Just another one... "pumping" cruise control

[Matt B]

While I took her for another spin today I discovered another unexpected behavior. When using cruise control I recognized a "pulsating" motion of the car. First I thought it may be because of very low RPM and rather low speed. Later on the Autobahn I realized that even the gas pedal slightly but very noticeably moves in and out about 2 times a second. This motion is very constant and seems to happen at all (engine) speeds.

Can it be a result of my recent speedo adjustment via module by Ecklers? To offset my 335/35 rear tires I accelerated the signal by 2%. Speedo is back to GPS correct reading now.
Or would you rather suspect a vacuum issues? Or something else?

[BigJohn]

Not unusual for manual cars back then.

[Paul Workman]

I don't have the answer. But, far as troubleshooting goes, I'd want to start with the all connectors leading to/from the cruise module (including power and ground connections. And, if they were all satisfactory and cleaned if necessary, and if that had no change, then I'd want to see if reinstalling the stock speedo calibration (chip) would solve the pulsing. And, do you have a the ability to do a scan to see what else is going.

Ultimately, you might want to swap out the cruise module for another module, if possible. And, if that results in fixing the problem, you have our answer and maybe BATEE (?) might be able to fix it (unless you are electronically inclined yourself!)

[secondchance]

I would be tempted to hook up vacuum pump/gauge on cruise control unit (near the brake booster), create vacuum and see if it holds. Never had it happen to me but if there is a slow leak, I wonder, could result in pulsing.

[Matt B]

That's what I'll do next. A ZR friend here in Germany who already helped me out with other stuff indicated that it might well be an effect of the speedo calibration box. Which makes sense in case the cruise control box expects a certain ratio of RPM and speed reading. I may have to play around with calibration, too.
Unfortunately I had no chance to try the cruise control with the original tires.

Sent from my J9110 using ZR-1 Net Registry mobile app

[WARP TEN]

The first time I had the rear gears changed to 4.10s (on the '93), I used an electronic speed calibration device (can't remember what brand). I had the same cruise control variance issue. Dumped the electronic unit and had Marc install the proper speedo gears and never had the problem after that. Ditto on the '95, which got the speedo gears at the same time as the 4.10s. --Bob