champs560

i have a 1988 560sl that will crank and run maybe 1-2 seconds and cutoff. It's getting fuel to the injectors fine but when i checked the cold start valve i'm not finding any fire. i've checked it with just the switch on and while the engine is turning over. i had to jump start the car and thought that i had damaged the ovp relay. i checked the fuse on top and it is fine. will the cold start valve cause as issue like this or do i need to look elsewhere? i read thet the cold start valve get it's power through the ovp.

slik560

The fact that it starts and runs, if only for a few seconds, points to the crank position sensor. Something is telling the motor to shut down. Fuel pump relay might be another cause.

champs560

thanks slik...where is the sensor located and can it be checked to see if it's good or do you have to replace it to find out?
thanks again

slik560

Driver side - bell housing. Held in with a single screw. There will be a cable that runs from the sensor to the control module. HARD to reach.

From what I understand, they can't be checked - they are either good or bad, and bad can mean intermittent - which is the aggravating part. Even a good mechanic can't diagnose it unless the car stalls while he has it in the shop.

The sensor unit is basically comprised two metals and if a sensor is bad, these "layers" can separate under high heat and lose contact. The car stalls. After a short cool-down, the sensors layers make contact again and it starts. That is a real hacked explanation of what my mechanic told me.

My mechanic gave me a way to check to see if it's the crank sensor: my car will die when running - mostly at stop signs, but occasionally when driving. He gave me a gallon jug of water...no joke...and told me to pour some water down over the driver side bell housing area - right behind the driver side cylinder head) the next time the thing quits. If it re-starts right away (I usually have to wait 5 to 10 minutes or so for it to restart) then the problem is likely the crank sensor.

So, I've been driving around with this jug of water, waiting for the car to suddenly die. Naturally, it has not happened since.

I've been trying to find a good price on a rear crank sensor (I think there is a front one that is not so problematic) but so far the cheapest is $200. I don't like going the Ebay route for a used sensor because I just don't trust used items.

One of the best things I've bought so far for my 560SL is one of the tech manual CD's. Not a diagnostic tool, but at least you can idetify and locate all the parts and pieces.

Good luck.

slik560

It just dawned on me that your starting issue is with a cold engine. Mine happens only when it's hot. It could indeed be a totally bad crank sensor, but I'd also look at the fuel pump relay. Does it start and then quit immediately or does it stall like it's running out of fuel? The relay located behind the glove box (it has to be removed) along with several other relays and there are few threads out there on other forums that can help guide you to it. Search Benzworld.org.

PS: All of my ramblings aside, it could still be the cold start valve. I'd have someone check it out before investing in an new crank sensor or fuel pump relay. Sorry - I interjected my own ideas and lost sight of your initial question!

champs560

i've had dealings with the fuel pump relay and it wouldn't crank at all...just spin. now it will crank and run for second or two and acts like you turn off the switch. The lack of fire at the cold start solenoid is what concerns me but i don't know if the solenoid would cause this........it was not driven for several weeks prior to this issue but ran fine before that.....thanks again