RENIX STUDS: Wierd Idle Issue

1989 4.0L Auto

OK, I just put my engine back in and think I must have forgotten to hook something up. Most sensors are new and wiring is pretty clean, so I think its something simple. Here's the issue:

Starts immediately and goes to correct idle of ~800 RPM on Tach.

If I give it some gas, it returns to idle at about 900 RPM.

Do it again, it goes to 1000 RPM. Usually peaks out at 1200 RPM idle.

Turn of engine and then turn on and idle jumps to 2500+ RPM. 2/3 of the time reving will bring it back to 800 RPM, sometimes turning off then on again brings it back to 800 RPM.

So, I'm thinking I might have missed a ground under the dipstick (only remember hooking up one wire other than bat cable).

Anything else to check? Its 110° here and when I get to the shop I want to isolate as quick as possible. It's gotta be 15° hotter in there.

If it ain't a ground, I'm thinking Power Latch Relay, then maybe TPS (which is new) and IAC.

I would say ground 1st, then TPS adjustment 2nd. also, check out the connectors on the TPS, make sure that everything is tight

Try switching the power latch relay with the ac relay. if you really wanna tackle the relay problem then just change them all they are about $6.00 at the local auto around here.

Latch relay sounds likely, but I thought that only affected reset of IAC. Would probably have no effect while actually running.

Any other ideas?

Nevada City Sparky said:

Latch relay sounds likely, but I thought that only affected reset of IAC. Would probably have no effect while actually running.

Any other ideas?

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Most of your test results indicate it is not the latch relay, or least not all of the problem is the latch relay. The 2,500 rpm thing might be the latch relay or dirty connectors at the relay.

Check the ground wire, pin B at the TPS/harness connector to the battery, with all power off. Should be less than one ohm. If not, then at least part of the problem is the ground. Sounds to me like it might be early TPS problems caused by wear at the idle position on the TPS, but a loose, variable ground will do it too.

IAC does not sound like the problem.

Says that TPS is new so must just be out of adjustment. Grounds are always good to check and add more if there is any doubt.

Ecomike said:

Sounds to me like it might be early TPS problems caused by wear at the idle position on the TPS, but a loose, variable ground will do it too.

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I'm putting money on the ground. I'm guessing i missed one when strapping on the dipstick grounds.

Ecomike said:

IAC does not sound like the problem.

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Thanks. I'll stay focused on TPS circuit.

No one mentioned distributor. Damn sure I got it in right, but I gotta tell ya, even when right, the rotor position just looks wrong. Didn't want to pull that sucker again, and when it does idle, it idles sweet.

Defintely not the distributor. If that was the culprit you would never get an idle or start for that matter.

Nevada City Sparky said:

No one mentioned distributor. Damn sure I got it in right, but I gotta tell ya, even when right, the rotor position just looks wrong. Didn't want to pull that sucker again, and when it does idle, it idles sweet.

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X3 on the ground having resistance. The TPS units on auto and manual equiped cars work opposite of each other. With the auto, as the throttle opens, the TPS resistance goes up.

Measure VOLTAGE between the TPS ground wire and the engine block. Going off memory, I think any potential over 0.1V indicates high reisitance. The fix is to tap the TPS ground wire and install a redundant ground under the hood.

OK, TPS Circuit it will be then. I adjust the TPS on the first engine I put in (long story). Probably need to check again once I ensure everything i hooked up OK.

Correction,

As the throttle opens the Resistance goes down, and the voltage goes up, from Pin B to pin C on automatics.

I think the Renix manual TPS is the same as the Renix manual engine ECU's are reported to be the same as the automatics. I seem to recall hearing the HO engine TPS works in the reverse direction or the renix TPS.

It is better from an electrical standpoint to measure the actual resistance, power off, between Pin B of the TPS connection using the back probe method on the assembled connector to the battery negative post. It should be less than 1 ohm. I trust this method more than measuring residual volatge on the ground wire as explained below. Both work, I just trust the direct ohm reading method more.

I ran a fresh ground wire directly from my TPS wiring harness connection directly to the negative battery post on mine.

Hypoid said:

X3 on the ground having resistance. The TPS units on auto and manual equiped cars work opposite of each other. With the auto, as the throttle opens, the TPS resistance goes up.

Measure VOLTAGE between the TPS ground wire and the engine block. Going off memory, I think any potential over 0.1V indicates high reisitance. The fix is to tap the TPS ground wire and install a redundant ground under the hood.

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mikeforte said:

Says that TPS is new

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I kinda missed that clue, must be brain dead from reading those TXXJA bylaws. Good catch Mike.

OK, so you made me go and look. After all, it's been over a year since I've done this.

http://www.naxja.org/forum/showpost.php?p=844996&postcount=16

For auto tranny:
http://autorepair.about.com/library/faqs/bl232i.htm

Vincent Ciulla said:

I do know the adjustment procedure is different for manual transmissions and automatic transmissions.

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I don't mean to be argumentative when I post this. I told poor phillip_g wrong in that thread, and came away learning something new myself.

The significance of the ground resistance is that the ECU compares a reference voltage to a return voltage. In the case of the auto tranny, the return drops as the throttle opens. If the return voltage drops due to resistance in the wires, it's still compared to the reference voltage; the ECU thinks the throttle is slightly open.

It's one of those unique things about the RENIX system; the auto is ass-backwards to everything else on the planet.

N-C Sparky, here is another link that may help you narrow things down: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Good luck, another Mike

As I recall there is actualy an error in their posted TPS calibration procedure on their site that I ran across once before. Ya, I found it, they have the wrong picture of the connector in the manual Tranny TPS thread at:
http://autorepair.about.com/library/faqs/bl608e.htm

They are showing the square four pin connector that is used on the auto tranny TPS, in the manual Tranny TPS calibration procedure.

Oh, and this one is flatout scarry, they are showing a picture of the MAP sensor in this page and calling it a 1988 Cherokee TPS!

http://autorepair.about.com/library/illustrations/bl736a-lib.htm

And this one is so far off base on it's "Usual Suspect" answers it's not even funny:

http://autorepair.about.com/library/faqs/bl051i.htm

I see now where you got the confusing miss information. But here is the deal. The Auto tranny method they are refering to is a calibration of the TCU side of the automatic tranny TPS, not a calibration of the ECU side of the TPS. If you calibrate the ECU side of the auto tranny TPS (it is three pins, not four pins) it uses the same calibration method as the manual tranny TPS calibration method. Many other sites list just the three pin ECU calibration for the manual and for the auto tranny TPS.

The Auto tranny TPS has 2 sides, the ECU side has a 3 pin flat connector, the TCU side has a 4 pin square connector. The TCU side is backwards compared to the ECU side. The TCU side works the reverse of the ECU side.
Personally after all my headaches with the Renix TPS I have come to the conclusion that about dozen extra tests including a dual calibration test of both sides of the automatic tranny TPS should be done, not just one side or the other, followed by a full throttle to idle voltage range test on both sides. For full details and links to better pictures, better idle calibration details and links see my story in the "Renix Files" thread that I sort of fathered here late last year.

Just a note of thanks on helping me dial into the problem. Got it solved in less than an hour. Grounds were all OK and TPS needed adjusting. After starting and going to 2500 RPM, I decided to "de-program" by pulling the battery cable while I got a beer (it was 109°). When I came back, went right to idle and subsequent test drives have kept it there.

A possiblility I may have had an air bubble behind the temp sensor on the first test drive.

Well I would just say COOL, but that would be cruel pun, being its so hot out there, LOL. So how about just great!

Must have been the beer that did it! :yelclap:

Nevada City Sparky said:

Just a note of thanks on helping me dial into the problem. Got it solved in less than an hour. Grounds were all OK and TPS needed adjusting. After starting and going to 2500 RPM, I decided to "de-program" by pulling the battery cable while I got a beer (it was 109°). When I came back, went right to idle and subsequent test drives have kept it there.

A possiblility I may have had an air bubble behind the temp sensor on the first test drive.

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Just for general info, I was having TPS issues, was driving me nuts. Everything tested out good sitting in the driveway.
Found a ground splice for the TPS and some other sensors between the fuel rail harness bend at the firewall and the C-101 connector, under the tape. Regular crimp splice, no solder and is was loose. Shake the harness and the ground resistance would change.

You are speaking of a non-OEM, aftermarket ground splice modification like a past ground repair that had gone bad, right?

Ecomike said:

You are speaking of a non-OEM, aftermarket ground splice modification like a past ground repair that had gone bad, right?

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It's where the TPS ground, the engine coolant temp ground, the MAT ground and (I'm guessing here) the MAP ground splice together then go through a single wire to D-3 pin (sensor ground) on the ECU.
The only connection on mine that was really acting up was the TPS ground, but it actually could have been any or all of the above. It wasn't an open circuit, it just had variable resistance. A bad crimp and no solder. Years of the motor rocking back and forth and flexing the harness probably loosened the wire ends, to the point of trouble.

8Mud said:

A bad crimp and no solder. Years of the motor rocking back and forth and flexing the harness probably loosened the wire ends, to the point of trouble.

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Found the same thing on mine when I went through the harnesses (I replaced all the coil jacketing and inspected all the wires) on mine. I found several splices that were not oldered, and one where the wires were just laying next to each other with a "splice on the side". Now that I have all those cleaned up, I'm getting a TPS input voltage of 4.9 V, measured connected, and a solid 5V when measured at the input plug.

I did that on version 1.0 of the rebuild.