Hello, I have a 99 ML320 and it has just begun to act up on me. I go to start it and all it does is turn over. this was a once a month thing, then once a week and now a daily occurance, if I wait 10-20 minutes it starts right up as if nothing happened. I did a search on "not starting" and many people suggested the overprotection relay. My question is this when the OVPR acts up does the engine still crank or does it not turn over at all? Also where is this relay located on the w163? I had 94 sl500 that used to do the same thing but it turned out to be the internal alarm, could this be a culprit?

 

Hi,
Could be several things.
Lack of ignition, lack fuel pressure, lack of spark, spark at wrong time, etc.
Difficult to say with any certainty without a little diagnostic work.
Don't throw parts at the car. This is VERY expensive.
Start by determining whether due to lack of fuel or spark.
Fuel rail has a schraeder valve on it to measure fuel pressure. Do you have pressure before, during and after cranking?
Using a spare spark plug determine whether you have a spark during cranking.

As a guess it is either the fuel pump relay or the Crank Position sensor.
Have you been having troubles during hot restart? If not then most likely the fuel pump relay.
Determine whether you have supply voltage at pump before and during cranking (during a no start of course)

Fuel pump is latched on once engine starts by an oil pressure signal to the fuel pump.. During cranking this is achieved thru a crank signal to the fuel pump relay.

Stuart

 

thanks for the advice, I never paid attention to whether it was a hot start problem, but since it only happens when I am not at home this must be the culprit. Plus the $150 investment in replacing the cps is cheaper than a couple of hours of trouble shooting at the dealer, I will change the cps and cross my fingers.


Thanks again
Valerio