BMW 2002 FAQ - Motronic or Jetronic Fuel Pump Relay for m42 pump?

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Thread Topic: Motronic or Jetronic Fuel Pump Relay for m42 pump?

Date: 7-9-08 09:58
From: citizenchan

Subject: Motronic or Jetronic Fuel Pump Relay for m42 pump?

A couple members helped me narrow down my fuel supply issue to the relay (I thought it was the pump and I was wrong, big surprise.. hah), and I found that the relay I have is for the motronic. I have a l-Jetronic from an 84, but a pump from the 90 318is.

So my question is: Should I try to wire up the appropriate relay, or does it really not matter that much?

The L-Jet relay has two + voltage to two pumps - I have one. It also has an additional input from the fuel regulating system, which is separate from the engine management system in the LJet. Plus the computer ground, ground to body, and ignition.

The motronic only has one power out to the pump, which is ok since I have the later pump. But it's got the other three - the computer ground, ground to body, and ignition.

I am thinking the input from the fuel system somehow helps to regulate supply or whatever as the engine runs. Would this relay be a possible cause of some driveability issues?

Thanks for the help everyone.

Date: 7-10-08 06:15
From: Mike87 in San Jose, CA

Subject: Re: Motronic or Jetronic Fuel Pump Relay for m42 pump?

The '84 and '85 318i had two pumps, one in the tank and one above the rear axle. The one in the tank is the volume pump which supplies fuel to the pressure pump under the car. Both are needed, although the car will run with only the pressure pump, but may have drivability issues.

There is an intank pump which does both volume & pressure that will work for that system. I think it came from a '90/'91 E30 but am not sure. Others on this board will know specifics of that.

Check your fuel pressure. You should have ~40-45psi all the time.
_________________
Good Luck,
Mike (#87)

Date: 7-10-08 06:39
From: citizenchan

Subject: Re: Motronic or Jetronic Fuel Pump Relay for m42 pump?

Thanks Mike. yeah, I've got the later single pump but the older LJetronic computer. I'm curious to know if you think it would cause any driveability issues having the motronic (later) fuel relay - which doesn't have an input signal from the fuel system, or if I really do need the relay for the older 84 that has the extra input.

On the motronic, fuel was managed by the ecu. In the Jetronic, fuel was a separate system, and the relay had input for that. So long as my fuel is hitting the pressure regulator at a high enough rate to achieve pressure in the fuel rail, I shouldn't need any other control over the fuel delivery. Correct? If so, I am thinking that the older relay was necessary solely because it had two pumps that had a special relationship that needed to be governed i.e. the electric undercar pump could be easily varied by the fuel system. If that's true, then the single pump is all-in-one and I should be fine, correct? Or do I still need a fuel system signal going regardless?

Thanks for your help.

Date: 7-10-08 08:08
From: Mike87 in San Jose, CA

Subject: Re: Motronic or Jetronic Fuel Pump Relay for m42 pump?

In theory, you don't even need a fuel pump relay. You could wire it directly to the key and have it on all the time. When I first ran my car that is the way it ran, then I added the fuel pump relay. All it does is turn the pump on and off. It does nothing more.

That said, you should be fine with what you have.

Check the fuel pressure and volume to make sure you have fuel pressure and flow per the Bently manual.
_________________
Good Luck,
Mike (#87)

Date: 7-10-08 01:29 From: citizenchan Subject: Re: Motronic or Jetronic Fuel Pump Relay for m42 pump?
Sweet, thanks a bunch Mike.

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