Cruise Control fix, the supposedly easy bit

[slotmeister]

Hi all. Have read plenty of posts on cruise control fixes after my '89's died. Had all the peripherals tested okay, replaced the capacitors in the brain-box (even though they were okay), but finally had some grown-ups determine that the microprocessor was dead.
I bought a used (and obviously the correct) VDO Tempostat brain-box via internet and plugged it in, but cruise control still inop. Seller assures me that the unit was tested, and this sainted fellow even sent me a second one, no questions asked. Have tried with this second unit, but also still inop.
My question is : are these units meant to be plug-and-play? Or am I supposed to recalibrate or reboot something?
If it's supposed to plug-and-play and it hasn't, I can start looking again at steering column stalk, etc.

[Chuck.H]

You mentioned you tested periphials but the only thing thats ever failed on mine has been the clutch switch next to the accelerator pedal... so I'd check that again :-)

And I use my cruise ALL the time! ( to avoid tickets).

Chuck.H
'89 TurboLookTarga, 364k miles

[slotmeister]

Thanks, Chuck. Yeah, cruise is a boon with the longer trips I seek out in my car, too. So the new unit should just fire straight up? Hmm, guess I'll get back to that clutch switch again.

[Chuck.H]

I can always tell when mine needs adjustment by putting my foot under the clutch pedal and pulling back slightly while trying to engage the CC - if this makes it work then I know it's the switch. Of course also check your brake switch is not stuck on.

Chuck.H
'89 TurboLookTarga, 364k miles

[slotmeister]

Aargh! Looks like adjustment would be the least of my worries. Have just pulled out carpet, footboard from passenger side - I'm in right-hand drive down here, remember - and found that the clutch switch and its L-bracket mount are floating around free. That is, the bolts meant to attach the bracket to the floor are missing.
In RHD, the mounting point for the bracket/switch (and the corresponding tab on the main pedal shaft) is pretty much smack-dab in the centreline of the car, i.e. pretty inacessible thanks to the centre tunnel.
Before I start pulling out the centre console, a/c knobs, hazard switch, lifting up carpet etc, can you tell me what lurks forward of it - that is, can this hard tunnel area forward of the console be accessed at all? It feels like it's steel.
Only other way to reach the bracket mount area will be by pulling out the a/c evaporator (or whatever it is) located behind the footboard, which would hopefully allow me to reach around into the tunnel area.
Hmm, thinking I'll probably install a new switch, while I'm at it ...
Hmm-mm, thinking I might get someone else to do it ...

[slotmeister]

Took a break from watching qualifying for tomorrow's Bathurst 1000 touring car race (huge deal down here) and got a bit archaeological with the clutch switch. Discovered that, in fact, the bracket is properly bolted to the floor, but the switch itself is cracked at its mounts, hence just sorta flopping around against the bracket.
Pulled the driver's (RH) side footboard out, but there's even less access from there.
Meanwhile, another question, besides advice on access: when I do get in there with a new switch, what's the correct adjustment for the switch against the tab?
I presume it's supposed to be set so that the switch lever is engaged, or lifted, just at the very end (rest position) of the clutch pedal's travel?
I do ask a lot of questions. Journalist. Get paid per word, too.

[Chuck.H]

Yes I seem to recall the switch needing to be closed when the pedal is at rest.

BTW, on mine, the CC is smart enough to let me shift gears while engaged and it will a) not over-rev the engine, and b) pick up the same speed in the next gear. I occasionally use this when I'm warming up my car and want to stay in 4th gear for a little while before shifting to 5th.

Chuck.H
'89 TurboLookTarga, 364k miles

[porschenut]

When the clutch pedal is at rest, the CC switch is in the open position (i.e., pressed in). That open position breaks the ground (or earth, if you prefer) connection and signals the brain to disengage the CC.

Access to the clutch switch is simple, directly from above it. There is a large hole in the center tunnel metal to allow it. You have to pull back the carpet in front of the console to access it.

When you install the new clutch switch, Loosen the two mounting nuts for the bracket, and simply move the assembly forward until the lever on the clutch switch is fully depressed, but not under too much pressure. A broken switch could mean that it was poorly adjusted previously, and the pressure against the arm when the clutch pedal was fully out stressed the switch housing too much, causing it to break.

[slotmeister]

Dave, that's great info on both access and switch adjustment, many thanks.
I'll have more of an explore next weekend. Think I'll have to pull the centre console out. Ahead of my centre console isn't a horizontal tunnel, but about a 1" gap then a sort-of vertical tunnel - feels like steel, with vinyl glued onto it. May be a shroud to cover ventilation hoses etc. If it unscrews, I'm in business.
Oh - and just to clarify re purpose of the clutch switch: Understood your comment on adjustment, but I'd have thought the switch being open, i.e. with pedal at the top of its travel, would have the CC engaged. It would want to disengage when the switch closes (clutch pedal depressed). Did I get that right?

[porschenut]

Originally Posted by slotmeister

Oh - and just to clarify re purpose of the clutch switch: Understood your comment on adjustment, but I'd have thought the switch being open, i.e. with pedal at the top of its travel, would have the CC engaged. It would want to disengage when the switch closes (clutch pedal depressed). Did I get that right?

Yes, you got it right. I said it incorrectly

When you depress the clutch pedal, it closes the switch (little armature extends), which completes a ground connection and signals the CC to disengage.

[porschenut]

I'm not sure if you have the Bentley service manual for the '84-'89 Carrera, but here is the page with the procedures for diagnosing a faulty CC. All of these tests are made using the connector pins on the CC brain harness connector.

The second test, for the clutch pedal switch, indicates the switch should be closed when the clutch pedal is depressed. A broken switch defaults to the normal position, which is open (infinite resistance).

[slotmeister]

Dave, you've gone above and beyond - many thanks! Yep, I have the Bentley book, was one of my first purchases after finally achieving my 911 dream 18 months ago.
I won't bother with the switch test because I know what the result will be. When I had the passenger footboard out, I could see the whole switch unit being pushed back and forth by the tab, as I pushed the the clutch pedal in with my hand. Switch may actually still work, but mounts are broken, so there's nothing to hold the body in place. Will get that new switch in there, adjust it properly and be good to go. Again, appreciate your efforts cheers, Michael