Hi everyone, I have a 92 R129 500sl that I have serviced extensively, rotors and caps, plugs and filters, it has been perfect all summer and autumn, I haven't been using it much over the last couple of months but have been starting it every two weeks or so and it has always started flawlessly , it has been under a cover outside, just a few days ago I went to start it and nothing! It cranks over perfectly but will not fire or even try to fire, it has been very damp and cold lately but normally this doesn't matter. Any ideas? Could the OVP fuse or the OVP itself cause these symptoms. Thanks forum
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First thing is to narrow down whether this is a fuel or spark issue.
So when you turn key to pos 2, can you hear the fuel pump buzz at back of car? If not look at fuel relay or ovp relay. Aftermarket alarm or immobilisers too If you have.
If it is buzzing then issue will most likely be spark, but this is very very rare.
So when you turn key to pos 2, can you hear the fuel pump buzz at back of car? If not look at fuel relay or ovp relay. Aftermarket alarm or immobilisers too If you have.
If it is buzzing then issue will most likely be spark, but this is very very rare.
The fuel pump appears to be working correctly, I haven't checked for a spark yet as the weather is still crappy, it's my first port of call, I don't think this car has an immobiliser fitted, the door lock is always iffy but this has never affected starting. I'll make some checks and get back on board. Huge thanks. Simon
Okay, having done some investigation the car has a great spark so it appears to be fuel, I slackened the fuel feed on the fuel distributor and hey presto no fuel...there is also a buzzing coming from the drivers side near the fuse box under the bonnet. Any ideas forum?
grober
MB Master
Don't know about the buzzing but you might have a problem with the fuel pump relay which supplies power to the pump. You could try shorting the main fuel pump connections to power the pump to see if the car will start [ caution this means the fuel pump will run continuously and wont switch off via the ignition switch- must be manually disconnected by link removal ] if this does the trick i you need a new FPR.
Great thanks, so would i do this on the 'MAS' relay? it appears to be the main relay controlling the fuel pump, the pump does run when the ignition turns on so i'm thinking this could be the issue, do you know if this car has a factory immobilizer fitted? i'm pretty certain its not this anyway…huge thanks so far
RichardF
Active Member
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The factory immobiliser stops the car from cranking (interferes with the starter circuit) so it's not this.
Could it be the battery? Sometimes a low battery will power the starter but unless it's 12v plus it won't power up the ecu's
Could it be the battery? Sometimes a low battery will power the starter but unless it's 12v plus it won't power up the ecu's
grober
MB Master
If you have the multifunctional MAS relay it may be more difficult to trouble shoot the problem. Substitution is a solution but these relays are quite expensive. Try doing a search on MAS relay on the forum.
grober
MB Master
The battery is brand new, I sprayed a little fuel in the airflow intake and the car started for a moment so it's definitely fuel starvation. So to be clear when I turn the ignition on the pump runs for a moment to pressurise the circuit, then on cranking nothing, this secondary current to the pumps must be controlled by the MAS unit? According to the pin diagram on the mas unit if I short 1 to 2 it should run the pump continuously? Is this safe to do??
Hi everyone, just a quick update, i searched and searched for a replacement MAS Relay and couldn't find one, so i bit the bullet and carefully took the old one apart! the connections on the circuit board looked a little cracked and dry so i re-soldered all of these and also cleaned the contacts on the little relay, reassembled and re-resined the box back together. re-fit it and BINGO!! Runs as sweet as a new nut/pin!! Thanks for all your pointers, if anyone else has issues like this i'd be glad to lend a hand.
Good result - well done!
grober
MB Master
Result! Many of these relays that are getting long in the tooth FP[fuel pump],OVP, MAS etc suffer from dry joints- cracks/bad connections on their small internal PCB's due to vibration - a quick run over with a soldering iron and problem sorted! Thanks for taking the time to report back.
