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T77911S T77911S is offline
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Join Date: Oct 2006
Location: MYR S.C.
Posts: 13,785
here, read this:

look at the fuel circuit as 2 seperate circuits. one is the 2 FP relays, the yellow relay(contacts), the rev limiter, and the pressure switch. the other circuit is the yellow relay(coil) and the AFM switch.
the AFM switch removes ground from the yellow relay, de-energizing it when the engine is running.
the FP circuit gets its ground through the contacts of the yellow, then the rev limiter then to the pressure switch.
all the yellow relay is doing is energizing the 2 FP relays. the yellow relay is also (?) what gives the 1-2 sec FP run before starting.

short description:
the yellow relay is controled or energized by the AFM switch. with the key on and engine off, the AFM energizes the yellow relay, removing the ground to the 2 FP relays. that ground is of course the boost pressure switch.

the drawing i have for an 86 shows the speed relay or rev limiter side cuts out at 7k, and the other side is for the O2 sensor. it shows it cutting out at 3k.

tim's relay had several broken solder joints. one was on the coil for the FP's. the others where on some of the major pin joints. some even for the O2 side. so for those that have bad relays' pull them apart and just resolder the points i mentioned.
you can apply 12v to pin 7 and ground to pin 5 to test the relay. both relays should pull in.
(i actually used a 9v battery)

here is how i would go about TSing this system: based on whats esiest to do first and each step does not fix it.
(if one pump runs and one does not, then the FP or the relay.)
the letters in () are to go along with the diagram above.

both pumps not running:
i would reach around and pull the connector off the AFM. as soon as the key is ON, no start needed, the FP's should run. (F)
then i would check to see if the boost pressure switch is grounded. with connector off AFM, ground the wire on the boost switch. (A1)
jumper 30 and 87a on yellow relay. (G5)
jumper pins 1 and 2 on the speed relay. (B3)
IF 1 and 2 run the pumps, check for 12v to speed relay. (M pin 7)
if 12v present, speed realy bad. bad solder joints is a problem on this relay.
jumping the FP relays really only lets you know the FP's work.
grounding (D4) just cuts the circuit in half. if the pumps run, then it is something on the left side. if they do not, then it is on the right side.

a note for the yellow relay:
unlike most realys, the yellow relay runs the pump in the de-energized mode. so the coil can be bad and the pumps can run. you might consider popping the cover off and cleaning the contacts with fine sandpaper. i bet there are a LOT of good/repairable yellow relays that have been replaced.
the yellow relay and speed relay are known for bad solder joints.

the FP circuit was different for the years on the 930 from what i am told. the 87 has a fuse in the wire bundle itself that feeds power to the speed relay. it is a pain to get to....from what i am told.
86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
Old 11-25-2014, 11:06 AM
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