regarding CPS...........

The local jeep shop that's done a lot of work on my '88 XJ 4.0 installed a new cps about 6 months ago...well in the last 2 weeks I developed a sporadic no start problem with no pattern...some times it started sometimes it didn't. Well lucky me, It didn't start while at the shop while ordering some other parts.
They tested and said there was no spark...they tested the coil, etc. and eventually changed the cps and it started right up. (we had already trying the old unplug and retry trick).
They told me that my recent cps shorted out as a result of either gettin some moderate coating from my bloy-by problem or by just getting wet all the time and they also said I was missing some plastic cover back there.
My question is: Is the cps really that fragile and prone to failure? Should I just buy another and carry a spare? Does the plastic cover really make a difference?

p.s. to add insult to injury, I had a high-altitude cps back at the house but I couldn't wait for that because I had my 2yr old daughter with me....figures

Sounds like bull to me. The cps sticks into the bellhousing so blowby should not affect it and I doubt the plastic cover would help. I recently had this problem and it turned out the female side of the connector on the engine wire harness was worn out. See the post below I have posted many times before. The info below is for a pre 91 model. Post 90s have a different amount of voltage and have a three wire cps system.

"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 my 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 replugging 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"

Good Luck. Greg

My guess is that the wire was touching something hot. If they were not careful with the routing of the wire when they replaced the CPS, then it could be the culprit. I don't see water hurting this thing. My rear main seal had my first CPS totalling coated but what killed it is heat on the wire leading to the unit. I actually found the shorted spot on it. The CPS is not fragile.

Greg Smith said:

[B 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.

Good Luck. Greg [/B]

Click to expand...

AC setting.

The CPS is plastic encapsulated and not fragile unless it is hitting the flexplate/flywheel or the wire melts on the exhaust manifold. The 87-90 are more common to fail due to heat cycling. The coil of wire that is inside can break over time. When the plastic expands the coil might go open circuit causing a no start.