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Discussion Starter · #1 ·
I'm trying to troubleshoot my cruise control on an 88 S/C. I found the cruise control computer but I'm still hunting for the speed sensor thingy. Will I find that under the dashboard's instrument panel? Any help locating this thing is appreciated.
 

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Something to realize immediately is that cruise will kick off with the parking brake (or low brake fluid), the clutch (adjust the button to kick it off), brake pedal (again, adjust for it), or a faulty voltage which kicks on the 3 idiot lights. If you have any of these out of whack, it won't set.

The speed sensor wire is purple with a white stripe and travels from the dash to pass kick panel, around the back of the cab, and back to the ECU.
 

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Discussion Starter · #4 ·
Thanks for the tips. I've looked through many of the threads for clues.
The cruise control power light stays on but the set button won't set a
cruise controlled speed and consequently the set light doesn't light on the stalk either. It failed in an intermittent fashion such that I could reset the
cruise at freeway speed by turning the motor off and doing a cold start at
freeway speed. Today, the set light never lights.

The brake fluid and clutch fluid levels are topped off.

The output diagnostic codes read off the the cruise/stalk (reference BGB, page BE-41) show:

No. 1 Set coast switch on - okay with prescribed indication code.
No. 2 Resume/accel switch on - okay with prescribed indication code.
No. 3 Vacuum switch on - okay with prescribed indication code.
No. 4 Each cancel switch on.... i still need to check this one out (requires
jacking the car up and engine idling) Note it passed for each
sub condition while I was driving around.
No. 5 Drive 25 mph or over - fail (nada, no light)
No. 6 Drive 19 mph or below - fail (nada, no light)

Failure of 5 and 6 above point to an abnormal speed sensor circuit but the
speedometer works just fine.

I was going to try and check the speed sensor
(pages BE-50 and BE-60) but needed some help finding it! Can the speed sensor wire harness be accessed easily from under the driver's side dash area?

Do you think I should still pursue No. 4 above which is the same as your
above recommendation: check cancel switches? I do not get the three
idiot light case nor do any warning lights stay turned on except for the intial start up.
 

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There are two switches on the clutch pedal. The one engaged when you push the clutch to the floor is the neutral start switch. It has no function in for cruise other than the other switch may be spiced to it's ground IIRC. It simply is a safety for starting the car (so you aren't accidently in gear). The cancel is unique to the cruise. It's depressed at the top of the clutch throw when the clutch is normally engaged. Check that it's adjusted properly. If it's not, it'll act as if the clutch is being pressed and deactivate the cruise (or not let it set). It could be that it has gradually been loosening and thus the intermittent circuit.

yes there is access at junction block 4 for the speed sensor wire--JB #4 is the light blue thing up by the clutch pedal that ties into the ribbon that travels to the passenger side. I wouldn't call it particularly easy access though. There is also access behind the passenger kick panel I believe. it is the violet-white striped wire.

I wired cruise into 2 NA vehicles before and I traced a lot of this wiring, shortcutting their circuitous route where possible. I'm relying on memories from a few years ago.
 

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Discussion Starter · #6 ·
White87,

I think you just saved me a bunch of time! Your description of the wiring
location is so valuable. I have had this car (correction 89 s/c) since 1991.
Changed out a leaky clutch release cylinder about a decade ago and I remember adjusting the clutch pedal height. What do you want to bet I adjusted the clutch switch right out of reach! Thanks for the detail on
locating the wiring harness. The car only has just under 100K miles on it
and I just don't want to believe a significant $$$ part has failed.
 

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No problem. I seem to be one of few people who have installed cruise in our Mk1 when it came without it. I went 9 years without cruise and only after browsing online did I realize it was even an option. I wanted it bad enough to source parts and wire it for highway travels.

Post back what your results are.
 

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Discussion Starter · #8 ·
Cruise Control Problem - findings

As the threads above suggested, the problem was with the clutch switch used for cruise control (thanks!). I had adjusted the switch some time ago after replacing the clutch release cylinder so that I would be happy with clutch pedal freeplay and also this kept the gears engaging fully.

The cruise control works perfect with the switch mount threaded in a few turns. However the tradeoff is that the gears on the 5-speed begin to slip when you hit the gas.

So, I spent this afternoon backing out the clutch switch turn, by turn, so that the cruise would still function and the clutch would not slip. I never got there ...
the cruise began to quit functioning and the clutch still would slip.

This setup is wrong and I'm pretty sure the push rod on the clutch master cylinder needs to be shortened. If I shorten this push rod
the gears should engage fully without slipping. Then I can re-adjust the
clutch switch (start with it wound out) and hope I can get both cruise control and drive gears that are fully locked up.

Does this sound about right? It looks difficult to shorten the push rod on the clutch master cylinder so I hope that's the right approach.
 

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So it sounds like you may have resolved everything now, and the clutch cancel was indeed the cause. yeah!

As for the clutch engagement, I know it can be adjusted. I wouldn't think that this switch would have that much affect, but anything's possible I suppose. It's been years since I've needed to do adjustments, so I can't comment on the slippage. Regardless, it sounds like you may have things under control now. Congrats.
 
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