300SE poor starting
 

I was given an 1989 300SE. It had been sitting for several years and had a dead battery. I replaced the battery with one I had and after some trying it started. When I got it home I replaced all the fluids and filters, cap, rotor, and plugs. I filled the tank with gas and a can of injector cleaner and drive it for about 275 miles. Now when I start it in the morning I have to hold the RPM at 2000 to 2500 for a few seconds and gradually bring it down to idle or it will stop running. After this first attempt it refuses to start untill the next morning when I must repeat this procedure. Once its warm and running it starts normally.

Any help or ideas will be appreciated.

Check your vacuum line connections
 

A car of that vintage that has sit unoperated for a prolonged time could have loose or blocked vacuum lines.

Ideally, you'd remove them, replace if necessary but at least check if the rubber components are not hardened or kinked.

I have a 1991 300SE that has been sitting for over a month. Now it will not start. The fuel pump is not being energized when the ignition switch is turned to the start position to crank the engine. The fuel pump does make some noise and the measured votage varies but for the most part it is not running while cranking. The fuel pump does run if I connect 12V directly to the pump terminals. I did verify that there is spark while the engine is cranking.

I have ordered a new overload protection relay and a new(used) MAS control unit. Hope they solve the problem.

After looking at the control circuits for the 1991 300SE, I really love the simplicity of my diesels.

Jack:

I've been down the road you're on now. I too have a '91 300-SEL.

To verify the fuel pump relay circuit in the MAS was the issue, I removed it and jumpered pins 1 & 2. They are closest to the outboard side of the MAS. When they're jumpered, you should hear the pumps run and if the car starts, that part of the MAS(fuel pump relay) was bad.

Be careful when you jumper them as it's a hot circuit and if the jumper wire gets just a hair caddywompus, you'll get a spark. I usually pull the grd. strap off the battery first, then set the jumper wire in place, then reconnect battery.

The MAS is a costly gadget. I'd like to find someone who could open and repair them.

Mike,

Thanks for letting me know that. I think I am on the right track. I found a used MAS on ebay for $49.00 plus shipping with the same part number as the one on my car (0115457932). The seller said there are a variety of MAS part numbers that will work the same.

Would you know what MAS stands for? It looks like they combined a couple of relays into one unit. I wonder if the knob on top allows it to be used for 1 or 2 fuel pump applications?

Also how do you replace the MAS?
It looks like there is a bracket attached to the firewall that needs to come off in order to disconnect or replace the MAS control unit.

The 1991 300SE is relatively new to me. I am just learning about it. I bought it on ebay and drove it home from PA to MA. It loves the passing lane and 80mph on the turnpike. Need to use cruise control to avoid getting a ticket.

Thanks,
Jack C.

The idle air bypass control may be stuck in one position. It's at the front of the intake manifold with two hoses attached. Try cleaning it as it may be full of crud. Also, check for vacuum leaks, such as the two hoses previously mentioned.


You've narrowed it down to the fuel pump relay.

It's not an overload protection relay, it's an overvoltage protection relay. If the voltage exceeds 22 volts, the fuse blows because a zener diode conducts to ground causing the fuse to blow. The ECU and ABS computers are protected in this way.

http://mb.braingears.com/126_DISC1/program/Engine/103/07-121.pdf

http://mb.braingears.com/126_DISC1/program/Engine/103/07-165.pdf

Jack:

There's a black knob on top that rotates counter clockwise. Once done, it's removed by lifting upward. Pretty simply. when it's out, you can jumper pins. There are a number of circuits controlled by the MAS - don't recall them off the top of my head.

Dont' know what MAS stands for. Perhaps someone else can provide this?

Thanks, it was the vacuum hose from the fuel distributor to the idle air bypass. It had rotted and fallen off. When connected, even in its poor condition, the engine started first time. Still available from the dealer, $10.

Thanks for the help.