Main Relay and Fuel Relay

Specific conversations and info for the BMW E28 M5 and M535i.
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Syndrome
Posts: 128
Joined: Nov 23, 2021 3:46 PM
Location: DC

Main Relay and Fuel Relay

Post by Syndrome »

I’m looking for the part numbers for the main relay and the fuel relay for the 1988 E28 M5. Does anyone know the correct numbers, please? Are these relays (which are both plugged in behind the glove box) the same part number? I think they’re both five-prong relays. Are they two different part numbers and types? What colors should they be?

The context is that now that I have replaced the two fuel pumps, filter and hoses back there, I get neither spark nor (seemingly) fuel. Losing my mind. It was starting but not running before, so I figured I should change the fuel pumps, which look to have been born with the car back in 1988.

Thank you.
Syndrome
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Joined: Nov 23, 2021 3:46 PM
Location: DC

Re: Main Relay and Fuel Relay

Post by Syndrome »

Please impart your knowledge! Thank you!
Galahad
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Re: Main Relay and Fuel Relay

Post by Galahad »

If the fuel pumps aren't powered, the ECU isn't trying to start the engine - check if there's voltage at fuse 1 when you're cranking. If you have spark but no fuel (or power at fuse 1 but no fuel), your fuel pumps are bad or you have a wiring problem. I don't know what the part numbers are for the relays, but they should be swappable with generic ones as long as the terminal numbering is the same
Fanclutchnut
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Joined: Jan 05, 2021 7:32 PM
Location: Westchester Co NY

Re: Main Relay and Fuel Relay

Post by Fanclutchnut »

Most of what you are sharing sounds like the Ecu isn’t getting signal from the crank position sensor. That could be caused by several things. You said the car has been sitting. If you’ve changed both sensors and you’re sure they are plugged in correctly at the firewall, you might want to look at the flywheel. It may be very rusts where the sensors pick up. Just a thought. Have you checked spark at the coil and plugs? Have you gone thought the fuse box and checked every fuse? Clean the fuse tips and contact points in the box. Tighten them and apply a little dialectic grease.

Here is a stepwise procedure I followed when we got our car. I forget the author but it has helped several people. I got it running in a couple of days.
It sounds like you have a mixture problem. Too much fuel I think. Pulling the relay let the fuel pressure bleed down and putting it back brought it back up to whatever it wants to be. That may be too high so a fuel pressure test is next diagnostic and FPR might be bad. The S38 does use a higher rate FPR but I don't think it should be over 45 at idle. You might pull the return line and make sure it's not blocked back to the tank, blow air through it. One possibility is the little hose that makes the bend at the front of the fuel rail is breaking down and clogging the return.

Just checked and the M5 has a slightly different current path for the ICV. It's powered by the fuel pump relay and therefore it will not turn on with the ignition on. The engine needs to be cranking or running. To test this jump the fuel pump relay (pins 87 & 30) and see if the ICV now vibrates.

I suggest you AC voltage test the crank sensors now.

I Download the 1987-1988 BMW 535i - M5 Electrical Troubleshooting manual. This one has the M5 wiring. Go to section Injections Electronics, page 1362-2 to 1362-3 and you'll see the ICV power coming off pin 87 of the fuel pump relay.

The AC voltage test of the sensors is done on the same pins as the resistance test. Simply set your meter to AC volts and then crank the engine. 4 volts AC for the speed sensor and .4 volts AC from the ref sensor come to mind for what you should be seeing. If you don't get any voltage from the ref sensor then it's time to verify that the ref pin on the flywheel is still there.

Edit: Forgot to mention what you need to test on the main & fuel pump relays. For the main relay confirm 12 volts coming off both 87 pins when the ignition is on. For the fuel pump relay confirm 12 volts is coming off pin 87 when the engine is cranked. I find a test light to be more useful as a test device here since a bright test light indicates both good voltage and good current flowing where as the multimeter will show you voltage but it can't tell you anything about the current.”
Mike W.
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Re: Main Relay and Fuel Relay

Post by Mike W. »

The main relay is unique and a 5 pin relay, it's bitten many before who haven't looked at it's pinout. The ETM shows the fuel pump relay a 4 pin, just like many relays and just like 535's. The main relay must have 2, 87 terminals, or an 87 and an 87b. If it has an 87a terminal, it's the wrong one. It might be this one.
https://www.autohausaz.com/pn/61368373700OE

One of the problems with part numbers are they have superseded part numbers so many times it's hard to say for sure. Plus there's BMW part numbers and Bosch part numbers and maybe others too. But the car will not run, not at all, without the right main relay. The 87b is an unusual oddball, focus on 2, 87 terminals.
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