CPS signal loss & fuel pump shut down logic

[parkerbink]

Sorry in advance for the long rambling post.

My '92 cabrio has been quite the challange. It took lots of work getting it to run, it was finally running great then I lost power to the fuel pump. As I had already done a bunch of work in this area I knew immediately what to check.

I have a good fuse, a new relay & socket (both were bad before) The pump is new & runs when given power directly. I have spark and power at #30 on the relay.

So I ordered a CPS as that is the next indicated thing. (a cheap one is out for delivery now, the 160 oe won't be here until Monday or Tuesday)

My question is what is the logic for a bad CPS to disable the pump? Is it that an out of time interference engine can destroy it's self if the valves are not running in time?

[varg]

Why? Because it doesn't need to be. There's no normal situation aside from a brief prime where it's a good thing for the pump to be running while the engine is not turning. It's a drain on the battery for one.

[parkerbink]

Replacement (cheap) CPS came & installed, no change. What is more likely, some circuit between the relay and the pump has died out of nowhere (car was running when this started) or the cheap CPS is junk?

Car starts with 12v to the fuel pump from an external source which seems to indicate it must be getting something from the CPS or how could it run?

[e30davie]

Got a wiring diagram? Time to start tracing wires in the fuel pump circuit with a multi meter.

[parkerbink]

Got a wiring diagram? Time to start tracing wires in the fuel pump circuit with a multi meter.

That was one way to go, I said fuck it, wired the pump to the rear window "defogger" so now I have a 30A fuse, the same relay and a switch. (don't care I lost the blower, it is a joke anyway)

[e30davie]

Does it turn off automatically when the engine stalls?

[parkerbink]

The pump? I doubt it but it has not stalled since I did it. It turns off with the ignition switch.

[jlevie]

The DME grounds the fuel pump relay coil when it sees timing data from a rotating engine. I can see that as being a safety measure in a collision that kills the engine. You would not want the pump to run if a fuel line was torn or dislodged.

Some wiring diagrams show power to the relay directly from the power point in the engine bay, others show the power from the main relay (which makes more sense to me) and is what I have seen on every 325i/is I've worked on. One pair of contacts in the main relay powers the DME and the other pair powers the fuel plump relay. So you could have a bad main relay or socket.
You can get the wiring diagrams fro: http://wedophones.com/BMWManualsLead.htm

[parkerbink]

I replaced the socket & relay when I first got the car and solved the issue, it just recurred.

I think the fuse socket on one side of the fuse has been pushed into the box. I took it apart to look but the wires are very tight and I was concerned I'd pull other wires out.

I gave up and wired it to the rear window blower (it's a cabrio) the circuit uses the same relay and has a 30 amp fuse. It is not correct but it is working and the car is not my daily so for now with it getting cooler and allowing me to enjoy the car I'm leaving it this way for now.

Thanks

[jeffnhiscars]

Did you replace the main relay as well as the fuel pump relays ?

[parkerbink]

I did, with the old & new one I have power at #30 on the fuel pump relay but no power to the pump when jumping 30 & 86. I think the issue is at the fuse, I can't see one of the blades in the fuse box the fuse connects to.

I like having the blower switch to kill the pump.

[TobyB]

Oooh, be careful-
on the cabrio, doesn't the rear defrost fan

<wait for it>

blow up?

Not the sort of thing you'd want in fuel systems!


sorry... I'll show myself out...

t

[parkerbink]

Never heard of that but I cut the wires from the fan (leaving plenty to put it back) and spliced them into the fuel pump. (after grounding the microswitch so it allows current with the top down)

I have driven about 80 miles like this and it's fine.