Cruise control

[thsupernge]

Guys,
I want to do a little troubleshooting on the CC for my 88. I know many of these rigs on the forum don't use it, delete it etc. A search turned up very little. I'm pretty sure it's vacuum-driven but that's about it. I'm not even positive the switch works right...the detents feel odd as well.

Appreciate the help/suggestions in advance!

[FrankM_]

It is a pretty simple system. First things to check are indeed the electrical connections to the control unit and the switches (stalk, brake, clutch).

Do you have the Bentley manual at hand or do you need a basic schematic ?


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[FrankM_]

This is the connection diagram that I made

The P1 .. P12 refer to the connector on the Cruise control unit
The SW1 here is just the key contact
SW8, 9, 10 are the switches in the stalk
SW2 is your brake light switch on the master cylinder
SW3 is the clutch switch near the pedal cluster (in the tunnel)
K3 is the Cruise control relay in the fusebox
F4 is the fuse

So you can easily test for +12V on the CC unit pins if you activate the switches (incl brakes)

In my case I needed to open the CC box to fix my issues : resoldering some bad connections and replacing the power and reference caps inside. Apparently the PO cooked the battery at some point (overvoltage) and the 2 central 47uF capacitors on the board can only sustain 16V. But before you open the box, please check all connections to the box are good.

Also check the resistance of the cruise control vacuum actuator. Between the two electrical connectors, you should see 12-22 Ohms from the coil windings.
And offcourse, check that you have a good vacuum without leaks going on or the valve won't hold position.

[thsupernge]

Wow, this is awesome guys, much much appreciated. I do have the Bentley manual but didn't see anything for CC? I always start with vac lines and work my way to the more complex. Let me see what I can figure out.

BTW, does the stalk switch have active detents or does it just move to select positions momentarily?

[FrankM_]

The stalk is momentary switching 12V to the CC unit.


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[thsupernge]

thank you, I was hoping so.

[uwanna]

Originally Posted by thsupernge

Guys,
I want to do a little troubleshooting on the CC for my 88. I know many of these rigs on the forum don't use it, delete it etc. A search turned up very little. I'm pretty sure it's vacuum-driven but that's about it. I'm not even positive the switch works right...the detents feel odd as well.

Appreciate the help/suggestions in advance!

Your '88 model does NOT use the vacuum servo, it uses a VDO electric servo
and a completely different CC module than the pre '88/89
Here's thread from someone who worked on his '88 unit.
Good info.
1988-89 Carrera cruise control

Good luck!

[GH85Carrera]

The Bentley manual is in error for the pedal switch diagnosis.

The cruise control is wonderful for people that actually drive their 911 on the highways for long road trips. It is possible the cruise control computer has a bad solder joint. There are several people with the right equipment to see the cracks and even replace a bad component. My cruise computer would just loose speed. After 10 minutes I was going 5 or 7 MPH slower than I want. I sent it to one of the repair shops and not it holds speed great. A couple of regulars on Pelican repair them.

[thsupernge]

Ok wow, I might be in for a little more than 'a little troubleshooting,' thanks guys!
I might start with the main failure points like switches and relays

[thsupernge]

here's a nice guide that might help...
https://www.porscherepair.us/porsche-911-1984-1989/cruise-control.html

[megerian]

Unfortunately that link doesnt apply to your car, at least not entirely. If you have an 88, they switched from the vacuum servo to the electric servo, so the pictures in your link do not match your car, and the troubleshooting steps are not accurate. If you decide to tackle the project, let me know. I have an 88 and I went through the entire process from beginning to end. While I cannot solder the box itself, I can at least help you narrow down the problem. For most people, the problem tends to be the cruise box, although I was one of the rare cases where the wires going from the speedo to the cruise box were incorrect, so the speed sensor was not being sent to the box.

Most people find that the stalk is not the problem, nor is it the electric servo, but its easy to determine that using a cheap multimeter. If you dont have one, you can pick one up at harbor freight and a complete novice like me figured it out in a few minutes.

The first thing to do is obvious, look under the dash, right below the radio, and find your cruise box. It connects to a wiring harness. Its simple to remove, and the numbered ports in the harness are the ones that you will test with your multimeter.

I have the correct troubleshooting guide for a 1988

[thsupernge]

Originally Posted by megerian

Unfortunately that link doesnt apply to your car, at least not entirely. If you have an 88, they switched from the vacuum servo to the electric servo, so the pictures in your link do not match your car, and the troubleshooting steps are not accurate. If you decide to tackle the project, let me know. I have an 88 and I went through the entire process from beginning to end. While I cannot solder the box itself, I can at least help you narrow down the problem. For most people, the problem tends to be the cruise box, although I was one of the rare cases where the wires going from the speedo to the cruise box were incorrect, so the speed sensor was not being sent to the box.

Most people find that the stalk is not the problem, nor is it the electric servo, but its easy to determine that using a cheap multimeter. If you dont have one, you can pick one up at harbor freight and a complete novice like me figured it out in a few minutes.

The first thing to do is obvious, look under the dash, right below the radio, and find your cruise box. It connects to a wiring harness. Its simple to remove, and the numbered ports in the harness are the ones that you will test with your multimeter.

I have the correct troubleshooting guide for a 1988

That's true, I just realized as I read further. I really appreciate all the help. I might just remove it and sent it in. If I do I'll hit it with the multimeter and may ping you for advice. I'm assuming that you check each port for continuity? I'm a complete novice as well when it comes to the dark arts (electricity).

[megerian]

Just to be clear, the main troubleshooting steps aren't actually testing the cruise box itself. They are testing that the input signals are making it to the cruise box. For instance, by using the troubleshooting guide, you will test the signals from the cruise stalk lever, the clutch, the speed sensor, the brakes, etc. In fact, you will have to remove the cruise box entirely to get access to the ports on the wiring harness to run the troubleshooting steps. Since its more common for the failure to occur in the cruise box itself, most people just skip the troubleshooting and send in the cruise box to be tested, and if necessary, repaired. So then if the cruise box is tested and good, only if the cruise control still doesnt work, would you really be forced to use the troubleshooting guide to figure out what else could be wrong. In my case it was the speed sensor.

[thsupernge]

Originally Posted by megerian

Just to be clear, the main troubleshooting steps aren't actually testing the cruise box itself. They are testing that the input signals are making it to the cruise box. For instance, by using the troubleshooting guide, you will test the signals from the cruise stalk lever, the clutch, the speed sensor, the brakes, etc. In fact, you will have to remove the cruise box entirely to get access to the ports on the wiring harness to run the troubleshooting steps. Since its more common for the failure to occur in the cruise box itself, most people just skip the troubleshooting and send in the cruise box to be tested, and if necessary, repaired. So then if the cruise box is tested and good, only if the cruise control still doesnt work, would you really be forced to use the troubleshooting guide to figure out what else could be wrong. In my case it was the speed sensor.

Exactly, that was my thought process as well. Thanks again for all the assistance and consideration here, much appreciated!

[thsupernge]

Quick update:
I sent the control unit to Kris Rose as suggested and I can't be happier with the results. Repaired better than original in one week, great communication and all for $100 including return shipping. This guy rocks!

[GH85Carrera]

Back in 1995 when I bought my 911 the cruise did not work. The brake pedal switch was just out of alignment. It is a real pain to twist my body into the shape to get my face down there to see what I was doing. A simple adjustment, and it worked for years. It was pretty easy to test the switch with a multimeter and I did not have the Bently manual at the time, so I just figured it out.

Years later when the cruise stopped it was just the main computer box that was NFG. I sent it in, and it worked great since then. It is nice to have cruise in the open road, low traffic areas of the country.

Glad yours was an easy fix. $100 bucks is a cheap repair for a G body 911.

[thsupernge]

Originally Posted by GH85Carrera

It is nice to have cruise in the open road, low traffic areas of the country.

Glad yours was an easy fix. $100 bucks is a cheap repair for a G body 911.

Totally agreed