No 12v to Fuel Pump
 

My 1972 350sl won't start. I check to wires to fuel pump, not hot. If I run a set of jumpers back to the fuel pump it will start, and the wires that I disconnected are now hot. There is something stopping the motor from starting and allowing the fuel pump to start pumping. In a perfect world the car will start and 2 or 3 seconds later the fuel pump gets 12v to keep it running. Any help would be great.

There is a device called "fuel pump relay". Its operation is to come on for a couple of seconds when the key is turned on then go off until it senses an rpm signal from the tach indicating that the engine is turning over. This is to prevent the fuel pump from continuing to pump fuel in the event of an accident where a fuel line is ruptured but the engine is stopped with the key still on - a very good design. It's likely that your tach is inop or the fuel pump relay has gone bad or you've got a blown fuse...check the fuse first, it's the easiest and cheapest to fix.

Don't just visually check the fuse. If it is original it is aluminum and they are famous for corroding at the contacts. Replace with a brass/copper? ceramic fuse.

The fuel pump relay is top left in the relay panel above the fuses. It is the same as all the others. You can swap another from the right side to test. Just don't try the one below it as it is the ECU relay.

I answered this in the Tech forum, but this is a better place for it.

BTW Mach4, are you sure the engine rpm is sensed from the tach? From memory (which is not always good!), I seem to recall that the djet ECU gets that from the trigger points.

First of all, you only get 12V to your pump for a second or two after you turn key on. To check if you have power to pump, you would have to jumper the fuel pump relay. Or connect meter to pump and have someone turn ignition key on while you watch the meter. It should just show 12V briefly.

If after testing you still don't have power to pump, then ...

There are two items that control power to fuel pump.

A fuel pump fuse.

A fuel pump relay.

Fuse is just one of those in fuse box. 8amp, I believe. Forget which one.

Relay is one in pic below (located above fuse box) You can switch it with one of the others (they are all the same) and see if you then get power to your pump.

http://www.benzworld.org/forums/atta...350-relay1.jpg

Yes I'm absolutely positive that the fuel pump relay gets its signal from the tach...unfortunately I was referencing my 107 (an '85) which I mistakenly assumed would be the same for the OPs vehicle. Obviously it's not.

Guess that's what I get for making assumptions - my bad and I apologize.

Accordingly, please disregard my post above.

In the case of D-jets, Graham is spot on, the "engine running" signal (TD) is from the trigger contacts.
In the case of K-jets, the TD signal comes from the EZL, and simultaneously to the fuel pump relay and the tach.

Frank, you lost me with D-jets and K-jets. Will test the relay tomorrow with my spare.

Thanks
Les

Don't worry about the kjets - you have a djet :)

For completeness, this was my post in the Tech forum re which fuse to check. (But do check them all and perhaps even replace using the type with copper/brass ends (not aluminum). You can get them at auto stores as well as in kits on ebay.

By "hot," do you mean warm to the touch or do you mean that they are electrically charged? If the former, you have a problem; the latter would be expected from the point of jumper attachment back to the relay.

By Hot I mean 12 Volts, not hot to touch. When I remove both wires from the fuel pump and run jumper cables from the battery to the fuel pump the car will start and the two wires I took off the pump now have 12 volts to them. If I remove the jumper cables the car stops running and the two wires are not hot.
I tested all the fuses and the fuel pump relay, all good.

Only one of the wires that are attached to the terminals on the fuel pump should ever have +12V present. The second wire, the brown one, is a ground, and should show continuity to ground. If the brown wire is not grounded, but instead has +12V present, there is a fault in the wire harness.

When the potential on both sides of a motor is the same the motor will not run.

Well I changed all the fuse's, the fuel pump relay, the Pump & Filter and stood on my head on the passenger side and IT RUNS. My thanks to all for the help.

Les

:clap:Glad to hear it :clap: