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.

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

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

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.

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1990 190E 3.0L

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

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?

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

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

Attached Thumbnails

 

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'91 300-SEL

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?

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.


http://www.peachparts.com/shopforum/tech-help/327927-mas-relay-diagram.html

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'91 300-SEL

Thanks for the info which may prove to be helpful. I have already jumpered pins 1 and 2 on the MAS, which was successful in getting voltage to the FP as a test. However, now I am either A) no longer seeing voltage at pin 1 on the MAS, or B) I have fried the FP (not sure which, because I didn't have a meter with me at the time). If it's A, I need to know if there is a fuse of some sort upstream of the MAS that I have fried. I already know I need to replace the MAS, wish me luck at the local pick-a-part...

Read post 3 in this thread. It's a start.

http://www.peachparts.com/shopforum/tech-help/266349-more-560sl-high-idle-stuff.html

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'91 300-SEL

OK, so its says:

"the ovp relay has a terminal #30 which is battery power all the time in mercedes language."

Is this voltage from terminal #30 then sent to pin #1 on the MAS?

I don't have a wiring diagram in front of me. Maybe someone who does will respond.

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'91 300-SEL

See post #14 in the following thread:

http://www.peachparts.com/shopforum/diesel-discussion/348483-russian-epc-back-again.html

The title says "Russian.....". Later into the thread whunter provides pointers to U. S. based web sites for various older MB doc.

Free 107 - 124 - 124 - 126 - 201 service manuals.

Beware - some downloads may be very slow. The "Circuit Diagram Index" located within "Electrical Trouble Shooting" for a '91 300-SEL took 13 mins to download and no, I am not using dial-up or DSL.

You'll need the circuit diagram to trouble-shoot your problem.

Querying thru the "Maintenance Manual" section for a '91 300-SEL returns "Document not found".

Hard to knock free.

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'91 300-SEL