Re: 1990 SAAB 900 non-Turbo - Saab 900 Bulletin Board - Saabnet.com
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Re: 1990 SAAB 900 non-Turbo
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Wed, 3 Sep 2008 05:54:22 Share Post by Email
In Reply to: Re: 1990 SAAB 900 non-Turbo, Mike Wiley, Tue, 2 Sep 2008 19:54:46
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There is only one fuel pump relay. That relay gets power from the same place as the fuel injection system, so if you can jumper the pump and it runs, the injection system is getting power.

That relay pulls in when the ECU senses the engine running, based on ignition pulses. Since the engine will run with the pump jumpered, the ignition pulses are working.

I suspect your relay really isn't working. Oh yes, it might make clicking sounds, but I'll bet the contacts are dirty and/or worn. Dirty contacts have resistance, and resistance limits the amount of current they can pass. Any electric motor takes more current to start running than it takes to keep running. So when the relay closes, the contact resistance and the high startup current create a voltage drop - so much drop that you get no voltage to the pump, and it doesn't run. And you read 0 volts on the pump.

I'm not sure of the location of the fuel pump relay in a '90 900, but if the relay box says Fuel Pump, then that's it. I would take out the relay and clean the contacts- pop off the case, and put a length of fine sandpaper between the contacts. Hold them closed, and pull out the sandpaper. Do that a couple of times - you just want to brighten the surfaces, not file it down. If that doesn't work, consider a new relay.

Power comes directly from the battery to the common contact on the relay (pin 30). The switched portion (87B) goes to the fuel pump FUSE, and from the fuse, to the fuel pump. The relay coil gets power from the same place as the main relay. the low side of the relay coil goes to ECU pin 20, which switches it to ground.

Try this - pull out the fuel pump fuse. With the meter measure voltage on one side of the fuse socket while cranking the engine. Then try the other side of the fuse socket. One side is connected to the fuel pump, the other to the relay. Yes, I know the fuse is out. The side connected to the pump will be 0 volts, as it should. I'll bet that the side connected to the relay will show 12 volts or so. Plug in the fuse, it'll drop to zero. That's because the meter presents no current load, and the effect of the contact resistance is minimal. That's assuming the contacts aren't completely bunged up.

I'm assuming when you say "as it seems to be working" you mean you know the relay is clicking, and that it is marked "fuel pump." If the relay isn't clicking, then it could be a poor connection in the wiring.

posted by 192.249....


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