M103 suddenly died on FWY at 70mph
 

Car (1991 190e 2.6) was running fine at 70mph on the freeway. With no warning the engine suddenly lost all power and the tach needle went to zero. Managed to pull onto the shoulder. Car would not restart, trailered home.


Where should I start? Check the crank position sensor...?

Lots of threads with similar problems. Here's just 1.;)
http://www.peachparts.com/shopforum/tech-help/152151-engine-shuts-down-while-i-driving-m103.html?highlight=m103+engine+stalling

Search "M103 engine stalling" for more.
Could be a few typical things. OVP, MAS sensor, Distributor cap.

In my case, repeated stalling was caused by fuel pump relay till it finally died. Fixed it and problem solved couple of years ago. No repetition.

One of my guesses would also be the fuel pump relay.

To be clear, the tach needle plunged to zero the instant I lost power, but the engine was still spinning as I slowed from 70mph...

New OVP relay was installed approx 6 months ago and appears to be fine.
Inspected the cap and rotor when I replaced the waterpump last month - both good. Could the FP relay or MAS sensor cause this kind of problem?

Fuel Pump relay or Crank sensor are my votes. You can test the crank sensor when cold.

Should be around 800 ?? Ohms from memory. You can pull the plug at the EZL unit and test here.

You should also be able to jumper the FPR and see if the pump runs.

When you turn the key to the on position, engine off, do you hear the pumps run ?

Appreciate the advice. Will try these things when I get home at the weekend and advise the outcome.

Has happened to my 103 twice now. Just shuts off and dies but I can get it into neutral and do a quick restart. What I found was I was running too lean on air/fuel mixture. However, it could be the fuel pump relay as well. Wiggle it around a little. Do you hear the fuel pumps when you turn the key? Your CPS is right behind the fan on the bottom on the base of the engine. See if its wet or covered in gunk. Cleaned my daughter's off during a water pump change and it starts first crank now every time...don't know why though. Mine doesn't and it's 4 years newer with less mileage. Best of luck.

The 103 has an engine speed sensor on the front of the block that supplies the so-called TDC signal to the tach and EZL. I think that's what sptt may be referring to, although that's not the CPS. If the signal from that speed sensor is interrupted for as little as 1 second, the fuel pump relay shuts off power to the fuel pumps. It's like someone reached over and switched off the ignition key. That's a safety measure to stop fuel flow and prevent a fire, in case the car has been involved in a collision and that's the reason the engine speed has stopped, even if, as in your case, the engine speed hasn't stopped at all, but just a malfunctioning speed sensor. Sure sounds like the symptoms you're describing.

Neil, the coil is a possibility also.

Our daughter was driving her 1991 300CE at 55mph when the engine suddenly died. Towed it to an indie who diagnosed a failed coil.

I don't think a 1991 would have the fuel pump relay. I think it appeared up to 1990 only?

The FPR was replaced by a MAS unit that control multiple functions (including fuel pump relay).

Can you hear the fuel pump buzzing when you turn key to position right before starting?

The front sensor is for the RPM for the cluster only. If it dies, the car will still run. The only thing you will lose is A/C. OVP is not your issue

Sounds to me like fuel pump relay or crank position sensor on the back of the engine that goes to the EZL. The back one is the sensor that is connected with the KE-Jetronic system. You can check resistance between the two pins at the plug. Should be ~1300 ohms IIRC.

I'm assuming since you said it wouldn't start the battery + charging system is OK. Listen if the fuel pumps turn on and if there is pressure. Pull a spark plug and check for spark...

Had exact same issue with my '86 300E. Car would run fine and all of a suddent engine will cut out. Pull over and a few minutes later car will start and run fine again.

Turned out to be the fuel pump relay - behind the battery next to the OVP.

103 Fuel pump relay.
 

One of the most troublesome component in a 103 engine is a OLD and failing Fuel Pump relay . Initially it will be a occasional stall at a stop sign and progressively gets worse ,there is always a quick restart . Eventually it fails completely.
I had re-soldered mine, worked well for a couple of years then failed completely .
It can be misleading as a possible ignition fault.
the OVP does fail too but will not leave you stranded , one of the symptoms will be difficult cold starts and low RPM's at idle.
mak
300se

I agree with Mak I recently went through all of the same head scratching and pulled the relay;went over the entire board with a soldering iron and it "cured the problem for a few months, before finally having a cow. Once replaced all the gremlins went away.

Cannot hear the fuel pump with ignition on. Cannot find a FP relay.
Behind the battery is the OVP relay, MAS unit and what look to be 2 x control modules - each with a large connector on top.

The crank position sensor reads ~1100 ohms, so assume this is OK.