1988-89 Carrera cruise control

[racerr1958]

I have noticed that there is no diagnostics on the 911 cruise control for the 88-89. This was the first year of the VDO electrical powered servo unit that replaced the vacuum unit.
Since I was having issues I decided to do a simple trouble shooting guide. Bentleys does publish an electrical diagram of the system and it is accurate.
The system consist of two major units; the control unit (under the dash) and the servo (engine room). These components are pretty much sealed but you can certainly take them apart and check for solder cracks and burned components on the board of the control unit. The servo unit can also be taken apart but there are very little serviceable parts in the unit. You can still grease the moving components inside. Try to swap these two components if the following troubleshooting does not help you out.
Start with disconnecting the control unit that is found under the dash on these cars. We are focusing on the harness that connects to the control unit and verifying the signals that the control unit receives.



CRUISE CONTROL UNIT

TERMINAL Ώ Ώ VOLTS
on off( push)
power 1 12v
power up cruis 14 ∞ 0
lever set speed 4 12v when stalk pulled up
cancel cruise 3 0 ∞
reset 6 ∞ 0 12v when stalk pushed down
clutch 12 0 27 ohms 12v when stalk pulled
brake 8 0 ∞ 12v
direct to drive 10
direct to drive 13
direct to drive 9
direct to drive 5
direct to drive 7
speedo 11 32 ohms


The test I took to verify the workings of my system are the following.


1 check volts at 86 & 85 on relay
2 check volts at 14 with unit on & off
3 check ground G104. Check ground from relay 87.
4 check ground at 85 on relay. Infinity with clutch out, 0 with clutch pressed.
5 85 and 87 in relay should have continuity when clutch is pressed.
6 85 in relay and 12 in harness should be continuity when clutch is pressed.
7 85 in relay goes to continuity, with clutch pressed.
8 check volts on 4, 3, 8 for stalk at ecu
9 check 11 going to speedo
10 check 10,13,9,5,7 for continuity to drive
11 check shorts to ground
12 check continuity between 11 and the speedo (wht/red wire)

Hope this helps those with cruise issues for the 88-89 Carreras.

[Chuck.H]

Did you find/fix your problem? All of my problems have always been traced to that problematic clutch switch, my second-to-least favorite switch after the ridiculous brake light switches.

My brake light switches got bypassed with the mechanical modification; one more failed or out of adjustment clutch switch and it's getting bypassed also.

HTH,
Chuck.H
'89 TurboLookTarga, 426k miles

[megerian]

Interesting, so the Bentley wiring diagram is accurate, but people have pointed out that the troubleshooting guide in Bentley is not accurate for 88-89

Cruise Control Questions


So, did you know that the Bentley troubleshooting guide was wrong, and that is why you did your own? I have a non-working cruise in my '88 and have had the brain redone and it still doesn't work. I tried the Bentley troubleshooting before I knew it was wrong, and got nowhere. So if your guide above is now the correct troubleshooting guide for the 88-89 cruise, that would be great!

[Mervo]

I am seeing no power with IGN on at 86 (and therefore at PIN 1)? Fuse 4 in Block 1 is fine. ??

[Mervo]

This was the most useful thread found. Most revisited the earlier model systems or the Bentley with its inaccuracies for the later cars. The Workshop Manual was the best.

I think I now know the CC system on these cars extremely well. Checked clutch cut out switch, brake cut out switch, exchanged CC module with a known good one, exchanged steering column switch with a known good one, tested CC relay and fuses ... etc. The unit just wasn't getting power at the relay.

Finally, I pulled out the whole fuse box and found the power wire underneath to the CC relay was disconnected. All perfect now and holds speed well. I guess you have to do these things for yourself, but what a saga.