Hello: I've exhausted myself reading threads on this issue. My 1990 300SE with the M103 engine would not start this afternoon. Over the last week I had noticed it was taking a little longer to start than usual. Well, usual for this particular engine was nearly an immediate start. After reading many threads from those who had similar issues, I've come to believe there is a problem with either the fuel pump, fuel filter or fuel pump relay in this vehicle.

I poured some gas directly into it and it started. I had someone turn the ignition on and off and heard no sound coming from the fuel pump and I was lying on the floor right under it.

So, what is the most likely culprit and how do I find out for sure which one it is? I have read about "jumping" the fuel pump relay. I don't know where that relay is nor how to jump it.

If you post a suggestion please do so in the simplest terms. I can certainly follow directions, but I need all of the steps in order to do so. I'm just learning these cars. Well, I'm just learning cars in general.

Thank you.

Well, you're lucky, I'm not playing golf all week; so I'll help you research.
First thing would be to jump the fuel pump wires.
Without checking, I'm gonna assume your '90 is like my '91.
Look just to the right of the brake master cylinder; you should see a retangular box bigger than a pack of cigarettes.
It should say MAS on it, and have a big plastic wingnut on the top.
Turn the wingnut Counterclockwise all they way, and just lift the MAS relay out and put it in a safe spot.
The pins underneath are numbered, you want sockets 1&2, the big ones closest to the brake reservoir.
Find a thick piece of insulated wire, and carefully short out sockets 1&2.
Don't need the key on or anything.
The fuel pump should run.

If it does run, then chances are you have a bad MAS relay.

If the pump does not run, then you have a bad pump or wiring.

I'll do a bit of memory jogging and get back to you. Seems if I don't work on something for a few months, these days I forget the details. Just got my Medicare card, so I guess I have a good excuse.

Let me know if the jumper worked.

DG

Hello sir. Thank you for your reply. I'm going to have to pay you a retainer soon.

Well, I was reading a little more as I waited for someone to reply to this thread. I read about the MAS relay and it sounded like something I should look at. Well, I wanted to remove it, so I wiggled it just a little and lightly pulled up on it. Nothing. So, that little knob on the top of it looked interesting and I gave it a whirl. Turning that produced a sound in the left front of the engine compartment. I thought I had screwed up a factory setting and returned the knob to its original position the best I could. Nervous that I had made the situation worse I went into the car to see if the car would still at least turn over. It did better than that. It started right up! I let it run and started it a couple more times. It seems great.

So, what exactly did I get lucky with? What is the function of that knob on the top of the MAS relay?

Thank you in advance.

Man, you are living right.
Obviously you had a little corrosion on some of the pins, and wiggling it around made contact.

Now, don't stop there. Turn that wingnut CC-wise about 1 1/2 turn, or until is stops turning. Then, the whole relay should just lift right up. When you get it out clean all the pins and sockets, and you should be good to go.

Be careful cleaning out the sockets, some are hot, even with the key off. Best to disconnect the battery. As stated in my earlier msg, the fuel pump is socket 1= 12 V.
Socket 2 = to the fuel pump.

DG

I deserved to get lucky after all I've been through with this car. I actually went out to pull a vacuum on the a/c as you suggested and that's when I found the car wouldn't start. I just wanted to back it out of the garage a little.

Of course I'll take your advice and clean the relay and surrounding area. There are a lot of parts that I'd like to change as part of my preventative maintenance. Maybe I'll purchase a new MAS just to have it when I need it.

Thank you.

I've had fuses appear to open up as a result of a bit of oxide buildup. They act open, but you just move them around a bit and they're good again.
Some of the guys here suggest installing new fuses every 20 years or so, even if they're working. Not a bad idea. And it's probably a good idea to unplug and replug all the relays occasionally, just for the reason you encountered.

DG