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Join Date: Mar 2019
Posts: 51
964 Cruise Control Help!

I am looking to make my cruise control function again on my 1990 C2. So far the diagnosing/troubleshooting steps I have taken are:

Swap Clutch switches. However, both were used.
Replace the brake light switch after breaking the original.
Replace all bulbs except the front turn signals with incandescent bulbs.
Remove any unused wiring in path.
Swap Cruise control computer from a 1997 993.
Replace the Vehicle Speed Sensor in the trans.
Replace the ABS relay in frunk.
Test functionality at cruise module from the inputs using a multi-meter.

NOTHING! I'm so frustrated by this. I really don't have any clue where to go from here other than thinking it may be that both clutch switches or wiring were bad based upon my multi-meter testing. Here are some YouTube videos that I made to try to help:

Hopefully someone can help me out because I'd really like to have the cruise functioning for Luft (assuming it's still going to happen).

Additionally, when I ordered the new brake switch it came with spade plugs so I had to cut the old wiring. I didn't make note of the wire orientation. I have attached some pictures of my current wiring. If anyone has an 89 or 90 and can help me out with making sure I have the pedal box wires correct, I would appreciate that.

Old 03-24-2020, 02:37 PM
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Registered User
Join Date: Aug 2014
Location: Ann Arbor, MI
Posts: 87
The cruse control not working in most cases is a sign that the clutch is not fully returning and contacting the microswitch to enable operation. That was the issue with mine. You probably already checked this, but it’s an easy check so thought I would mention it. It’s been a few years, but I think you can relocate the the switch slightly to ensure the switch contacts are engaging.
1989 Carrera 4 Coupe - Guards Red
Old 03-24-2020, 05:39 PM
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Dmitry at Pelican Parts's Avatar
Join Date: Aug 2016
Location: Los Angeles, CA
Posts: 5,773
Another member suggested: Check the linkage at the throttle cables in the engine compartment. There is a small turnbuckle type thing that ties the cruise cable into the gas pedal cable. Mine came off and caused my cruise to quit."

Here are some other threads for check out too:

Cruise control not working

cruise control problem

Cruise control
Old 03-25-2020, 11:23 AM
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Join Date: Nov 2013
Location: Torquay , UK
Posts: 75
Dont think the brake switch has a polarity as such since it is just an in line on/off switch.
Is your speedo and rear spoiler automatic up/ down function working OK? Believe the cruise control gets a signal from wheel speed sensor then via a frequency converter relay in the front fuse box, then to the speedo and from there to rear spoiler, cruise control and if fitted On Board Computer in the tach.
Old 03-25-2020, 12:45 PM
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Join Date: Mar 2019
Posts: 51
Paperboy, This is the interesting thing with my car (I think) I do not have a frequency converter relay. From the diagrams and research I was able to find in the 89 and 90 they use the Vechicle Speed Sensor on the trans rather than the abs sensor in the wheel. Needless to say, I have cleaned the driver's side ABS sensor and it seems fine to me. The speedo and spoiler work fine from what I can tell. The speedo was intermittent until i replaced the VSS now its been rock solid. Also, no OBC in the tach for me.

Dmitry, I'll look into the linkage. I've check as much as I can with my fingers to feel around back behind the intake manifolds to see if I could feel something disconnected back there but I couldn't feel anything suspicious.

Another interesting note is that almost every other picture I've seen of the clutch switch there are only 2 wires. BUT, my car has 3?

Old 03-26-2020, 09:39 AM
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