Weird E28 M5 question: main & fuel pump relays

[danielletiller]

I've been having my M5 rebuilt over the course of the last five years while daily driving it. It has always had random issues with cutting out and shutting down: the battery was arcing against the battery box (rust and no tie-downs), it had a bad AFM, then a bad ECU, and the fuel system was generally unhappy. Over time - and after a complete engine rebuild - all of this was worked out, except for what seemed like one nagging problem that I could never pin down that would still cause the car to die occasionally.

It stalled for no rhyme or reason that I could tell. Full tank, low tank, hot, cold, at idle, on the highway -- didn't matter. It happened irregularly, and it would always fire right back up. Other symptoms included sputtering and not turning off for a few seconds after killing the ignition (this happened a handful of times), hard starts (geometry dash liteagain, a handful of them), and it turning over too abruptly (the engine would come to life with barely a tick of the starter, and the idiot lights on the dash would complain at me until I revved it).

Over the last week, the car started stalling more. Eventually it stalled so often that it was undriveable and then died for good, and I had to have it towed to my mechanic.

My mechanic asked me what I had done to it because he thought I had made a change to the relays when I bolted the new DME back into place. Turns out it had two main relays in there instead of a fuel pump relay and a main relay. Neither of us had noticed the second silver relay instead of what should have been an orange one.

Back in 2020, I had the fuel system overhauled. The guys who did it charged me $58 for a new fuel pump relay; I didn't look into it, but assumed it was fine. That cost is in line with a main relay.

Both of us are surprised it was running, and it's been running like this for four years. Does anyone know enough about the relays to know if what I experienced was the result of the wrong relay being used? I would have thought the car would simply not work.

tl;dr

[Blue Shadow]

The fuel pump relay on the 535 is just a normal 12V relay, turn it on and it latches the power to the load.
The main relay has two powered outputs. Check the numbers to see when they are powered.

Check here to see how the pins should be connected: https://schemesnet.com/relay-5-pin-diagram/

I don't know if the extra output on the 5-pin relay is going to be a problem when used as a fuel pump relay.

[austin8753]

my M5 will randomly stall when i push the clutch in, when slowing to a stop. it always fires right back up. i have noticed that it does it more when the air conditioner is activated. the only thing that is an unknown on my car, is the DME. you say yours has been replaced, was it with another used one, or a "reconditioned" unit? i'd be looking there, if it were me.

[Shawn D.]

Hi danielletiller, I see this is your first post. Welcome!

Please read and heed the Forum FAQ and Tech FAQ sections (it can often be obvious if you haven't).

Also, do not post technical questions in "General Conversation," post them in the proper technical section.