Hi All,

My 1992 230E (automatic) randomly cuts out. One minute driving normally, the next it just suddenly stops. No spluttering or anything. On occasion it starts running again before I get to stop.

Other points which may help:
1 - EZL (I think that's what it's called) was replaced last year
2 - It seems to be a fuel supply related issue as when I put the ignition on afterwards, normally I cannot hear the fuel pump prime. After a couple tries it sometimes pumps again and then it starts
3 - Other times I tap the fuel pump relay a few times and then the fuel pump runs and it starts, but this may be coincidence
4 - It had an immobiliser installed which has been bypassed. However, there's an earth wire linked to the immobiliser box which if disconnected the car won't start. So it still seems to be partly wired in and may have an effect? Maybe they just shorted the switch which deactivates it? I can hear a clicking from the immobiliser box under the steering wheel from time to time.
5 - Auxillary fan has been acting erratically. It sometimes switches on as soon as I put the ignition on or it comes on when the car is clearly not hot yet. It sometimes switches on and off in close succession like someone was flicking a switch on and off. I was thinking this is a loose connection and that if there's no signal it runs by default as a precaution?

How do I go about diagnosing this? It's not safe having my car cut out at 120km/h (75mph) in the fast lane on a freeway...

Thanks,
Jason

Well as I guess you are aware it is most likely an electrical problem. Do the easy stuff first - cleanliness of battery terminals, clean up all earth points and protect with petroleum jelly. If the car at some time or other used to work with out these problems I wouldn't be ripping stuff out just yet - especially with a bypassed alarm (sounds like a nightmare)...

Look also for electrical connections that are close to heat sources - I once owned an old V6 Ford that had an electrical plug that rested on the top of the engine... the engine got warm - the contacts broke - you'd just hear the air whistling by and then a loud bang as the engine mis-fired and started again. Great.

That's quite a collection of symptoms, but I don't think they're connected, unless the immobilizer modifications have totally screwed up the electrical circuits. The sudden engine cutoff at highway speeds suggests an interruption of the fuel pump electrical supply. That would lead me to OVP, fuel pump relay (or MAS, if so equipped), and engine speed sensor (TDC signal). ABS light coming on during these episodes?

__________________
1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

Tell me about it! I thought I'd list all symptoms in case one of them is the root cause...ABS light does not come on. No lights are on when it cuts out but when I switch off and on then the normal pre-start lights come on.

How do I test the operation of relays? Do I take them out and apply current across the relevant connectors for the coil portion?

Jason, there are tons of threads on both OVP and fuel pump relay on the forum, you just need to spend a little time with the search feature.

I included the engine speed sensor in the list of possible suspects, because a loss of TDC signal of just 1 second shuts off the fuel pump relay instantaneously. It's like someone reached over and turned off the ignition key. That's a fire prevention safety feature in case the vehicle was involved in a collision as the reason why the engine speed suddenly went to zero. Of course, with a malfunctioning sensor, the car doesn't know that you weren't involved in a collision, so the fuel power supply is immediately cutoff via the pump relay.

__________________
1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

Ok, thanks. I'll have a look around the forum again.