Cruise control wiring help request

[tmadden2]

I’m trying to locate where NR158 (circled) is and what it does in a 1986 911. The factory manual shows it on the diagram, but omits it from the list of components and locations. I’m working in from the ends and zeroing in on that section of the wiring as the potential culprit for why cruise has stopped working. At the front end of the car, the fuse, relay, clutch switch, brake switch, steering column stalk, and amp all work. I can get continuity between pin 7 on the cruise amp wiring harness and one side of the servo. At the back end, the resistance between the two contacts on the servo check out and the Bowden cable is in good shape. I have recently replaced the servo to no effect. I have also tried a second known-good brain to no effect.

The head-scratcher is that I can’t get continuity between pin 3 of the cruise amp wiring harness and pin 7. This should test the resistance of the servo, but it just shows an open circuit. It seems like there’s a break somewhere between pin 3 of the cruise amp harness and the servo wiring harness. Possibly at NR158? If you have any other tests, thoughts or suggestions, I’m all ears. Thanks!

[schoward]

I was very recently tracing wires for the Cruise on my '81. I believe that may be a white or black plastic plug that simply connects the two sets of wires heading to the servo if I recall. I found it hanging out of the harnesses strapped up behind the pedals behind the driver floorboard. In fact I seem to recall the brown with the white stripe (BR/WS) just dead-ending not too far from the plug like the diagram shows.

As usual, the brain was the culprit in my issue and I put it all back together. Cruise working again and left all wires as is, including the dead end one. Cruise, including the cruise switch for changing gears works just fine so that must be by design as the diagram shows.

This is worthwhile to check. I had a very similar plug deteriorate on the alarm many years ago that caused it to sound randomly. Turns out the insides of plug corroded, intermittently losing connection if wires jostled. My cruise one apparently was fine.

Check up behind the pedals/driver floorboard. The floorboard has to come out if I recall.

[Uwon]

Following..

[megerian]

This thread is not for the same year, and may not even be helpful, but I am referencing it because my troubleshooting guide led me to think the servo was bad, and it was fine.

Speed sensor for Cruise Control

[steely]

Sorry not clear on your question so don't think me a smart-a&&.
NR 158 is the shorthand (reference designator) name of the harness itself.
The dwg would be something like 911612158xx
It mates to NR157.
Maybe that is where your problem is - at the 3-pin mating connector.
I assumed since it sounded like you took readings at the motor end of the harness, then you already touched NR 158.

[targa80]

Have you looked at the relay tempostat pin 87a and pin 30. In order to have a reading between pin 7 and pin 3 on the tempostat control module you need the relay tempostat to be in the normally closed position. If the clutch switch is closed it supply’s 12V to the relay coil and opens the contacts 87a and 30. You may have a clutch switch alignment issue.

[tmadden2]

Originally Posted by steely

Sorry not clear on your question so don't think me a smart-a&&.
NR 158 is the shorthand (reference designator) name of the harness itself.
The dwg would be something like 911612158xx
It mates to NR157.
Maybe that is where your problem is - at the 3-pin mating connector.
I assumed since it sounded like you took readings at the motor end of the harness, then you already touched NR 158.

I’ve been looking all over the car for a three-pin connector like the one in the wiring diagram. Working from back to front, I looked in the engine bay, transmission tunnel, under the auto heat box, behind the center console stack, and behind the pedal cluster. I just don’t see anything that looks like a three pin connector. I’ve got a tone generator that I’ve been using to trace through the cabin from the servo end. I can follow the signal as far as the wall behind the pedals and then I lose it somewhere. It seems like somehow the wire from the servo has to make it up under the dashboard to plug into the cruise control brain wiring harness but i can’t follow it anymore without starting to cut open insulated bundles, which I really don’t want to do.

[tmadden2]

Originally Posted by targa80

Have you looked at the relay tempostat pin 87a and pin 30. In order to have a reading between pin 7 and pin 3 on the tempostat control module you need the relay tempostat to be in the normally closed position. If the clutch switch is closed it supply’s 12V to the relay coil and opens the contacts 87a and 30. You may have a clutch switch alignment issue.

I’ve tested it a few different ways and it seems to test okay with a multimeter when I hook it up to the relay plug in the frunk, but if that’s the only way to get a signal between 3 and 7 on the cruise amp wiring harness, I’ll keep playing with it.

[targa80]

Try doing a point to point resistance test I.e. pin 3 on tempostat control module connector to pin 30 on relay tempostat connector. Disconnect each of the connectors iin the cruise control circuit then testing each wire from point to point will eliminate the need to damage the harness.

[tmadden2]

Things got busy here, but I’ve finally had a chance to run through every connection the wiring diagram (highlighted in yellow). The failure seems to be the connection between 87a at the relay terminal and its corresponding terminal on the cruise servo. There’s no continuity, which also explains the open circuit when I try for a connection between 3 and 7 on the cruise amp harness when the relay is and the ignition is on. I’m working from the 1984 diagram now because it looks more like what I’m seeing in the system (e.g., no third piggybacked wire on my clutch pedal switch).

I have a 12v tone generator that gets me as far as the bundle under the transmission tunnel. After that, I can’t pick the tone up anywhere between there and the fuse panel. If anyone has any advice on where this white with green striped wire runs around the dash, floorboards, etc, I’d be grateful.

[schoward]

I tried tracing that one starting from the fuse panel. I lost it behind the gauges/clock even with removing the gauges (its down in there somewhere in a black bundle). It appeared as if the whole fresh air fan / plenum assembly would have to come out in the frunk to continue to follow it. It then re-appears in the footwell / tunnel if I recall.

[tmadden2]

Thanks for the suggestion on getting behind the dash. None of the wiring diagrams show this two-wire connector anywhere. I discovered it behind the pedal cluster after tracing the signal from pin 87a with the Power Probe tone generator. The connector was zip tied very tightly to the back side of a large bundle of wires. There was no way to see it in all the dozens of times I’ve had my head down in that footwell. I clipped the zip tie, straightened the wires out, and hit the contacts with a shot of Deoxit. I’ve got continuity between pins 3 and 7 of the cruise amp for the first time since I’ve started this mystery project. Test drive tomorrow morning to see if that’s what did it. I’ll report back either way.

[tmadden2]

Well that did it. The two-wire connection behind the footboard was the culprit. Cruise is back in business for the first time in a while. During this process, I unhooked the clutch pedal switch entirely to have one less variable to worry about. Any reason to put it back in, or just let it ride as-is?

[rwest]

Doesn’t the clutch switch stop the engine from over revving when you push it in to change gears?

[tmadden2]

Originally Posted by rwest

Doesn’t the clutch switch stop the engine from over revving when you push it in to change gears?

Not sure. I figured it only came into play if cruise was active and you tried to shift at the same time. I typically cancel cruise with the stalk or brake pedal before shifting, but if it’s a safety thing, maybe I should do battle with the footwell one more time to get it back in place.

[targa80]

The system is designed to disingage the cruise if the clutch is depressed. Eliminating or by-passing that function would be a bad idea. Without the clutch switch in the circuit and you disengaged the clutch to neutral the cruise would still be engaged, what will the engine do with no load while the cruise is still engaged????

[ClickClickBoom]

Clutch switch is for shifting, works like a champ. Without it when you push in the clutch and speed deteriorates the throttle will open up, trying to get the speed back to programmed set point.