Well, he seems to be happy with the diagnosis.
On an automatic, when the engine stalls, I would expect to see the rpm drop to zero if the engine stalls as the engine is no longer running.
Gilly

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The ‘tested’ replacement MAS unit did not solve the issue. So I decided to dig a little deeper.

First, some important additional background info. About 4 months ago the car started to cut out - once every few days - for no apparent reason. About 3 months ago I found a length of wire in the harness behind the battery that had melted insulation. The wire color was white with red stripe. None of the surrounding wires were damaged.


So I replaced the damaged wire from the MAS unit plug..


.. to near where it goes around the back of the engine - under the vent box. This appeared to stop the intermittent cut-outs.
Note; the above picture shows how the area where I joined the new-to-original wires looks now. But it was fine when I did the repair.

This evening it became abundantly clear the rest of the original wire – which goes across to the drivers side front fender - is bad. It got hot at some point in the past and melted the insulation.





I would like to obtain a good harness if anyone has one available. Car is a 1991 190e 2.6.
Alternatively, I will have to rebuild the old harness.

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Neil.


1991 190E 2.6 170k
1995 E320CA 57k
1999 Chevy Tahoe 98k
2000 R1150GS 45k
1983 300CD 178k - sold, but not forgotten

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...

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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.

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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.

Something bad happened to make that happen to that wire.....I'd try to figure that out too....

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Agreed.

Most of the exposed wire is green from oxidation. So it appears this happened some time ago. Could have been much worse and caused a fire!

The bad wire appears to go under the fuse box. I'm also checking for other damaged wires and shorts before making any repairs.

__________________
Neil.


1991 190E 2.6 170k
1995 E320CA 57k
1999 Chevy Tahoe 98k
2000 R1150GS 45k
1983 300CD 178k - sold, but not forgotten

The bad (white + red stripe) wire broke apart right where it enters the underside of the fuse box.



I cannot move the fuse box enough to see how to unplug the wire bundles from the underside of the fuse box. Any ideas how these plugs lock in place?

__________________
Neil.


1991 190E 2.6 170k
1995 E320CA 57k
1999 Chevy Tahoe 98k
2000 R1150GS 45k
1983 300CD 178k - sold, but not forgotten

Discovered that the plugs on the underside of the fuse box simply pull out with a bit of leverage.

After removing the wiper assy, blower box cover and various other parts, I gained access to the harness and cut away the protective cover to check if any other wires were damaged. Fortunately they are not, just bits of melted plastic here and there from the bad wire.

Have now run a new wire from the fuse box to the MAS & OVP sockets and recovered the harness with Split Flex Tubing (1" diameter).

Also discovered that Pin 15 of the OVP got fried. So will need to order a replacement before seeing if these repairs are good.

__________________
Neil.


1991 190E 2.6 170k
1995 E320CA 57k
1999 Chevy Tahoe 98k
2000 R1150GS 45k
1983 300CD 178k - sold, but not forgotten

Replaced the OVP relay and all is well. Have now driven approx 500 miles with no bad signs from the replacement wire, MAS unit or OVP relay.

In conclusion, the OVP relay did it's job by shutting down the MAS unit when the white + red stripe wire with melted insulation finally came apart under the fuse box. I was fortunate that this burnt wire did not damage any other wires in the harness.

As mentioned previously, the melted wire appeared to be very old damage. Probably caused by a temporary short many years ago - perhaps when the alarm system was fitted.

I now have a spare MAS unit (the original was fine) - although the plastic cover is cracked.

__________________
Neil.


1991 190E 2.6 170k
1995 E320CA 57k
1999 Chevy Tahoe 98k
2000 R1150GS 45k
1983 300CD 178k - sold, but not forgotten

Anyone know if the MAS can be R&Red without damage with the battery still connected?

p.s. MAS sends output signals to:
-fuel pumps (pin 2)
-start valve (pin 8)
-AC comp cut-in (KE) (pin 9)
-AC comp coupling (pin 22)
-TDA engine speed (Tach?) (pin 11)
-Air Pump (pin 19)
-O2 heater (pin 20)

You can check to see if MAS is getting 12 volts at pin 1.

Our '88 M103 did this... it would not start after I shut it off at the gas station. We eventually got it started a few days later, but it ran very poorly. Turns out that the coil had went bad.

Good job finding and fixing the wire.