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Fuel pump relay question

Posted by Dave_G 
October 24, 2012 12:18PM
Today while trying to leave work at lunchtime, my car suddenly would not start. Fortunately it was lunchtime, so I took some time to diagnose it and determined that the fuel pump was not getting power. I pulled the fuel pump relay, jumped sockets 30 and 87, and I heard the fuel pump come to life. Going further, I was able to start the car like this and it idled fine, so after a few seconds I shut it off. This led me to diagnose the fuel pump relay as faulty, and I have a new one coming tomorrow.

My question is this: I need to get home after work today. Am I going to break anything if I drive the 4 miles home with a jumper in place of the fuel pump relay? As long as I start it right after the jumper is in place, and pull the jumper immediately after shutting it off, I don't see how it could damage anything, but then I don't see lots of things that are obvious to others.

__________
Dave
'91 325iX
October 24, 2012 01:32PM
Unless you have leaky injectors, there's no reason you couldn't have the fuel pump running all the time. The fuel pressure regulator maintains the required pressure within the injector fuel rail, and any excess fuel is recirculated back to the tank. With the pump running full time, the fuel just goes around in a circle back to the tank.

That said, it's a safety issue in that pin-30 on the fuel pump relay goes straight to the battery and is hot all the time. In case of accident, or whatever, the fuel pump will continue to run until somebody thinks to pull the jumper wire out of that socket.

What you could try instead is to jump pin-86 to pin-87 in the fuel pump relay socket. Pin-86 only becomes hot whenever the Main-relay closes. That means you could turn the fuel pump off by turning the ignition switch off. This puts more load on the main-relay wiring though. Normally it's just the extra juice required to close the fuel pump relay, but now the main-relay would be powering the fuel pump directly.
October 24, 2012 01:43PM
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Ferdinand
That said, it's a safety issue in that pin-30 on the fuel pump relay goes straight to the battery and is hot all the time. In case of accident, or whatever, the fuel pump will continue to run until somebody thinks to pull the jumper wire out of that socket.
Yeah, I did think about that. It probably wouldn't happen, but then nobody ever thinks they're going to get into an accident.
Quote

What you could try instead is to jump pin-86 to pin-87 in the fuel pump relay socket. Pin-86 only becomes hot whenever the Main-relay closes. That means you could turn the fuel pump off by turning the ignition switch off. This puts more load on the main-relay wiring though. Normally it's just the extra juice required to close the fuel pump relay, but now the main-relay would be powering the fuel pump directly.
That's an interesting idea, but I'm wondering if I would then risk frying the main relay.

Another idea I had was to swap in the O2 sensor relay, which is an identical relay right next to the fuel pump relay, and jump the O2 sensor relay instead. It sounds a little safer, but I have less of an idea of exactly what the O2 sensor relay does, and what would happen if I jumped it.

__________
Dave
'91 325iX
rkj
October 24, 2012 01:55PM
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Dave_G
Quote
Ferdinand
That said, it's a safety issue in that pin-30 on the fuel pump relay goes straight to the battery and is hot all the time. In case of accident, or whatever, the fuel pump will continue to run until somebody thinks to pull the jumper wire out of that socket.
Yeah, I did think about that. It probably wouldn't happen, but then nobody ever thinks they're going to get into an accident.
Quote

What you could try instead is to jump pin-86 to pin-87 in the fuel pump relay socket. Pin-86 only becomes hot whenever the Main-relay closes. That means you could turn the fuel pump off by turning the ignition switch off. This puts more load on the main-relay wiring though. Normally it's just the extra juice required to close the fuel pump relay, but now the main-relay would be powering the fuel pump directly.
That's an interesting idea, but I'm wondering if I would then risk frying the main relay.

Another idea I had was to swap in the O2 sensor relay, which is an identical relay right next to the fuel pump relay, and jump the O2 sensor relay instead. It sounds a little safer, but I have less of an idea of exactly what the O2 sensor relay does, and what would happen if I jumped it.

Dave, I'm pretty sure you can put the 02 relay in the pump slot and leave the 02 slot open for the ride home (the computer will just go to a default setting). Rick
October 24, 2012 02:00PM
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rkj
Dave, I'm pretty sure you can put the 02 relay in the pump slot and leave the 02 slot open for the ride home (the computer will just go to a default setting). Rick
Hmm. I thought about that, but at some stage in the troubleshooting process when I put the O2 relay in the fuel pump relay socket, I turned the key and did not hear any sounds from the fuel pump. (I don't think I actually tried to start it, though.) I interpreted that as meaning that the car didn't want to run without all of its relays (or jumpers) in place.

I'll try double-checking that when I get a chance later.

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Dave
'91 325iX
October 24, 2012 02:23PM
There ya go. Rick's idea is the best solution. Swap the O2 sensor relay in place of the fuel pump relay.

The O2 sensor relay is only for pre-heating the sensor. It's not required otherwise and shouldn't affect any computer settings. But if you're worried about it, then you can use the jumper wire from pin-30 to pin-87 in the O2 relay socket instead. Pin-30 in the O2 sensor socket receives power from Pin-87 of the fuel pump relay, ie only ever when the fuel pump relay is activated.

In normal operation, I believe the fuel pump only runs for a couple of seconds and then shuts off waiting for an engine cranking signal. Just turning the key on might not be enough, you'd actually have to try starting it.

The coil in the fuel pump relay pin-86 receives power whenever the main relay closes. But the ground end of the fuel pump relay coil pin-85 is grounded through the Motronic Control Unit. So there's an electronic logic circuit involved in determining when to close the fuel pump relay. Sticking a jumper wire from pin-30 to pin-87 bypasses all that and runs the pump full-time. But plugging a working relay switch into the socket means the Motronic Unit takes control as designed.
October 24, 2012 02:27PM
Well, alright then. I'll let you know how it works out when I try to get home this evening. smiling smiley

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Dave
'91 325iX
October 24, 2012 02:40PM
On second thought, I wouldn't use the jumper on the O2 sensor heater.

I was wondering, if it's powered directly from the fuel pump relay anyway, why the need for a separate relay for the O2 heater? Again, the ground end of the relay coil is routed through the Motronic Unit, so there is a logic circuit controlling when that relay closes or opens. Bypassing it with a jumper means the O2 sensor heater would be on full time, which is not needed.

The sensor will heat up all by itself, just not as quickly, within a few moments of starting the engine. Probably not going to do it any good to have the O2 heater powered on all the time that the fuel pump is running.
October 24, 2012 04:30PM
I put the O2 relay in the place of the fuel pump relay, and... it worked perfectly. I couldn't even really tell that anything was missing. It's funny that there are two identical relays rubbing shoulders under the hood, and one of them is needed to run the car, and the other one is almost optional! (I'll still be replacing it tomorrow, though.)

__________
Dave
'91 325iX
rkj
October 24, 2012 04:51PM
Quote
Dave_G
I put the O2 relay in the place of the fuel pump relay, and... it worked perfectly. I couldn't even really tell that anything was missing. It's funny that there are two identical relays rubbing shoulders under the hood, and one of them is needed to run the car, and the other one is almost optional! (I'll still be replacing it tomorrow, though.)

Yes, it's that easy. I usually carry a spare relay for each circuit. I actually needed it one night when I had the ladies on board and I was able to keep my boy scout image alive... A Very good night for the rickster smiling smiley
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