RPM's go way up when put in drive and 4lo only with brakes on

3 days before winterfest and I am having a problem I should have tackled when I put it on the trailer last fall. I finally figured out the sequence that annoys it though. If I have my truck in 4lo and in a forward gear all is fine. But as soon as I mash the brakes the rps's go up to 2500-3000 who knows how much higher till I put it into neutral or if I release the brake it will come back to earth. If I just touch the brakes so the lights are on and the truck thinks it is braking all is fine. If it is in reverse no problems regardless of the brake. If it is in any transfer case mode other than 4lo it is fine.

I am above average when it comes to working on cars and this has me stumped where to even look. It is a modified truck axle wise and suspension, it does have a mild built 4.0 but most other stuff is still jeep. I thought vacuum related and went through every remaining line on the truck today and did find a leak which fixed my defrost only mode for the climate control but I knew that was a vacuum leak. No other lines were suspect and most are new.

I have a 44 front and 8.8 disk rear and the brakes have always sucked, they stop it but there is 'no pedal' I used the recommended master cylinder and after getting one bad one out of the box and replacing that they are better but still suck. Only reason I say this is one random crazy ass idea I have is that the diaphram is cruddy, it's the only origional brake part left in the system.

Any ideas?

Oh yeah 1990 cherokee so yes it is a Renix truck.

If the vacuum line going to the booster was leaking it would cause the RPMS to go up. weird in 4 lo only though

Does it have a vacuum operated disco on the front Dana 30 axle?

I am going to look over the booster, the check valve, and that hose in the morning. The hose looks good but I am not sure about the connections.

It has a D44 front and it is a np242 so it never had a disco 30.

turbo6justin said:

If I have my truck in 4lo and in a forward gear all is fine.

But as soon as I mash the brakes the rps's go up to 2500-3000 who knows how much higher till I put it into neutral or if I release the brake it will come back to earth.

If I just touch the brakes so the lights are on and the truck thinks it is braking all is fine.

If it is in reverse no problems regardless of the brake.

If it is in any transfer case mode other than 4lo it is fine.

I have a 44 front and 8.8 disk rear and the brakes have always sucked, they stop it but there is 'no pedal' I used the recommended master cylinder and after getting one bad one out of the box and replacing that they are better but still suck. Only reason I say this is one random crazy ass idea I have is that the diaphram is cruddy, it's the only origional brake part left in the system.

Oh yeah 1990 cherokee so yes it is a Renix truck.

Click to expand...

I read some of the suggestions.

A D30 Vacuum leak will not cause runaway RPMs, it's too small. I've had them fall off with no discernible affect other than HVAC not working right.

I don't think a brake booster leak would cause runaway RPMs either. A small rise in a RPMs, but not runaway. To test, yank hose off pinch it, and see what happens as you open the port.

The highlighted part is what bothers me.
It suggests that this symptom might be related to the chassis twisting under load and pinching or pulling something. It might not be a bad idea to check wire and control cable routing.

Sounds like this truck hasn't been "Stock" in a while.

I think you hit the nail on the head about wiring while the jeep is "twisting and turning"

Brakes dump a fare amount of energy into the ground at the oil dip stick, and the notorious poor grounds of a 1990 Renix, may be the culprit, spiking the TPS sensor output voltage to the ECU when the poor, loose, dirty ground contact is jerked, moved and sees a voltage spike build up back voltage on the sensor ground, sending the apparent TPS output voltage to the ECU higher!!! Also the sensor grounds piggy back with crimps onto one wire in the harness and they come loose over time!!!

Morning update is second verse same as the first. Pulled off the brake booster line and checked it, no leaks and the check valve works as it should. Put it back on and pinched it off, truch still has the same problem. There are only so many things that can cause the idle to change so for kicks I unplugged the IAC, it stalled, restared it with it unplugged and it would idle just fine a little rough but okay. Tried it again and it still wanted to run away plugged the IAC back in and disconnected the cruise control cable no change. Pinched off the EGR because it is easy no change.

There are only so many things that can cause to idle to rise, physical would be some sort of twisting of the throttle blade, I think I have eliminated all off that. Electronically the IAC is suppose to control it and I was thinking that direction but it reacts the same without the IAC.

I had another problem last year and went over the whole truck, every connection was cleaned, all the routing was checked, it looked good to me. So I kind of doubt a cable or wire stretching problem. It also will start to runaway without any torque on the chassis. Put it in drive, push the brake and away she goes, most of the time. It can be induced everytime by the smallest tap of the gas pedal.

It has not been stock for a long time but the engine controls and drivetrain is remarkably stockish.

As I was typing there were some more questions about the grounds. As I said I had a problem last year and went over everything. The result of that was a lot of waisted time but I did clean every existing ground especially over by the dipstick as well as every other factory ground. I did add one 2 AWG ground between the head and chassis.

The IAC controls the idle air flow only, but does so at the instructions from the ECU. Are you saying that with the IAC disconnected, that applying the brakes make the idle go up?

If so, then I would suspect the ECU is increasing the fuel injection rate based on a bad signal from the TPS that is caused by a ground problem on the TPS wiring.

I understand what you are saying about the IAC. My thought was that it was working properly but was getting an 'incorrect' command for some reason from the ECM to open it up. Like I said there are only so many ways to raise the RPM, move the throttle butterfly or open up the IAC passage are two sure fire ways.

How should I add a better ground for the TPS? Black wire of the 3 wire connector is ground? just solder a jumper wire from the backside of the engine side of the plug and run it to ground?

I just went out to double check and yes the rpm will go way up with the IAC disconnected.

turbo6justin said:

I understand what you are saying about the IAC. My thought was that it was working properly but was getting an 'incorrect' command for some reason from the ECM to open it up. Like I said there are only so many ways to raise the RPM, move the throttle butterfly or open up the IAC passage are two sure fire ways.

How should I add a better ground for the TPS? Black wire of the 3 wire connector is ground? just solder a jumper wire from the backside of the engine side of the plug and run it to ground?

I just went out to double check and yes the rpm will go way up with the IAC disconnected.

Click to expand...

I ran a new ground wire from the TPS harness side on mine to the battery negative post. Cruiser54 recommends opening up the wiring harness and soldering the crimps. He has a whole post on it here it somewhere, "Cruiser54 Renix tips" IIRC, Google it or search here for the meat of it.

http://www.naxja.org/forum/showthread.php?t=1072534

https://www.google.com/search?q=crueser54+rrenix+tips&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-USfficial&client=firefox-a#hl=en&client=firefox-a&hs=4NP&rls=org.mozilla:en-US%3Aofficial&sclient=psy-ab&q=cruiser54+renix+tips&oq=cruiser54+renix+tips&gs_l=serp.3...2874.3309.1.4264.2.2.0.0.0.0.203.379.0j1j1.2.0.les%3B..0.0...1c.1.4.psy-ab.Xg33rbE7H8E&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&bvm=bv.42965579,d.dmQ&fp=783d027d1bd68a5f&biw=1280&bih=612

He has some great write ups.

turbo6justin said:

I understand what you are saying about the IAC. My thought was that it was working properly but was getting an 'incorrect' command for some reason from the ECM to open it up. Like I said there are only so many ways to raise the RPM, move the throttle butterfly or open up the IAC passage are two sure fire ways.

How should I add a better ground for the TPS? Black wire of the 3 wire connector is ground? just solder a jumper wire from the backside of the engine side of the plug and run it to ground?

I just went out to double check and yes the rpm will go way up with the IAC disconnected.

Click to expand...

Be real interesting to see what the TPS output voltage on the 3 wire flat connector is to the ECU versus a real ground at the battery when the brake peddle is pressed!!!

Boy I liked that TPS grounding issue so much. I went and put a 14awg ground from the sensor side of the plug to the battery and gave that a try. No luck.

I would love to go through all the crimps but it is going to start snowing here in about 2 hours and that is going to make this more difficult and don't figure I can get through all that in a few hours.

turbo6justin said:

Boy I liked that TPS grounding issue so much. I went and put a 14awg ground from the sensor side of the plug to the battery and gave that a try. No luck.

I would love to go through all the crimps but it is going to start snowing here in about 2 hours and that is going to make this more difficult and don't figure I can get through all that in a few hours.

Click to expand...

LOL, I think I used 18 gauge. Sorry it did not solve the problem yet. Keep at it!!!

It is amazing how useless a $20,000 machine becomes when a 50 cent ground goes bad, LOL!

I am at a point where I do not know what to do next. Last fall when I was having what might have been a grounding problem I went ahead and did all the ground clean up mentioned in those articles. Since that time it has been put on a trailer and thats it. The grounds look shiney perty, actually kind of gooey from dielectric but you get the idea. I added the extra battery to radiator support and head to firewall cable. At that time I also went through every connector and rechecked that they were still clean and greased with dielectric. If there are ground splices along the fuel rail I have not done anything with that. And getting to my c101 connector is a bitch because I have to remove a home brew snorkel that is very tightly in there. No it has never seen water even close to that.

FWIW before I added the new ground for the TPS I was getting .4-.5 ohms between it and the battery ground. Not perfect but not too bad either.

Some meters read .5 ohms for a dead short, it is the meter internal resistance showing up.

When you put it in 4-lo, could the linkage be messing with the O2 sensors wires somehow?

I don't know enough about the 4-Lo wiring (if any) to help, but I would look for chaff wires and signals getting crisscrossed if it only does it 4-LO, but be sure to look at the C-101 and O2 sensor wires....

What is common to the brakes, and 4-Lo, wiring or otherwise? So the idle only speeds up in gear, with 4-Lo and brakes on?

So I haven't really got too much further on this in the last day and a half. Leaving for Winterfest in a few hours but I will be revisiting this soon.

Thanks for the help so far!

I promised a final post on this one and here we go. No it wasn't a grounding issue, no it wasn't a Renix issue. It was a mechanical issue. With the help of some extra eyes and feet at Winterfest I was able to see that the throttle itself was being opened. A bit of brainstorming and one useless throttle cable later and my buddy figured it out. I have a remote resivoir for the ram assist and it was run close to the throttle, well close was too close. In 4 low in a forward gear only the motor would torque over a bit and make contact, the contact would push the throttle further which torqued the motor more and so on until the good condition motor mount was out of movement which was about 25% throttle.

Sometimes some fresh eyes and a little time are all it needs.

With that said I have the familiar 3000 RPM high idle after start up and can't figure that one out. Latch relay is okay, IAC is spotless and seams to be working, Grounds were cleaned again, TPS has its own ground, Injectors do not leak down, nothing I could find around the intake or TB gasket area. Not sure where else to look.

Bad TPS more than likely.