1991 300SEL fuel pump/MAS diagnosis

[ThePhoenix]

I am helping a friend with a 1991 300SEL that is not running because of fuel delivery issues. He had purchased a second hand fuel pump which he did not install. I installed it for him, but there was no voltage coming to it. A little sleuthing on the boards here led me to the Mass Air Sensor, which incorporates the fuel pump relay on this car. One of the users here suggested jumpering pins 1 and 2 on the MAS loom as a test. Doing this produced voltage, so it looked like the MAS was the culprit and not the pump.

However... in my haste to get the car running (my friend was two hours late to show as I worked on it in cold dark conditions with his mom watching, loads of fun), I left the ignition key on while connecting the wires to the pump. At this point the MAS was still jumpered, and as soon as I connected the wires to the pump it started whirring away. However, as I tightened up the connections (which took a while considering the working conditions) it just stopped and it won't do anything now. Did I fry something? The pump perhaps? Should I have not left the MAS wiring jumpered that long? Or is there a fuse or another relay involved?

If I can get an MAS at a wrecking yard and that's all it takes to get her going, he will be stoked.

[oldsinner111]

Does your fuel pump have fuse protection in the trunk or relay

[ThePhoenix]

Originally Posted by oldsinner111

Does your fuel pump have fuse protection in the trunk or relay

That's a good question that I'd like to know the answer to.

[lsmalley]

wouldn't there be a fuse or relay behind the smaller fuses in the fuse box? Or what about the ovp? Isn't the ovp designed to protect against issues like this? Check the fuse cover to find out the fuel pump/mas relay or fuse and also check the fuse on top of the ovp.

[ThePhoenix]

Originally Posted by lsmalley

wouldn't there be a fuse or relay behind the smaller fuses in the fuse box? Or what about the ovp? Isn't the ovp designed to protect against issues like this? Check the fuse cover to find out the fuel pump/mas relay or fuse and also check the fuse on top of the ovp.

What's an OVP? I see it mentioned regularly.

I did a quick visual of all fuses in the box and they looked good. I need to go back with a volt meter, which would have been handy last night. Any idea where to find a schematic which will tell which relay might be the culprit?

[1991300SEL]

OVP - located in the middle back close to the firewall.

http://www.pelicanparts.com/catalog/images/INT-Images/2015403745.jpg

[ThePhoenix]

Originally Posted by 1991300SEL

OVP - located in the middle back close to the firewall.

http://www.pelicanparts.com/catalog/images/INT-Images/2015403745.jpg

Cool, thanks for that. I did check the fuses there and they were good. Any idea whether it's pin 1 or 2 on the MAS plug that should have voltage? Is the OVP responsible for sending voltage to the MAS plug?

[1991300SEL]

Look at the 5th post in the following thread. There are 3 thumbnail photos. View the one on the far right. It has the MAS pin layout. You jumper 1 & 2 with a short-looped piece of wire. I used 18 gauge I believe. Be sure to get a straight shot going down with the jumper wire. One time I did this & got a spark. One of the wires must have gotten caddy-whompus.

I had a similar problem once and suspected fuel pump relay(FPR). Jumpered 1 & 2 and the car would start and continue to idle. Changed out the MAS. Ouch! - $$$$$

Not sure of the OVPs relationship with the MAS.


MAS relay diagram