marc3800

My 1990 xj trail rig keeps dying when I am going slow and try to get on the gas. It starts right back up, but this is annoying when you are on the trail. It doesn't do it all the time. It never dies whenever I am hammering on it. I was thinking about changing the crankshaft positioning sensor.

stev-o

Great idea change the cps

tjwalker

1. Be sure all tuneup hardware is up to snuff. Plugs, wires, cap, rotor, fuel filter.

2. Clean your throttle body and idle air control. More below

3. Test your throttle position sensor. More below.

4. Load test battery. Stalling is a common symptom of a marginal battery. Not at all uncommon

5. I don't think this is a crank sensor. Test it before replacing it. More below. If you do replace it, buy it from Jeep for best reliability and longevity
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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 3.5 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 it’s mounting holes with the first drill bit that just won’t fit through the original holes. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.
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RENIX (87-90) TPS ADJUSTMENT

Before attempting to adjust your TPS be sure the throttle body has been recently cleaned.
It's especially important that the edges of the throttle butterfly are free of any carbon build-up.

IMPORTANT NOTE: Using one lead of your ohmmeter, probe the B terminal of the flat 3 wire connector
of the TPS . The letters are embossed on the connector itself.
Touch the other lead of your meter to the negative battery post.
If you see more than 1 ohm of resistance some modifications to the sensor ground harness will be
necessary. The harness repair must be performed before proceeding.
I can provide an instruction sheet for that if needed.

MANUAL TRANSMISSION:

RENIX manual transmission equipped XJs have a three-wire TPS mounted on the throttle body.
This manual transmission vehicle TPS provides data input to the ECU. The manual transmission
TPS has three wires in the connector and they're clearly embossed with the letters A,B, and C.

Wire "A" is positive.
Wire "B" is ground.

Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors..
Note the voltage reading--this is your REFERENCE voltage.

Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your
OUTPUT voltage.
Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For
example: 4.82 volts X .17=.82 volts. Adjust the TPS until you have achieved this percentage. If
you can't achieve the correct output voltage replace the TPS and start over.

AUTOMATIC TRANSMISSION:

RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a three-
wire connector, same as the manual transmission vehicles have, and it is tested the same as the
manual transmission equipped vehicles--FOR ENGINE MANAGEMENT RELATED ISSUES.

However, the automatic TPS also has a four-wire connector clearly embossed with the letters
A,B,C, and D.. For the automatic transmission equipped vehicles the four-wire connector
provides data to the TCU.

Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your
REFERENCE voltage.

Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT
voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage.
For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage.
If you can't, replace the TPS and start over.

So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and
the other side feeds the TCU. If you have TRANSMISSION issues check the four-wire
connector side of the TPS. If you have ENGINE issues check the three-wire connector side of
the TPS.

For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is
stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.
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The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality

freegdr

check wiring to the cps for bare spots cracking possible short to ground

stev-o

the wiring does run right where its hot could have gotten a short from the heat

5-90

Before you do that, check the battery cables. I had a similar problem with my 87 - what had happened was that the cable to the starter motor relay (and thence to main distribution) had corroded internally, and the connection was intermittent.

Engine vibrations caused it to lose contact every now and again, and that drove me batty!

I finally found it by accident one day, I was adjusting something-or-other underhood, and had the engine running. I bumped that cable (it's the smaller-gage lead off of the positive battery post, if you're still OEM.) and stalled the engine. LIGHTBULB! I started again (had a bump switch hooked up, so I didn't have to go anywhere,) and repeated the problem. Same action, same result.

I went ahead and replaced all of my mains, since I had cable floating around. Showed them off at a meet & greet the following week (had to point out what to check - another RENIX owner was having the same problem,) and thus was Works in Progress born...

Something else to check - the fuel pump ballast resistor (if present, it will be at the driver's front corner of the engine bay, on the fender liner) is a wirewound resistor, and sometimes the wire can break. Remove and inspect visually. This resistor was added by Chrysler mid-1988 (voluntary recall on older models) for NVH reduction - my 1987 never had it, my 88 had it but I removed it and put in a 12AWG jumper wire. It's a bit less common, but a break in that wire will also cause a stall due to vibration.

But check the battery cables first - that's far more likely. The cables I make are sealed against that sort of thing happening again...

Oh - and if your CKP wiring has come loose from the clip on the driver's side rearmost cylinder head screw and melted through on the #6 primary (that's happened to me as well...) you can make a quick repair by splicing in a bit of 18/2 SJOOW wiring to replace the burned-through patch. Join it well, I suggest crimping and heat-shrink to seal, and that can get you going for a couple of whiles. I had to do that, and it ran until the sensor itself failed - about 40kmiles later. (I put the patch in, put a bit of heat-resistant sleeve over the repair, tied it up farther out of the way, and forgot about it while I was dealing with other things. I'd literally forgotten about the repair until the sensor itself failed, and I had to replace it...)

ryno137

Had this problem with mine, it would stall when I pressed in the clutch to slow down for stops. Turns out that the PCM gets a signal from the brake light switch and the vehicle speed sensor to determine that the vehicle is slowing/stopping which in turn activates the Idle Air Control motor to maintain proper idle speed. My brake light switch was disconnected because the clip was missing. I plugged it back in after finding the clip and it hasn't stalled since. Might be something to check out.