Sudden Cut-off/92' 300E?

[300EJax]

Posting this for a co-worker who recently purchased a 1992 300E and is experiencing occasional problems with her car just completely shutting off while she drives down the street. She took it to a MB shop here in Jax who admitted they have had similar problems with 300E's in the past and replace a myriad of parts before they can stop the problem.

She states that the car will just shut down on occasion and completely lose all power and does so at a variety of speeds and driving conditions. The MB tech suggested throttle system as a general culprit.

Any suggestions or direction for her?

Thanks in advance- Mark in Jax

[MattBelliveau]

Fuel pump, ignition switch, relay...or the dreaded, and notorious...... mid-90's w124 wiring harness.

[Mike Murrell]

As stated in previous post - a number of items may cause this.

I'd bet on it being.

1. Fuel pump relay
2. Fuel pump(s) - that car may have 2.

The fuel pump relay(FPR) on the 92 300E is incorporated into a MAS unit. Photo at next URL:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1U40N9POF1VD0ZS7DG&year=1992&make=MB&model=300-E-001&category=P&part=MAS+Control+Unit

My car has same motor and many of the same elect. as her's including the MAS. Have had to replace MAS and fuel pump.

When FPR circuit in MAS bit the dust, car would not start.

In my case fuel pump would cut out after driving in warm/hot weather for approx 15 min/miles. Like clock work, car would die after that distance/period of time. Required substantial cool off period before it would restart. No problem during cool weather.

The problem she's experiencing sounds a great deal like a fuel pump relay, but this MAS unit is about $500 @ a dealership. You can buy remans for about $350. No way I'd spend that kind of $$$ on a used elect. item.

If the car dies and jumpering pins 1 & 2 inside the MAS restarts the car, the FPR circuit in the MAS is bad.

Here's a write-up I have saved on my PC for checking FPR circuit in a MAS unit::


MAS unit near firewall. You remove it by loosening the black knob on top counter clockwise. There may be arrow-like depiction on top of the knob indicating the direction to be turned. Once out, you locate pins 1 & 2. In my case, they are the 2 outer pins. One is across from the other.

OK - here are the steps I took:

1. Remove MAS unit
2. Detect battery ground cable(negative cable) - more on this later.
3. Make a jumper wire. Bend into U-shape
4. Insert one end of wire into one pin; the other wire into the other pin. The "pins" are actaully going to be small, shallow recesses where the MAS prongs would go into when connected.
5. Reconnect the battery grd. cable.
6. Hopefully you'll now hear the pump(s) running at the rear of the car.
7. Try to start car with key. If car starts with pins 1 & 2 jumpered, your MAS FPR circuit is likely bad. In my case the vehicle would start with pins 1 & 2 jumpered, but would not if I reconnected the MAS.

Reason for item 2 above - The FPR circuit is wired hot sorta like your rear brake lights. If when inserting the jumper wire, you fail to hit the 2 holes just right, you may inadvertently ground one of the wires and if you do, you will get a loud pop/spark. Don't ask how I know this. It would appear that I survived this mishap as all circuits involved continued to operate except of course, the faulty FPR.

According to the BuyParts function at this site, her car has a 12 valve M103 motor. That engine did not have the W124 wire harness problem that the M104 motor experienced. The M104 motor has 24 valves and a more sophisticated elect. setup.

[softconsult]

And remember this point. If the MAS is replaced hang on to the old one.
It has a value to rebuilders.

I only learned that after the fact.

Steve
'92 300E 245K with replaced MAS and many other goodies as they reached the end of their useful lives.

[Mike Murrell]

Originally Posted by softconsult

And remember this point. If the MAS is replaced hang on to the old one.
It has a value to rebuilders.

I only learned that after the fact.

Steve
'92 300E 245K with replaced MAS and many other goodies as they reached the end of their useful lives.

Steve - I'd be interested in knowing of a legitimate rebuilder. Do you? I still have the faulty unit and would like to have it rebuilt to carry as a spare. Such a repair would hopefully be less than the $357 this site gets for a reman.