topographical sensor
 

Anyone know how to disable the topographical sensor in my 1990 W126? I know it was able to be switched off in the Euro models. I find it makes the Cruise Control surge on the highway. I would like to turn it off and see if it helps.

I don't think there is anything like that on a MB.

The cruise control is designed for each model car.
Starting in 89 MB used the same CC computer & customized them with a resistor pack.

Wasn't there an altitude sensor used in the 1980s, at least on the M103s? But that was for fuel mixture purposes. I don't know if it had anything to do with the CC circuit. What do you think, Doc?

I'm not doc, but if it's screwing with the mixture settings "on the fly", that might cause the surging problem he's talking about if it's faulty.

I know cars today have baro sensors (sort of).

Many CIS fuel systems used a barometric sensor...has nothing to do with CC system.

If the engine surges with cruise OFF & ON look elsewere.

Another place to look when it surges while cruise is on is the speedo cable. If the cable causes the speedo to bounce the cruise will follow this behaviour. Our 300E used to do this, mostly when cold outside, which is a big clue to look at cable.

And on the diesels, too.

It would be called a barometric sensor rather than a topographic sensor, of course.

L-Jetronic measures density of air via the flap or hot wire, so performs this function without separate sensors.

Of course W126 cars use electronic speedos & process the speed signal that way.

well I read that the 126's had this topographical sensor that works like an inclinometer that tells the trans to hold back when sitting on a hill or when descending a hill. I figure if it works on the trans it must effect the cruise. Here is a quote from Wikipedia on the 126

Drivetrain technologies
* Four-speed automatic transmission with a topographical sensor that monitored the vehicle's position (whether on an incline or decline) and gas pedal position. The result was an extraordinary capacity to "hold back" acceleration when coasting downhill and to maintain the S-Class' position without using the brake pedal after coming to a complete stop on a steep incline. The transmission also featured a "limp home mode" in case of electronic failure and was programmed to start in second gear so as to permit smooth, wheel spin free launches. Second gear start could be switched on and off in European models with a S/W switch to allow more performance orientated driving versus winter driving.
Anyway that was my question. My speedo does "tick" at low speeds though. The cruise is constantly speeding up and slowing down ever so slightly. Might be normal for all I know just doesn't seem to be the way Mercedes would design something. I had a truck once that had "Soft Cruise" to avoid that problem, it would not correct the speed in slight variations.