Cruise Control Servo Unit

[Porsche80]

Does anyone know how to test if the Cruise Control Servo unit is bad ? I have a 1980 911SC and no info in the Bentley Manual that I can see

[island911]

swap it with a known good one.

those things have little valves inside. little rubber-stopper valves. The rubber seems to take a set, after the years, causing the adjustment to be off. (they are adjustable, if you cut into the servo unit. Watch out though; the adjustment screws are gooped in place. )

[Porsche80]

I am trying to determine if it is my Servo that is bad as everything else seems to checkout. I had Beckman check my ECU (Brain) and it was good. Clutch switch checks out. All diags from Bentley Manual checks out. Is there anyway that you know to see if the cruise control works without the servo unit?


Thanks

C

[island911]

all you can check is the voltage signal.

If all that other is good, it does sound like the servo has a problem.

fwiw, the servo guts are rather simple: one valve opens, lets the vacuum line pull the diaphram . . the other opens to bleed-off the vacuum.

I suppose that you may want to check that the vacuum line is pulling a vacuum. And you can test the diaphram by disconecting the vac-line, push the diaphram center in, put your finger over the vac-line nub, and see if the diaphram holds its position. (if there's a tear in the diaphram, it will just spring back out)

[Porsche80]

Ok, the Servo has 2 leads going to it. How do I test to see if it is getting voltage. Do I turn on the ignition and engage the cruise control on the column and have another person check for dc voltage between the 2 leads on the Servo. As far as checking to see if the diaphram how to do with the Servo in car or is that possible.


Thanks

[island911]

You can probe terminals 3 & 7 . . . I ran the test wires thru the dash (popped out the clock) and to a volt-meter. (terminals 3 & 7 are the only two wires that go back to the servo unit. . .grey and white wires, btw.)

What I found (while driving) is the control unit changes voltage quickly, when the speed is not the "set speed" (it drops to ~2v when trying to loose speed, climbs to ~3.5 when trying to accel . .. if I bump the accel on the cruise lever/switch it hits 6V. )

[Porsche80]

ok, I will look at trying that. BTW Do you have any pics of how the vacuum hoses are hooked up. I may have the large hose hooked up incorrectly. My wrench was setting the CO last year and I was checking around and saw the large vacuum hose laying next to what I beleive is the Aux air valve and hooked it up. Also noticed that the 2 smaller hoses-one goes to the throttle body and the other goes to some type of solonoid. This solonoid appears to have to opening the vacuum hos is in one and notheing in the oter any thoughts.

BTW Thanks for all the help


C

[efhughes3]

Bentley has a detailed test procedure that will supposedly ID the culprit in the system.

[greglepore]

The test procedure was actually posted on another thread, but I have to confess that after trying for weeks, I gave up. New brain, nothing; all switches check out, and applying 9v to the servo got it to move and increase engine speed. Now that its gone, I don't really miss it.