code 78?

[wctouring7]

I have a 98 318ti that I am getting a CEL on, the Peake tool shows code 78 "vehicle speed signal not present" If I clear the code, the CEL will go out for a little while (usually about 50 miles) then come back on. I am guessing the speed sensor in the diff is good seeing that my speedo and on board computer work fine. I checked for continuity from the DME pin 83 to the obc pin 4 as well as continuity from the obc pin 9 to the inst. cluster (x17 pin 2)....continuity is good. What is the next step? Any sugestions?

[Wayne at Pelican Parts]

It may be an intermittant problem - the speedometer needle doesn't dip at all? I did have a message like this on one of my cars, but the speedo stopped working and the plug to the sensor in the diff was loose...

-Wayne

[BMWCowboy]

Pull the sensor out and clean it

[wctouring7]

Wayne, the speedo needle does not seem to dip at all, but I am probably not staring at it when the cel comes on.

Cowboy, sounds like a plan...I will give that a try and if that does not work maybe I will just replace the sensor since they are pretty cheap.

[Wayne at Pelican Parts]

Originally posted by wctouring7
Cowboy, sounds like a plan...I will give that a try and if that does not work maybe I will just replace the sensor since they are pretty cheap.

That's probably what I would do first...

-Wayne

[wctouring7]

Finally replaced the speed sensor....and I still have the same problem. I am noticing a problem that seems to be related. Every once in a while, when accelerating hard through the gears I get missfire above 5k rpm, it acts as if I am hitting the rev limiter and the engine stops accelerating. This only happens very intermittantly and it seems the CEL comes on soon afterwards with the code 78. Just to cover the basics, I am putting in new plugs anf a fuel filter tomorrow. I am wondering if I have a bad DME?

[Wayne at Pelican Parts]

Chances are the DME is fine, if it's giving out codes, and the car is running fine most of the time. Many people spend useless dollars replacing DMEs that are not the problem...

Try resetting the code, and then unplugging the sensor completely, and see if the computer gives the code again, right away?

-Wayne

[wctouring7]

Ok, I unplugged the speed sensor on the diff and reset the trouble code. Other than the speedo not working the car ran fine and the cel did not come on right away ( I drove it for 5 miles). So I am thinking that the DME is never getting the speed input signal...its just that the dme does not check/read the speed signal constantly? So that is why I can go anywhere from 20-50 miles before it sets the code?

Looking at the wiring diagrams, I see the signal goes from the speed sensor to the inst. cluster then to the on board computer and from there, to the DME. Maybe my obc is bad? It seems to work fine although the fuel mileage feature is not accurate.

Thanks again for you help.