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Xj Transmission Woes

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Was on my way to work in my 89 4.0 AW4 NP242 Cherokee this morning and for the first time in its 190,000 mile existence, the transmission appeared to slip some between 2nd and 3rd gear (when going slightly downhill none the less). I continued to drive it around the neighborhood troubleshooting and it won't do it under a load... you either have to let your foot slightly off the gas prior to the shift point or be going down a hill with a little power prior to the shift point to notice it. Occurs in "power mode" at 2600rpm (give or take a 100) and won't do it in "comfort mode", yet (I assume because the rpm's are lower). BTW it's only slightly noticeable, I just noticed it because I'm obsessive about my Jeeps and have been driving it for 16 years. I last changed the fluid (drain and fill) at 150,000 and has been done regularly since new. The fluid is a little dirty; it's slightly overdue for a change, which I'll do when I get home from my trip. Perhaps that'll pro-long it's life (or kill it quicker)? Either way, I won't know until I have a little more time to dedicate to it. At this point I have to assume an internal clutch is wearing out and anything short of a new transmission will just be a band-aid... Two days ago I replaced the ignition switch (actual switch on top of column under instrument cluster) and was wondering if I knocked something loose when I had everything apart. Since the lower part of the dash was out, obviously the TCU was removed and the transmission "park" safety cable was unhooked from the column as well... but the symptoms have me thinking its completly unrelated. Could a faulty tps or tcu cause transmission slippage at the top of a shift point??? Otherwise the transmission functions just as it did when it was new. I don't wheel this one, it's just an extra vehicle we typically leave home for the sitter to drive or haul the boat with. It's a very nice, stock, fully loaded laredo that I try to keep nice. Has anyone else had similar issue with their AW4? Worst case scenario, I check my connections/grounds, tps, tcu and change the fluid. If it goes away, great, if not, I'll try to find a decent re-man....

On the bright side it gave me the opportunity to drive my Comanche 75 miles to work instead. What a fantastic vehicle! 240,000 miles on the original 4.0/BA-10 combo and gets 21mpg on the hwy with good reliability....


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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: With the Key OFF, and using the positive (red) 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 black lead of your meter to the negative battery post. Wiggle the wiring harness where it parallels the valve cover and also over near the MAP sensor on the firewall. If you see more than 1 ohm of resistance, or fluctuation in your ohms reading, 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.


RENIX manual transmission equipped XJs have only a flat three-wire TPS mounted on the throttle body and it provides data input to the ECU. It 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. Loosen both T-20 Torx screws attaching the TPS to the throttle body and rotate the TPS until you

have achieved your desired output voltage. Tighten the screws carefully while watching to see that your output voltage remains where it is supposed to be. If you can't achieve the correct output voltage, replace the TPS and start over.

Sometimes, after adjusting your TPS the way outlined above, you may experience a high idle upon starting. If that happens, shut the engine off and reconnect your probes to B and C. Start the engine and while watching your meter, turn the TPS clockwise until the idle drops to normal and then rotate it back counterclockwise to your desired output voltage.

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

However, the automatic TPS also has a square four-wire connector, clearly embossed with the letters A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module. THIS SQUARE FOUR WIRE CONNECTOR IS USED FOR TRANSMISSION/SHIFTING RELATED ISSUES ONLY. 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.

FOR AUTOMATIC TRANSMISSION RELATED ISSUES: Check the square four-wire connector side of the TPS.

If you have ENGINE ISSUES check the flat 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.

Revised 09-22-2012

TPS is definitely a player in this. I would check the square connector using this:

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Thanks for the response cruiser. Since it was obviously slipping I just assumed the transmission was the culprit... good to know that the tps could still be an issue. Is there anything else I should look at since I just I took the dash apart for the ignition switch replace? Too bad I don't live in Prescott anymore, had a lot of great times there back in the day... I'll certainly look at the tps when I get home. I replaced the tps in the comanche about a year ago; your write-up made it a breeze.... Thanks

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Well, I got home from my trip and drove it last night.... Functions perfectly without doing anything... I'm going to clean and refresh thr grounds in the engine bay and monitor it from there. Additionally, sometimes when you crank it the tach will bounce off of its stop, giving erratic readings when it should be zero. It reads correctly as soon as it fires up... Whenever the tach does this, the starter will almost act like it won't continue cranking... I've got a new battery and terminals but think there may be a ground issue, perhaps at the dipstick stud.... I'm going to start there and clean/re-seat all of my connections in the engine bay too. Can't help but think that the cranking/tach/sudden transmission issues are related to a poor common ground. Anyway, the transmission appears to be fine.... Thanks!

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After driving it a couple days its now apparent that what I'm hearing is just the fan surging. Engine Rpm remains the same but if I hold the shift until 2800 occasionally you hear the fan clutch increase in speed or surge. Relieved its nothing significant. I replaced the fan clutch last February, looks like its time to replace it again

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