HELP!! The damn TPS Dilemma!

Ok everyone, first let me thank you all who have helped me in the past. Here we go-

1989 xj stock 4.0 automatic NP242

Had an original issue of transmission slipping out of gear at random times. Did the whole unplug the TCU and the tranny shifts fine manually. had it checked out by a pro shop and he said the same thing. Long story short, clearly an issue of the TPS. Changed it to the Duralast. As you all know, that changed nothing. Changed to the Crown, no change. So, with everyone's advice, I go the original NOS part from Mopar. A little better, but still kicked out of gear every once in a while. Read ALL of Cruiser54's tips and performed them ALL!!! Great info btw and you are the man! I particularly paid attention to the grounding issues and directed most of my efforts to ensure the grounds were as flawless as I could see. I use a digital ohm meter and am getting right at or just under 1 ohm of resistance at the TPS now. Still the same issue. Here's the kicker. I upgrade the ground strap to a 4 guage battery ground cable grounded directly to the block where the battery ground is connected. I ground the TPS directly to that and get the 1 ohm or less as expected. Now, the engine revs up to 3000 rpms at startup and stays there for up to 30 sec before idling down. Really bad. And sometimes it will not idle down. So, I pull the IAC, clean it, clean the port, clean the TB butterfly, etc. All the usual things. I even watched the IAC move (with the air cleaner removed) to ensure it operated. I watched when I first started it up and it revved, then the pintle moved out and iot dropped down the idle. So is it not that?? I cleaned EVERY connection I could see with QC Electronics cleaner and wire brushes (again under the advice of Cruiser54) to ensure I don't dump money into anything unnecessarily. Still the same result. And, btw, I think I have a bad MAP sensor because my MPGs are like 9.

So, now I have a high rev at idle, f'd up TPS situation, and bad MPG. Somebody please help! I changed the TCU, as well, back when the TPS issue was first discovered to eliminate that suspect. Any ideas??? ECU?? CTS, O2 sensor, SOMETHING?? I love this jeep and when it runs, it runs great!!! Only 116k on it and all original w no rust ANYWHERE!!! Need this thing. Please, Please, Please. I will try anything to get this thing fixed.

Thank you

How did you adjust your TPS and what were your final numbers?

Just basic stuff, I have no idea what you know and what you don't know. The TPS is actually two TPS, one half is for the trans and one half is for the ECU (motor management).

The voltage to the trans half starts out high (usually around four volts) and drops as the throttle is opened.

The voltage to the engine half starts out low (ideally around 0.8 volts) and rises as the throttle is opened.

The TPS is solvent an water sensitive. You have to be careful with any sort of spray around the TPS.

The TPS has two grounds (trans and engine half), with two different routes, but both eventually end up at the same spot.

One ohm is acceptable, but seriously it should be close to zero. Touch the two leads together on your meter and see if it ever hits zero ohms.

A quick setup for a test is to set the engine half of the TPS to 0.8 volts and then check to see where the trans side ends up, typically around 3.6-3.8 volts.

The ideal numbers are 4.15 volts for the trans and 0.8 volts for the motor side of the TPS. But the ideal numbers are rarely (never) seen. Both the ECU and TCU work with the numbers off aways. Mine typically ends up at around 3.86 on the trans side and 0.06 volts on the engine side of the TPS, runs and shifts fine.

You say slipping out of gear, does it downshift or go into neutral?

Have you adjusted the kick down cable? And/or checked your your shift linkage/cable adjustment?

Your poor mileage could be anything from a vacuum leak to a bad sensor.

When your engine flairs to 3000 RPM is it with a hot motor, a cold motor or all the time?

Thank you for the reply. Good questions. My first test on the trans side of the TPS was 4.82 volts on the power side and i adjusted it to be .83 volts of that 4.86 on the output side, as required for the automatic. That, however jacked up the output of the engine side of the plug. So, i adjusted it again, to meet the .17 of the input voltage on the engine side. This worked better. and the engine dropped to idle faster then 30 seconds, but now it still revs up to 3k and sometimes drops and sometimes does not. Problem is exactly that. When I get the trans side numbers are correct, the engine side is wrong, and vice versa. Again, this is with all 3 TPS's that i've tried.
The slipping out of gear is more like it goes into neutral and will just rev, so i need to slow down to about 15-25 mph and it will kick back in to gear. It will do this sporadically at cold or hot engine at anytime it feels like it. i have been unable to pinpoint a specific scenario where this will happen.

Kickdown and shift linkage all correct and adjusted.

This should answer all of your questions. I'm stuck. ?? I tell you I did check the resistance for a reference directly on the ground terminal with both sides of my meter and I DID NOT get 0 resistance. It is 1.1 or 1.0 ohms. I changed all of the battery cables including the ends to brass marine type and the terminal ends are clean and shiny. All other ground connections are clean and shiny as well. Maybe the battery is bad, not being able to get a good close to zero resistance?? I get 0 ohms on the meter when i touch the leads together too. Thanks for your help and let me know what you think.

CB

To set the trans side multiply the input voltage by .83 .

To set the engine side multiply the input voltage by .17 .

Sometimes you have to fudge a little, which is acceptable as long as the numbers aren't to far out. The input voltages for the engine half of the TPS and the trans come from different places ( the ECU and TCU) and are seldom the same, but often close.

I've never seen the trans go into neutral sporadically. The only time I've ever had anything like it was bad transfer shift bushings that sometimes left the transfer shifter half way in a gate and it would occasionally slip into neutral.

It takes a combination of the shifter and the solenoids to hit neutral in the trans, first, reverse park and likely other gears. The trans popping into neutral could theoretically be either mechanical or electronic. Or maybe something just as simple as a bad power connection to the TCU, just a wild guess.

That surge at startup can have numerous causes.

The IAC sticking. It may not be sticking in the front of the plunger, it may be grit behind the plunger. A little bit of space beside the plunger (stopper, piston) and the IAC housing. Try spraying some WD-40 in there and see what you blow out form behind the plunger. I've gotten a bunch of grit out of there before. Don't try to move the IAC plunger (piston), numerous ways to screw that up. I once launched an IAC piston 15 feet across the driveway.

Low voltage is always a possibility when the iAC starts acting up. On numerous occasions I've had charging or battery issues that I never really noticed until the IAC started acting up. The B+ latch relay is responsible for parking the IAC for the next start, you might want to check the spade pins and/or the relay socket for corrosion.

A 3000 RPM flair is most often something to do with the TPS. Or the TPS wiring or grounds. My last major issue I found by shaking the harness, I had a bad crimp in my TPS ground circuit. I've ruined a few TPS doing a motor wash, water in the TPS will sporadically idle you to 3000 RPM or above. Solvent or contact spray and the TPS also don't get along.

I'm out of ideas for the moment. Any new ideas or an epiphany I'll post up.

Thank you. I'm going to have my tranny guy look into the shift bushings

As for that TPS, I'm going to try again to make the correct adjustments, replace the IAC, replace the Latch relay, possibly the battery and verify the power to the TCU during operation. I'll keep you posted.

Thank you, again!! Any other thoughts would be great!!

So, I replaced the IAC first and foremost before doing anything else. The high idle remained. Once again, I adjusted the TPS to be in the appropriate range for the trans; I got 4.62 in and 3.83 out. Perfect. But when I check the engine side of the TPS is get 4.92 in and .64 out (or .13 x 4.92). So, adjust ing either side, knocks the other side out of whack.

He's the enigma, with the engine idling at startup at about 3k, I manually adjusted the TPS, just to see if that was the culprit, and she idled down. Now, I adjusted to startup correctly, but the numbers are all out of whack. I will go double check them and post in 5 min

now I get 4.10 output (or .88%) on the trans side and .38 output on the engine side (or .07%)

At this point, she runs and drives good and starts up with not high idle at all. idles normally at around 600-750 rpms but will still slip into neutral when driving at all different speeds or times, cold or hot.

Please advise!!!!

When you set your output voltage, where are you grounding? Testing to battery, chassis or the TPS dedicated ground is likely to produce different results.

Just a quick check, put your meter on a low voltage scale and check the TPS ground wire to battery ground for voltage. This is a quick way to check the health of your ground circuit.

That hot start high RPM flair is a typical Renix sickness. For me it was caused by a bad splice on the TPS ground circuit (engine side). Best guess is I had a sporadic voltage backup from another circuit on the same splice. I've heard of other causes for this. Even a bad regulator dumping excess fuel into the intake manifold. Or an issue with the gas tank purge system. The main reason they had the recall to install a brake interlock, so you could only shift from park with the brake applied was to prevent accidents from the that RPM flair at start up.

You have to keep in mind that the TPS is two halves or in effect two TPS sandwiched together. The two halves also have separate ground paths that end up at the same place (in a normal unmodified harness).

I'm at a loss why your tranny is dropping into neutral. Is it totally random or does it happen in a certain gear or certain RPM? Most times the shift is subtle in the AW4, it may be hard to tell which gear you are in, but during normal acceleration you can often see where it shifts on the Tach.

I think you have two separate issues here (unless your TPS has gotten wet).

I've never had my transmission randomly shift into neutral so I#m unlikely to be much help. Just a wild guess. maybe your speed sensor is faulty and the system is going into coast mode? More likely IMO is a bad connection someplace. The first time I disconnected my TCU I found three pins that were just sitting in the connector and not locked into place by the tiny tab that holds the pins into the connector. I was surprised that the TCU was functioning at all, my issue was random shifts. Oil soaked connectors have to always be suspect.

Now I gently pull the wire on the back of a connector to make sure the pin is locked into place, for most any connector and especially for the TCU and the ECU.

A shift solenoid ohm check is likely to be a good idea. Check the ohms at the TCU connector. This checks the harness and the solenoids together.

You may have 13 plus volts at the battery, by the time the voltage gets to the ECU or the TCU it can be substantially less than this, under nine volts and function gets flaky. Low power in or poor ground, same outcome.

If you have a constant high RPM at startup and it isn't a flair (temporary), it is likely to be a vacuum leak, maybe a wet TPS, maybe excess fuel from someplace, maybe something to do with the IAC. A quick check can be if/when you have an acceptable idle, unplug the IAC and see what happens during the next start, you may have to give a *little* gas pedal at start up.

Swapping out components and sensors may not be a cure, the harness and connectors in the Renix are, in my experience, more likely to be the issue than the sensor itself. The majority of the sensors in my 87 and 88 are original, I've been through the harnesses/connectors multiple times cleaning and repairing.

Always set the tPS on the "engine" side and let the "trans" side end up where it ends up.

Thanks, guys!!

First the flair up. I will set the TPS back to the correct .17 of the 4.92v, as Cruiser says and see what happens. If it does not correct the flair up, should I suspect the TPS? Even after I've gone through 3 of them, including a genuine Mopar one?

I am determined to find this issue. This jeep rocks and I won't give up!!

When I test the TPS voltage, I am doing it according to Cruiser's tips of leaving the harness connected and back probing the ground on the TPS itself, not grounding to the battery or anywhere else.

I will check the TPS ground today as you suggest as well as the ohms at the TCU for the solenoids and harness. I replaced the speed sensor, but like you said, it's probably more likely a wiring or connector or harness issue somewhere.

Thank you again, and I will keep you posted.

Sorry, I missed this question of yours.

The tranny drops into neutral at totally random times. And, it will sometimes catch right back or sometimes I will have to slow down to like 15-20 mph or sometimes even have to shut it off completely and restart. This HAS to be an electrical issue!

Have you done my Tip 1 and 5? 10?

Ye, sir!! I'm that guys who's too proud to ask for help, so I'll read and try everything before I ask. This is why I'm stumped!! Even My wife wants to know who you are because I keep saying, "look right here in Cruiser's TIPS...!!"

chefbuldog said:

Ye, sir!! I'm that guys who's too proud to ask for help, so I'll read and try everything before I ask. This is why I'm stumped!! Even My wife wants to know who you are because I keep saying, "look right here in Cruiser's TIPS...!!"

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LOL. What kind of ohms reading did you get when doing Tip 5?

I got right at 1.1 I know it's high. But I changed both battery cables, put brass ends on the terminals, cleaned the terminal ends, and even relocated the battery ground cable to be on the same stud(behind the dip stick) as all of the Trans ground connections.

I disconnected all the grounds at the firewall and coupled them 3 and 3 wires. 1 set of 3 I coupled and soldered into a 6 gauge wire directly to the battery ground terminal. The other set of 3, I soldered and ran up to the firewall ground where the strap used to be. I replaced the strap w a 4 gauge battery type cable and ran that directly to that same ground bolt behind the dipstick.

So, ultimately all of those grounds are solid, clean, and connected with new large gauge wire DIRECTLY to the battery ground. you know, as I'm typing this I'm starting to think my battery may be on it's way out??

What next? And thank you for taking he time to help me with this. I'm sure you have netter things to do.

Set the tPS to the proper setting on the "engine side".

Just wanted to thank you guys. Jeep's running awesome! reset the TPS on the engine side. and no flair up revs at start.

Still have this thing slipping into neutral intermittently, so I'm really starting to believe I have a bad solenoid. Not sure which one, obviously, so I'll buy the kit and change all 3. For now, shifting it manually and it still drives awesome!! I'm getting about 14mpg, but I think we'll get it up to about 16-17mpg once I get trans computer plugged back in.

Any other thoughts of what that could be besides a solenoid going out before I drop $300 and some change?

chefbuldog said:

Just wanted to thank you guys. Jeep's running awesome! reset the TPS on the engine side. and no flair up revs at start.

Still have this thing slipping into neutral intermittently, so I'm really starting to believe I have a bad solenoid. Not sure which one, obviously, so I'll buy the kit and change all 3. For now, shifting it manually and it still drives awesome!! I'm getting about 14mpg, but I think we'll get it up to about 16-17mpg once I get trans computer plugged back in.

Any other thoughts of what that could be besides a solenoid going out before I drop $300 and some change?

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Engine side is good. You've done my Tip 10?

yes, sir, engine side is good. I have done the Trans plugs. Cleaned both with QC Cleaner and a small brush. I actually left them to dry overnight before plugging them back in. Still the shift into neutral randomly. KILLING ME!!