The RenX Files: High idle problems

[Ecomike]

Almost sounds like your wiring from TPS to ECU has a problem. I can not drive mine with the TPS disconnected. If I give it gas with no TPS connected to the ECU it bogs down and back fires. Also sounds like all those used TPS's may have problems. I would also look for the source of the voltage drop from the ECU to the TPS. The ECU has a 5 volt output to the TPS, and the return voltage to the ECU from the TPS tells the ECU where the throttle is at so it can estimate increased or decreased air flow before it gets to the O2 sensor. It speeds up throttle response .

 

[Muad'Dib]

Almost sounds like your wiring from TPS to ECU has a problem. I can not drive mine with the TPS disconnected. If I give it gas with no TPS connected to the ECU it bogs down and back fires. Also sounds like all those used TPS's may have problems. I would also look for the source of the voltage drop from the ECU to the TPS. The ECU has a 5 volt output to the TPS, and the return voltage to the ECU from the TPS tells the ECU where the throttle is at so it can estimate increased or decreased air flow before it gets to the O2 sensor. It speeds up throttle response .

Now im very concearned about the ECU side of the TPS. Cause as i said, i unplugged it, and everything was great.

Ecomike, when i was stating the voltages, i was talking about taking the normal readings from the TCU side of the TPS. From what i have heard and read, 4.65 volts is fairly normal for a Jeep 18-19 years old.

Do you have any suggestions for me to check out the ECU side and its wiring??

 

[Ecomike]

Now im very concearned about the ECU side of the TPS. Cause as i said, i unplugged it, and everything was great.

Ecomike, when i was stating the voltages, i was talking about taking the normal readings from the TCU side of the TPS. From what i have heard and read, 4.65 volts is fairly normal for a Jeep 18-19 years old.

Do you have any suggestions for me to check out the ECU side and its wiring??

My 87 and 89 both read about 5.00 volts +/- .02 volts on a high quality meter. Check the output voltage at the ECU itself.

The TCU side has no effect on idle, only the ECU side affects idle. That is why I preached in this thread about the need to check both sides of the TPS, including the 5 volt input to both sides, the ground on both sides, and the TPS span for high, low and smooth operation. Some have report getting by OK with a 4.65 volt input, but what if your input is jumping around due to a poor contact in the harness, and reading 4.95 one second then 4.5 the next on the input to the TPS on the ECU side?

I check both live voltages, and then check wire resistance (using a very long set of leads) with the power off to try and isolate problems.

Is your O2 sensor working? Is it getting into closed loop operation? When I disconnect the 87 ECU side of my TPS it will back fire if I stomp on the gas in park. Have not tried on the 89.

 

[Muad'Dib]

My 87 and 89 both read about 5.00 volts +/- .02 volts on a high quality meter. Check the output voltage at the ECU itself.

The TCU side has no effect on idle, only the ECU side affects idle. That is why I preached in this thread about the need to check both sides of the TPS, including the 5 volt input to both sides, the ground on both sides, and the TPS span for high, low and smooth operation. Some have report getting by OK with a 4.65 volt input, but what if your input is jumping around due to a poor contact in the harness, and reading 4.95 one second then 4.5 the next on the input to the TPS on the ECU side?

I check both live voltages, and then check wire resistance (using a very long set of leads) with the power off to try and isolate problems.

Is your O2 sensor working? Is it getting into closed loop operation? When I disconnect the 87 ECU side of my TPS it will back fire if I stomp on the gas in park. Have not tried on the 89.

My 02 Sensor is working correctly... but im assuming this only because i just replaced it less than a couple months ago. As far as i can tell it gets into closed loop.

Can you give me an idea on where to look in my FSM for information on the ECU side of the TPS? Or do you happen to have a cheat sheet on what needs to be checked where.

Im thinking its time to check the readings at the ecu to see what its seeing and outputting. The ECU side just has Input, output and ground right?

I also wonder if the ground for the ECU side of the TPS is important to be good at the ECU, or if its ok just to have a jumper at the TPS. The same goes for the TCU side. Does this make since?

For example, lets say that the ground wires for both sides of the TPS are weak\corroded, or split (higher than normal resistance). If you put a short jumper to ground at the TPS, then you get a good ground at the TPS, but is it important that said grounds are good at the TCU\ECU respectively...???

 

[Ecomike]

First off it really sounds like you need to read (and take notes) this thread cover to cover. Most of your answers are right here.

Both sides of the TPS, and the ECU have separate ground wires. They are all important and significant. I have had to replace the TPS ground on both sides of mine, and on the TCU so far.

 

[Muad'Dib]

First off it really sounds like you need to read (and take notes) this thread cover to cover. Most of your answers are right here.

Both sides of the TPS, and the ECU have separate ground wires. They are all important and significant. I have had to replace the TPS ground on both sides of mine, and on the TCU so far.

I have skimmed through this thread many times. Unfortunatly i just dont have the time to read and take notes to find what could easily be consolidated into one decent post... especially from the people who probably have the information off the top of their heads.

Ill have to take another look at this problem at a later date when i have more time.

 

[Ecomike]

I have skimmed through this thread many times. Unfortunatly i just dont have the time to read and take notes to find what could easily be consolidated into one decent post... especially from the people who probably have the information off the top of their heads.

Ill have to take another look at this problem at a later date when i have more time.

I had it off the top of my head once, but it's now buried and scattered about in this rather short brief thread, LOL. :laugh:

And the summary you speak of simply leads to another dozen detailed questions and issues that you already asked and that are already answered here. I beat on the Renix TPS issue, high idle problem, for over a year, documented my ideas, thoughts and tests and results in the earlier parts of this thread.

I understand not having the time. Take my word for it, if you don't have time to scan this thread for my more pertinent posts, you don't have time to track down the high idle TPS and related gremlins either.

I followed other posts from here and there including posts and links from here, and I still spent a year fighting the damn thing before I finally fixed all 6, or was it 8?, things that were all contributing to the random, high idle on mine.

As I said in the prior post, the first thing you should do is check the wires, point to point for continuity, all 6 of then from the TPS to the ECU and the 2 ground wires, and then check all the ECU grounds at the ECU. All grounds should read less than 1 ohm between the ground connection / wire and the Battery negative post. Be sure to wiggle wires and the wiring harness during ground and continuity tests. I have found bad ground wires and replaced the wires, 2 on the TPS (I ran the new ones straight to the battery Neg post), and one on the TCU just on my 87. Then the TPS and IAC also were bad, as were many CCV lines.

Once you have done that (checked all the wires), if it still acts up I would buy a new TPS in your case.

 

[Muad'Dib]

I stayed up late tonight to read this thread in its entirety. I found great information on what to check on the TCU side, but absolutely nothing on the ECU side. I'm talking about pin outs. Id like to pull the ECU out and check reading there as i am going to do at the TCU now that i have the proper information from here on page 4. I guess ill have to resort to my FSM.. sometimes shit is just to hard to find in there!!

 

[Ecomike]

You do have the Renix Fuel Injection manual right?

I posted data in this thread on the ECU side of the TPS, actual ohm readings, mounted and unmounted, for WOT to idle positions (Power off, wiring harness disconnected) readings, about 2200 to 6600 ohms, respectively IIRC. The Renix Fuel injection FSM has the WOT to idle voltage readings for the ECU side, which should read about .83 volts at idle and about 82-87% of ECU input voltage at WOT on the third return wire from the TPS to the ECU at WOT. The lundford (IIRC, ? SP) web site has a nice brief on the TPS pin outs, and Renix Fuel Injection manual has the ECU pin outs and values, and the TPS pin outs.

When checking my TCU I had to replace one bad ground wire and one 12 volt unfused wire that keeps the TCU memory alive when the ignition is off.

 

[Muad'Dib]

I found what i was looking for in the Electrical Section of my FSM (which is a whole other book).

It shows that most of the sensors grounds splice off of one from the ECU... which most of us already knew was true for the TPS.

I may have to scan and post the 3 important pages that i found.

Interestingly, the ground splice also runs to the diagnostic connector under the hood. This (from looking at the schematic) would be a great place to also check for a load, and or high resistance. IIRC it was pin 8 on the bigger connector (Right smack dab in the middle). I feel it would be a good quick test to see if its a ground problem, or if you should start digging deeper into the +5v supply.

At first i though it showed the +5v for all the sensors coming from one splice, but i was wrong... it appears that they each have a seperate wire going to the ECU.

I dont remember which pins on the ECU are for the TPS, but i will scan those pages tonight and post them.

 

[Ecomike]

Keep in mind that wires have been known to fatigue and break, connector female pins get loose and dirty. While you may get a good reading at one end of a ground, or voltage input, or return, it can degrade along the return path. So a good signal or ground at the diagnostic connector may be miss leading!

Don't assume anything as you run the tests.

I have preached before I will do it again, that the best time to do tests is during the random gremlin malfunction!!!!

 

[PuddinHead]

About 80% of electronic failures/problems are wiring related whilst the remaining 20% is component related. If a problem is intermittant that just screams wiring to me. Running new wires back to the ECU could cure a host of engine problems related to sensors. Wiring can look good and even test good with a standard volt-ohm meter, but still be corroded inside the jacket.:lecture: When dealing with these small DC voltages this kind of corrosion would change current flow nearly all the time.

 

[Ecomike]

About 80% of electronic failures/problems are wiring related whilst the remaining 20% is component related. If a problem is intermittant that just screams wiring to me. Running new wires back to the ECU could cure a host of engine problems related to sensors. Wiring can look good and even test good with a standard volt-ohm meter, but still be corroded inside the jacket.:lecture: When dealing with these small DC voltages this kind of corrosion would change current flow nearly all the time.

Surface wire corrosion inside the wire by itself is not a problem in my book, but many have reported spots in the wiring where hidden taps have loosened, and where all but one or two threads of multicore wire are fatigued and broken leading to voltage looses. I can also see large ground wire problems causing feed backs into the TPS ground when head lights, brake lights, other large loads are dumped to a poor overall grounding system in the XJ body.

 

[Muad'Dib]

Sorry for the delay, here are the two schematics i found to be helpful:

http://www.jeepstrokers.com/TPS and ECU Wiring Diagrams 1990 FSM.pdf

 

[doodahman]

I fixed my high idle problem. The 2 rearmost nuts were slightly loose on my manifold. All of a sudden, it started ticking and when I tightened them, voila, no more high idle. Stupid vacuum leak.

 

[CJR]

I had this problem awhile ago. It turned out to be a cracked intake manifold that the previous owner had cleverly covered with aluminum epoxy. The epoxy perfectly matched the aluminum manifold and I never saw the crack. My son's sharp eyes picked it up. As the manifold heated up, it would separate behind the epoxy and cause erratic vacuum leaks and high idle. Once I replaced the intake manifold, the problem went away. Hope this helps.

Best regards,

CJR

 

[Muad'Dib]

Well to update, i pulled the TCU, and tested the TPS wires and they all passed with flying colors. So did the wires for the ECU side. I went ahead and installed a ground wire at the TPS for the ECU side for safe measure just as i have already done for the TCU side.

Voltage from the TCU is the same at 4.66V (at both directly at the TCU, and after the wiring to the TPS). Voltage from the ECU is 5.05v.

For shits and giggles i also checked the other ECU +5v outputs (to the MAP sensor for example), and i got very close readings. They were all over 5 volts (on average 5.05 volts).

Given the facts above, it made since that something was up with my TCU. I checked the input voltages to it (switched and unswitched +12 volts), and they were spot on.

At that particualar time i installed another TPS i had laying around. It cured the idle issue, but some other weird things started happening. My "RPM Float" came back. It would sometimes act like it was staying in gear too long.. just normal weird i know ive got a crappy TPS problems...

I dealt with it unit i could finally convince myself to just try a new TPS. I got it from Rock Auto. Its an Airtex brand. Model number 5S5212. The nice thing about this TPS from Airtex is that it came with new hardware. They were phillips head screws. Instead of those old torx ones from the factory. One big thing i did notice before installation was that when moving the arm on the TPS, it was much stiffer than any other one i have touched before.

I installed the TPS, and adjusted it to output 4.15 volts to the TCU per my 1990 FSM.

I took it for a quick drive tonight, and all seems perfect. No more problems with the high idle (which i expected)... but was surprised me even more was the was the throttle response changed. I cant say it was in a good or a bad way, but it was definetly different. Another problem it fixed that i have dealt with for awhile was something i describe as "RPM Float". After acceleration, if you let off the gas and just let it coast, the tach would drop to about 1,000 RPM's, and then Jump up to about 1,300 RPM's. It would cycle like this until you slowed down enough, or accelerated again. You could hear the engine rev when it was doing this also. Like you were giving it a slight amount of gas when you really werent. Anyway, its nice to know what was causing that problem.

Now only time will tell if i get some weird problems again or not. Now that i have a new TPS for the first time since i have had the Jeep, if a problem arrises, then i think im going to try a different TCU that can output close to 5 volts to the TPS and see what happens then.


I hope what i have written here helps someone in the future.

 

[The Lure Washer]

Hey Mods Can we get a sticky!@

 

[Ecomike]

Nice update and detail, you are officially awarded one ataboy!:yelclap:

I suspect a hand full of us could fill a book with a list of weird things and symptoms a goofy, used, but not totally dead TPS can cause. Now you have me wondering if TPS is not off just enough to be causing my final still a little low on MPGs and not quite full power output effects.

Sounds like you got the beast fixed for now! Congrats.

 

[roktoy]

Guess I’ll bump this great thread back to the top…….

First, let me say that this is my first forum post here and I want to thank Ecomike and others that have contributed to this thread…..it’s been very helpful.
I just purchased an ’89 XJ with 4.0L & AW4. It drove fine initially but when it warmed up after a few minutes of driving, the tranny would no longer shift out of first (and the idle was running high). I did a little research on here that led me to suspect the TPS may be bad.

I measured the impedance on my TPS (warm) and got these readings:

TCU Side:
Pin A to B: ~6.6k (idle) to ~2.4k ohms (WOT)
Pin A to D: ~200 ohms, constant
Pin B to D: ~6.5k (idle) to ~2.3k ohms (WOT)
ECU side:
Pin A to B: ~47 ohms, constant
Pin A to C: ~2.4k (idle) to ~3.6k (mid) to ~2.4k ohms (WOT)
Pin B to C: ~2.4k (idle) to ~6.3k ohms (WOT)

It appeared I had some internal shorting and other issues, but the readings were erratic as well. This pretty much convinced me the TPS was faulty, so I decided to replace it. I bought an Echlin one from Napa for $45. Before I installed it, I decided to check its impedance to see how a new one compared. I measured:

TCU Side:
Pin A to B: open circuit, constant
Pin A to D: open circuit, constant
Pin B to D: open circuit, constant
ECU side:
Pin A to B: ~4.0k ohms, constant
Pin A to C: ~6.3k (idle) to ~2.4k ohms (WOT)
Pin B to C: ~2.3k (idle) to ~6.4k ohms (WOT)

It seemed odd to me that the TCU side measured open between its pins and a little further looking on this forum led me to this thread and Ecomike’s measurements showing the TCU side was a second reverse pot. I returned the first TPS to NAPA and got a replacement. The second unit measured:

TCU Side:
Pin A to B: ~2.4k (idle) to ~6.5k ohms (WOT)
Pin A to D: ~4.1k ohms, constant
Pin B to D: ~6.5k (idle) to ~2.4k ohms (WOT)
ECU side:
Pin A to B: ~4.0k ohms, constant
Pin A to C: ~6.3k (idle) to ~2.4k ohms (WOT)
Pin B to C: ~2.3k (idle) to ~6.4k ohms (WOT)

All seemed right with this one. Goes to show that you probably need to check a new sensor before installing it. I have a habit of checking parts most times anyways (brake cylinders, starters, alternators, etc.) before installing them on a vehicle to save possible headaches later.

I installed the new TPS. I also checked the ground wires for the ECU and TCU and found them to be floating up to nearly 0.7V. I added a temporary ground wire over to the battery negative terminal and the idle sounded a bit better, so I took it for a test drive. All tranny gears had returned. It shifted and drove fine.

After my test drive, the idle was still a bit on the high side but I was pretty pleased with the progress. I took the ground wire I had simply wrapped around the negative battery terminal off and soldered on a lug. I placed the lug under the ground bolt on the firewall. This time when I fired up the motor, the idle was near normal speed and smooth. Seems this minor improvement in grounding made some significance as well. I plan to go over the other grounds when I get the chance.

I once again checked the old TPS I had removed and was getting readings that were pretty close to normal. So…I stuck the old TPS under the hood on the hot engine to heat soak for about 10 minutes. After that time, I rechecked the TPS readings. They were not identical to the previous warm readings, but they were still flaky. Seems that in my case, the TPS operation went south as the unit got hotter.

I next decided to cut the TPS open. Maybe someone has done this before, but I had not seen it. Inside, a small plastic drum rotates in the outer housing. On the drum are small metal fingers that rub against resistor material on a small flexible circuit card that is curved inside the outer housing. Evidently these parts mechanically wear and result in poor contact or maybe loose foreign debris plays some havoc with the resistor values.

In any case, thanks much to everyone that’s contributed to this thread. It has been immensely helpful and I now understand a lot more about this system. Maybe my story can add to this thread and help someone else down the road.

Jay