your help.

CPS location

The best way to find the connector for sure is to find the CPS and follow the wires back the harness. On your 90, look in the gap between the engine and the firewall on the drivers side. Shine a light down on the bellhousing. The CPS is at about 2 o'clock on the bell housing at the joint with the engine. The wires come up over the valve cover and connect into the main harness on the drivers side near the back. It's a three way, two wire weather pack connector right next to the two wire connector for the manifold temp sensor connector to the harness.
BTW, my no start problem was caused by an unplugged wire into the ignition module. I'm still not sure on the intermittent problem.
Fred

Re: Hey Eagle

Fred said:

If you have the part number of the kit, I'd appreciate it. Neither of the dealers in town has heard of such a thing, but then again they didn't know how to order speedometer gears either.

Fred

Click to expand...

I am constantly amazed at the things many Jeep dealers don't seem to know.

P/N 83100066

For the benefit of other viewers who may be joining this thread mid-program, remember that this kit is only for the Renix style CPS.

Thanks!

I'll try that.

Fred