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1987 740t cruise control 700 1987

posted by cx on Sun Oct 30 10:38 CST 2005
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last visit: Wed Oct 8 07:56 CST 2014

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I just finished the process of wiring harness replacement and vacuum line replacement in my 740t. Cruise control hasn't worked since i've had the car so I thought now would be a good time to get it functioning. I replaced the vacuum lines through the firewall to the "t" that provides vacuum to the switches/valves on the brake pedal and the clutch pedal. Now when I step on either or both of the pedals I get a hiss/vacuum leak from the pedal valves(either pedal alone ...low hiss, both pedals at same time twice the hiss). Is this telling me both vaves/switches are junk or are they supposed to give an audible hiss? With both pedals in the "up" position there is no hiss at all. My cruise control vacuum diapham under the hood is junk so i plugged the vacuum line going to it until I can get a new one. Any suggestions are appreciated.

Dan

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1987 740t cruise control 700 1987

posted by Bram Smits on Sun Oct 30 12:47 CST 2005
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Probably fine. They usually don't give a very audible hiss, but it is the point of them: in the up position they're airtight (keeping the system under vacuum if the pump's active). If you depress the pedal they'll give the computer the signal to stop the pump (you do have the electrics wired up as well, don't you?) and release vacuum so the bellows lets go.

Bram

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1987 740t cruise control 700 1987

posted by jpezie on Mon Oct 31 00:16 CST 2005
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last visit: Sat Oct 24 00:20 CST 2009

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Bram,
Do you have diagrams of all the vacuumlines available.
This would help in a lot of discussions.

Kind regards,
John

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1987 740t cruise control 700 1987

posted by Bram Smits on Mon Oct 31 00:52 CST 2005
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I don't have an "official" diagram handy, but here's the quick'n'dirty: drawn on paper, digital camera, reduce size and upload...

If you've got an automatic then obviously you don't have the clutch switch and the T-piece in the interior side.

Bram

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1987 740t cruise control 700 1987

posted by someone claiming to be jpezie on Mon Oct 31 03:57 CST 2005 [RELATED]

Very clear in it's simplicity.
Thanks a lot Bram.
Kind regards, John

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1987 740t cruise control 700 1987

posted by Bram Smits on Mon Oct 31 04:35 CST 2005
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It is. The bellows controls the throttle, the vacuum pump controls the bellows, and the computer reads the speedo signal to control the vacuum pump. The pedal switches immediately drop vacuum to release the bellows (vacuum part) and tell the computer to turn off the vacuum pump.

It's pretty much fail-safe: if there's a vacuum leak or a signal missing, cruise doesn't work. Normal failure mode for those switches is a vacuum leak or failure to make contact when the pedal is up (ie. the computer is always told the pedal is depressed).

It's a beautiful system, and 95% of problems with it can be resolved by finding and fixing the vacuum leak.

Bram

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