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TPS bad or not

97XJ Jeeper

97XJ Jeeper

Semper Fi
Location
Southeast Iowa
when i am going 30-35 mph in drive about every 20 seconds it will downshift for a second and then go right back into 4th. i know it downshifting not slipping because when i drive in 3rd it does not do it at all. it norm starts when it starts to get warmed up and when holding the throttle to keep me at that speed on level ground, when climbing a hill it dont do it because it stays in 3rd. so is this a bad tps or would a tranny flush help. thanks for any ideas.
:cheers:
 
replaced tps still doing it, try a flush or any other ideas
 
urban_offroader said:
this is normal.
Click to expand...
cant be normal anybody else think this is normal?
 
I would do a ground wire connection clean up...overhaul next, unless you have already done that.
 
i think i am going to have the tranny flushed and put a new filter in. why not prolly needs it anyway. i need to put mercron in it right, is there a sub just in case i cant find reg merc? (small town)
 
I have the same problem. I thought it was the tranny also since I had just drained and refilled it, but I took it to a transmission place and had them take a look and they found nothing wrong. They did say my TPS was operating 4ohms higher than usual intermittently which sounds like exactly the cause. So I swapped an old TPS and it still does the same thing. I don't think this is normal because I never used to have this problem until recently.
 
Just curious if this is really downshifting or if it's the t/c intermittently unlocking. If you drive along normally in 4th gear, you can tap the brake pedal and the t/c should unlock and see if that seems like the same thing. It's possible that the brake pedal sensor is gummed up and the trans computer thinks you are hitting the brakes, causing the t/c to unlock. On the older XJs, I know it's a white switch high up on the pedal that also has a vacumn line from the cruise control connected to it.
 
tc on mine dont lock up untill about 42-43 mph
 
4 ohm difference at the tps to me would suggest a bad ground like has already been mentioned. I'd think a tps if failed would be off a lot more than 4 ohms.

I recently had some tps hesitation/stalling issues myself. A good cleaning of that ground strap where it gets coated with leaking oil (an electrical insulator) and she runs like brand new money.
 
Last edited:
The TPS is supposed to make a steady, progressive, linear change in resistance as you increase/decrease the throttle. Intermittent changes in the resistance is more critical than being a few ohms off.

A bad ground could make ohms be off, depending on how you measured it, but it wouldn't effect intermittent resistance as it turns.

I believe the ECU learns the limit values of TPS and adjusts, NOT sure on that, but IIRC, there is a rather large margin for what is acceptable for the the reading of ohms on the TPS.

What makes a TPS bad is when moving the throttle the resistance jumps up and down instead of the steady linear increase/decrease in resistance. That is what your looking for when you test it with a multi-meter.

The TPS is just a reostat, a little film of varying resistance that contact runs over, the contact wears the film and coating thru and has dead spots. So while changing throttle, you hit one of those dead spots and resistance makes a big jump, and that makes the ECU think you slammed the throttle closed or open.
 
Rick Anderson said:
The TPS is supposed to make a steady, progressive, linear change in resistance as you increase/decrease the throttle. Intermittent changes in the resistance is more critical than being a few ohms off.

A bad ground could make ohms be off, depending on how you measured it, but it wouldn't effect intermittent resistance as it turns.

I believe the ECU learns the limit values of TPS and adjusts, NOT sure on that, but IIRC, there is a rather large margin for what is acceptable for the the reading of ohms on the TPS.

What makes a TPS bad is when moving the throttle the resistance jumps up and down instead of the steady linear increase/decrease in resistance. That is what your looking for when you test it with a multi-meter.

The TPS is just a reostat, a little film of varying resistance that contact runs over, the contact wears the film and coating thru and has dead spots. So while changing throttle, you hit one of those dead spots and resistance makes a big jump, and that makes the ECU think you slammed the throttle closed or open.
Click to expand...

Great explanation Rick! Just one point to add. A poor, variable ground, where the ground is loose and dirty and changing in resistance can cause the engine speed to suddenly change as well. In my experience the ground (bad or loose variable grounds) problems usually manifest themselves as a variable idle speed, high one minute, OK the next.

I had a lot of trouble with my ground and idle speed. Only took an 8 ohm increase in ground resistance (versus a normal 0.8 ohms using my meter) to cause a high idle on mine. In fact it chaged the TPS 0.80 volt calibration I had done into a 1.07 Volt idle setting running my idle up to 1300 rpm. I fixed the loose, dirty ground, recalibrated the TPS idel to 0.82 volts and no more problems.

During that time I never had it act up while in gear and driving, but if the ground was really loose, I think it is possible, just not probable as it would show up as high idle problem first (in my opinion).

Therefore, the TPS "dead spot" test proceudre is probably much more relevant for this guys problem with the sudden downshifting.
 
I hadn't thought of grounds varying resistance, a bad, frayed, dirty, loose ground can do that, and yes would do the same as spikes in resistance in the TPS.

One thing to try as well, pull the ground ribbons or cables and clean the ends and the contact point on the engine/frame. Seen lots of rust build up on a ground, between the contact points, and cause problems with it working.
 
ok hayes manual says the aw4 takes mercon did some searching and alot of people say dexron III. which is it? i dont have a owners manual. would like to find out going to flush and put a new filter in this weekend.
 
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