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Old 04-07-2004, 09:48 PM
Mike Murrell's Avatar
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Question Diagnosis Verification Requested-300SEL No Start When Hot

Drove back from the big city to home this afternoon. Ambient temp about 90F. Parked car in garage, went inside for a couple of mins., then back to the garage and off again in MB(see signature for model/yr.). As I was backing into the street, the engine died. Car cranked easily, but would not start. An alert neighbor gives me a push back into the driveway. Car ran great to & from trip to town - 70 miles total. Idled great, accelerated great, etc.

Suspected the infamous OVP I suppose 'cuz it gets blamed for everything else; I was also suspicious of the fuel pump and fuel pump relay. I normally hear the fuel pump come to life when I turn the ign. switch to the first position. Not the case this time. Then I referenced a post I made awhile back that M.B.DOC was good enough to reply to. A pointer to that thread is further down in this one. That info helped with the diagnosis.

I let the car sit about 1/2 hr. then reattempted to restart. The car started, ran for about 20-30 secs.; then died again. Would not restart. Wait awhile, attempt restart - starts - then dies a bit later. I went through this scenaio a number of times. Same deal each time it does start - dies not too long after it starts. This part of the diagnosis was done with the MAS unit attached.

No ABS light on while running.

I pulled the MAS - near the brake fluid reservoir. The fuel pump relay in my 126 in inside the MAS; not over by the battery as it is on some older cars. I jumpered the number 1 and number 2 female ports. Got a little spark - must be wired hot and best of all, I can hear the fuel pump. Jump inside and the car fires right up. I intentionally let it idle a good while. It ran the whole time - never died as before after running for a short period with the MAS plugged in.

From where I sit, I'd say I need a new MAS. Is my diagnosis flawed?

Thanks for your time.

MAS units that contain Fuel Pump Relay
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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Last edited by Mike Murrell; 04-08-2004 at 06:27 AM.
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Old 04-13-2004, 02:31 AM
Mike Murrell's Avatar
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So far my diagnosis/resolution has proven correct.

Faulty MAS control unit - part # 011 545 79 32.

Part replaced.
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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"
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