CPS dead=No spark?

Ok so my XJ (1988, 4.0,auto) dropped dead on me while driving home ....just instant death and wouldnt start even though it turns over fine. I have verified fuel but testing for spark was a negative. I have checked the yellow wires on the ignition module ... large one gave close to battery voltage, smaller gave what i assume is voltage supplied from the ECM (cant remember the exact number but i think was 5 or 6 volts) with ign on and not running. I checked the resistance of the coil-to-distributer wire and it seemed ok for it's length.
After some internet scouring i believe I should be directed towards the crankshaft position sensor that everyone loves so much. I tried the unplug,wait,replug trick and nothing. ohms between the two terminals is within , what i have been told is, the acceptable parameters. I think it was 190 ish? I then tried checking the AC voltage on the same plug while cranking and got a bout a quarter of a volt. That is what i have read considered a replacement reading for this sensor, correct?
Anyways my question is basically the title of the thread. Does a faulty CPS usually result in a absolute no spark scenario? and also there is an intermittent "sucking" noise coming from somewhere around the intake while I crank it over...is that normal? Thanks for any help~!

A dead CPS means no spark.

Thank you...will get replacement tomorrow (hopefully ) and cross my fingers. I am hoping that this will also fix my dies-when-wet problem. when it is raining or moist outside The jeep would just cut out, no sputtering , same as what happened before i towed it home the other day. When this would happen it would stall and the fuel gauge would go up to max. Could this be related or a different issue like bad wiring at/near the fuel sending unit?

Renix CPS Testing and Adjusting



Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It won’t start with the CPS disconnected.

You should get a reading of .5 AC volts.

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from Napa or the dealer.

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

A little trick for increasing the output of your CPS is to drill out the upper mounting hole to 3/8” from the stock 5/16”, or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.




Revised 01-26-2013

Thanks Cruiser ....I did both tests , resistance was within range but volts AC only showed about .25 so i am hoping i can get a replacement tomorrow

Prevenge said:

Thanks Cruiser ....I did both tests , resistance was within range but volts AC only showed about .25 so i am hoping i can get a replacement tomorrow

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A little trick for increasing the output of your CPS is to drill out the upper mounting hole to 3/8” from the stock 5/16”, or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

Since i was getting signal from my cps , just not enough, I decide to try your tip cruiser.wanted to do so before i get the new cps. I reamed the top hole out a bit but still nothing. Its below freezing here and dark so i will wait to see what the results are on the meter tomorrow. I just wanted to ask a couple more questions. Is there a way to test the ignition coil with a meter? And secondly, in a no spark situation is it normal to have a prominent hissing noise like a quick burst of pressure being relieved every 3 turns or so?

Hissing noise is not normal. Got a loose spark plug?

I have not found a reliable coil/ICU test.

I checked that all the plugs were seated...they are. I think I may have a bigger problem. I pulled the breather off and determined that the sound is coming from a blowback of fuel/air coming back through the throttle body....as much as i dont want to know how bad it could be...what could be making this? head? valve(s)?

Prevenge said:

I checked that all the plugs were seated...they are. I think I may have a bigger problem. I pulled the breather off and determined that the sound is coming from a blowback of fuel/air coming back through the throttle body....as much as i dont want to know how bad it could be...what could be making this? head? valve(s)?

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Valve, valve spring?

Do a compression test.

I do not have a compression tester...until i find one i could borrow is it worth pulling the valve cover off for inspection?

Prevenge said:

I do not have a compression tester...until i find one i could borrow is it worth pulling the valve cover off for inspection?

Click to expand...

Up to you.

screw it...I am going to take a look. Nothing else to do since my cherokee isnt running , right? Um ...since this post started as a no spark issue then wouldn't it also be possible that there may be a problem with the cam? Thanks again for the help.

cruiser54 said:

Valve, valve spring?

Do a compression test.

Click to expand...

Like Cruiser said do a compression test "and" if you suspect a valve issue, a vacuum gauge really shows up bad valves quick.

Dropping a cylinder is unlikely to kill your engine. I've dropped multiple cylinders for various reasons (sometimes on purpose just because I was curious) and it sill ran, though not well.

People get fixated on the sensor. I've found at least half the time, maybe more than fifty percent of the time, it is in the wiring.

Ohms law, volts divided by ohms equals amperage. A CPS .5 volts, 1 ohm resistance, equals .5 amps, 2 ohms of resistance equals .25 amps etc. Your .25 volts with 1 ohm of resistance equals .25 amps. Volts isn't everything, you sometimes need enough current flow (amps) to work an electronic switch.

The CPS wires between the CPS and the first connector are a common failure spot. These wires tend to cook on the exhaust manifold if they aren't tied down and /or routed correctly. Or the harness connectors collect a layer of oxide on the contacts inside the connector that induces resistance (common in the Renix).

If you want to kill two birds with one stone, look up which pins are the CPS connections at the ECU and test for voltage and resistance there.

Most times when my CPS failed it got quirky before it quit. Starts got long and iffy. And/or it would start cold but not when it was hot. And/or the motor would run like it had a terrible miss sometimes. It would eventually fail completely, but I really can't remember it ever happening without getting quirky first.

Very few things would stop me dead in my tracks without any warning. The CPS on the Renix, in my experience wasn't one of them, unless it was the wiring cooking on the exhaust. Of course anything is possible.

Thanks , I am gonna talk to my boss and get a compression tester and vaccuum tester now that you suggested it. My mind is telling me that the cps should go onto the backburner as far as importance goes considering the blasts coming out of my throttle body. I may be wrong but that sensor being out shouldnt cause the computer to do anything extreme enough to cause that , correct?

Prevenge said:

Thanks , I am gonna talk to my boss and get a compression tester and vaccuum tester now that you suggested it. My mind is telling me that the cps should go onto the backburner as far as importance goes considering the blasts coming out of my throttle body. I may be wrong but that sensor being out shouldnt cause the computer to do anything extreme enough to cause that , correct?

Click to expand...

Is it blowing air or is it igniting and popping out if the intake?

You said no spark, so I assumed it was blowing air and not flames?

Ya , air and i would assume some fuel cause i do have fuel at the test fitting. That was about the extent of that test though as my tools are limited to a socket set and wrenches so I did not check the pressure. No ignition just a blast out of the throttle body. also ...I just noticed the vacuum line on the TB side of the MAP(?) sensor was pushed out and the smaller plastic line off the valve cover was broken. Probably all related but it is cold out so stuff is brittle and i could have broke it while messing around with it

If that little plastic line from the TB to the MAP is broken, the ECU can default to full rich. It may run badly, it may try to run or it may just flood the motor so bad with fuel it won't run.

You also have to make sure that little plastic line from the MAP to the TB is plugged into the right hole. Memory tells me it is the top hole, but I'm old as dirt and don't much trust my memory anymore. Only one of the holes goes all the way through to vacuum.

That little plastic line off of the valve cover is less critical, if it is plugged or broken the air filter tends to get soaked in oil. May affect your idle some.

its the bottom one...fooled with it before. my mechanical knowledge is limited but luckily some guy , who also drives a cherokee , and who i had never met before came over tonight and offered to help me tomorrow. Literally just met him on facebook an hour ago while asking about parts. He figures it MAY be the timing chain and is gonna gimme a hand so long as i go wheeling with him next summer. fair trade? lol jeep owners are great.

update-I feel like a dumbass. First i thought it was the cps , then discovered i did actually have spark contrary to what i first believed. Then i thought my timing chain was out or guide was gone or something. So i was preparing to do that job, found TDC and the balancer timing notch was lined up perfectly. This made me immediately look over at the distributor and slap myself. Sure enough ...the rotor was half snapped off but stuck in a way that my timing was advanced by about 15 degrees . Should have been one of the first things i looked at. But...now I know and knowing is half the battle. Thanks for all the help in my silly goose chase ....5 bucks later and .....FIXED.