e-300. 1997. CRUISE CONTROL SURGING. ?

[HERBIEMERCMAN]

hi everyone for some years my cruise control has controled the cars speed very smoothly,for 180k miles, for the past 10k miles it has been surging, it still maintains the speed but does it in fits and starts, rather like power on, then power off, it is just like applying the power then braking slightly all the time to maintain a constant speed. anyone any idea how to rectify this problem. ? is the cruise control related to the autobox controls. ? many thanks.

 

[Number_Cruncher]

Check that the throttle body is free and isn't gummed up, also check lambda sensor isn't becoming sluggish. If the lambda sensor is switching too slowly, the engine will alternately be too rich and too weak, and will surge - when the sensor is working normally this rich / weak oscillation happens too fast to be felt, and it all averages out nicely.

A good way to rule out the lambda sensor is to try the test from cold - the lambda sesnor does nothing until it warms up.

 

[television]

There are small connections between the box and cruise, but not much, they both need a speed signal, if the box is corrupting the signal that could cause the fault, does the speedo or rev counter fluctuate at the same time,, OK I know that very few diesels have a rev counter

 

[Number_Cruncher]

>>OK I know that very few diesels have a rev counter

Ah, sorry, I didn't realise that this was a diesel. Everything I wrote about lambda sensors is therefore not valid.

 

[television]

>>OK I know that very few diesels have a rev counter

Ah, sorry, I didn't realise that this was a diesel. Everything I wrote about lambda sensors is therefore not valid.

Thats OK I do it all of the time

 

[HERBIEMERCMAN]

hi malcolm, thanks for info. what is "the box" and where is it located ? the rev counter and speedometer stay steady, it's just the car that surges. herbiemercman.

 

[GEORGEROV]

Hi Herbie, My 500 started doing the same thing then gave up the ghost completely a few thousand miles later. Have a look at the cruise stalk itself. Contacts on the switch were worn out and needed replacing .

 

[television]

hi malcolm, thanks for info. what is "the box" and where is it located ? the rev counter and speedometer stay steady, it's just the car that surges. herbiemercman.

The gearbox,, I will look up the known faults soon

 

[simon_wall69]

I have had an occasional misfire and cruise control issue on my w124, which is same engine but without turbo. The man at the garage blamed the misfire on build up of carbon from the previous owner pootling around in it. I have put two injector treatments through the car and driven with a very leaden foot until all of the vast plumes of black smoke stopped appearing.

Touch wood, seems to have worked so far - and the cruise control is back to being smooth again (although, could be coincidence - I don't use it very often.)