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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,274
There is a 4th relay in the fuel pump circuit...

I havn't read all the posts in this thread but I recently dicovered there is a 4th relay never mentioned anywhere before in the 1987 930 fuel pump wiring circuit that has to be working correctly or the pumps don't run... in my car anyway.

It is the aluminum colored "speed relay" under the driver seat. It is the factory rev limiter and 4 of the many wires going to it have to plugged in to it and making good contact or the pumps shut off.

Under the driver seat on the floor over on the left side is this aluminum color clear anodized "speed relay".
The part# is 930.618.123.00 and it's $314.25 from pelican.

To the right of it is a black plastic "fuel enrichment" relay that adjusts the lambda pulse valve to a higher frequency which richens the fuel mixture around full throttle on 1986 and later lambda equipped USA cars.
Then to the right of that is the larger K-jetronic ECU box.

I have had an intemittent problem with the fuel pumps shutting off randomly in my car since I bought it.
I've jumped both red relays in the front and the yellow relay in the rear and they still wouldn't run and then they'd just come back on when they feel like it. I also pulled 12" of the black tape off the rear wire harness under the relay console and then retaped them all looking for burned wires... very frustrating.
I finally wired in a toggle switch that bypassed both red fuel pump relays and gave both pumps 12 fused volts directly from the battery for if and when that happened so i wouldn't need a tow truck.

So last weekend I'm removing ALL the heavy factory sound proofing from inside my car and while the driver seat is out I started experimenting with removing the plugs and then individual wires from those relays under the seat while the ignition is on and the pumps are running and also while the motor is running to try and figure out what they do.

I thought the speed relay only cut out the ignition around 7200 rpm but it's also tied into the fuel pump circuit. If you remove either of the 2 white wires, or the red wire, or the brown ground wire from this speed relay the fuel pumps shut off.

I accidentally found that if I tapped this relay while the pumps were running my fuel pumps would shut off or come back on intermittently... great.

I removed the relay and bent the edges of the aluminum cover back along the bottom and took the aluminum cover off it and there are 2 small mini relays and some circuit boards inside and they were all clean and working fine.

It turned out that the male pins on the relay were not making good electrical contact with the female pins in the plug.
After gently scraping and cleanng the round male pins on the speed relay with the edge of a knife blade and scotch brite pad it worked perfectly again and the pumps keep running when moving or tapping on the speed relay so it seems that was the problem even though the pins looked clean.

As far as I know, no one has mentioned this relay for fuel pump problems before so reach under the seat and try moving the plug on the aluminum color relay while the ignition is on and the green plug is removed from the air flow meter...maybe the pumps will come on?

This multi pin plug is the type that is used in several other areas on the car and can be taken apart to resolder wires to the pins and it fits on the relay pretty tight.

hope this helps.
Old 03-19-2010, 10:25 AM
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