Fuel system electrical issue

[trojwl]

My 82 911 SC will not start. After extensive troubleshooting I determined that pin 85 on the fuel pump relay was the culprit. This pin is the ground side of the relay coil.
With relay out of socket, I turned engine over seeing what was happening with the ground. I have continuity to ground (from socket hole #85) before doing anything. When I turn the ignition, the continuity of pin 85 to ground goes away thus the relay will never energize the fuel pump (i.e. pin #30). It stays gone until I turn off ignition. It's like things are happening opposite of what you would expect.

All the voltages at the relay look correct. The ground is definitely eating my lunch.

I know the "RPM rev limiter" is also part of the ground circuit but I can't find any info on this. Also I physically moved the air flow sensor plate up and down and saw no change in ohms to ground. Not knowing how the RPM limiter figures into this is difficult to say the least.

Any input would be appreciated.

I'll probably take it to my mechanic as I know nothing about the flow sensor or the RPM limiter. At least I have saved them the time trying to locate the problem.

[steely]

It could also be the FP relay itself.

[boyt911sc]

Search for DKLever48's thread about FP relay and FP relay socket tests. Everything you need to know is well covered in that thread. Test terminal #85 of the FP relay socket using a test light. Keep us posted.

Tony

[boyt911sc]

Originally Posted by trojwl

My 82 911 SC will not start. After extensive troubleshooting I determined that pin 85 on the fuel pump relay was the culprit. This pin is the ground side of the relay coil.
With relay out of socket, I turned engine over seeing what was happening with the ground. I have continuity to ground (from socket hole #85) before doing anything. When I turn the ignition, the continuity of pin 85 to ground goes away thus the relay will never energize the fuel pump (i.e. pin #30). It stays gone until I turn off ignition. It's like things are happening opposite of what you would expect.

All the voltages at the relay look correct. The ground is definitely eating my lunch.

I know the "RPM rev limiter" is also part of the ground circuit but I can't find any info on this. Also I physically moved the air flow sensor plate up and down and saw no change in ohms to ground. Not knowing how the RPM limiter figures into this is difficult to say the least.

Any input would be appreciated.

I'll probably take it to my mechanic as I know nothing about the flow sensor or the RPM limiter. At least I have saved them the time trying to locate the problem.

trojwl,

What tool were you using doing the troubleshooting? Multi-tester or a test light? Repeat the test using a TL. The switching of the ignition switch (ON/OFF) has no direct effect or relation to the AFS (air flow switch). Terminal #85 is grounded via the AFS. On the contrary, the fuel pump will be energized when you turned the ignition to ON position and losing ground contact (not normal). Do you realize that the FP would be energized and running when power is delivered to terminals 87A/86 and #85 has no ground contact unless the FP is defective?

Tony

[strictly]

i recently spent 2 hours diagnosing what was a fairly new fuel pump relay! (and yes it did click).

A quick check on your starting, is take fuel pump relay out, and bridge 30 to 87a, the pump will come on. Have a go at starting....it starts? okay so back to 86 and 85. No start - check you have power at 87 with the starter turning and check spark. You can also use a black relay as a quick test of the red one, they will all work! If the FP did not come on, then put 12v straight to 30 on the relay socket, if the pump now runs, there is no 12v to 87a is just power to the WUR, AAV, an TTV via the ignition/relay back panel.

86 you can follow from a wiring diagram (30 and 87a as well). Basically 86 and 87a should have 12v, ignore if any of them show continuity to earth/ground i have seen that on different cars, and it has always puzzled me, electrics are weird. 87 is starter (aka 50) for fuel pump feed when cranking

85 as tony says is the ground for the AFS; when the engine is turning, the airflow plate is lifted, thus breaking the connection, and turning the FP on. You can play around with the AFS connection, by taking off your air boot between the FD and TB. With the ignition on pushing up on the AFS plate (and a good FP relay) will turn the FP on, if it dosent, unplug the switch at the back, and check continuity on the red/brown wire (to 85 on the FP relay i believe). Shorting the red/brown wire should make the FP run (or maybe thats the black/brown wire), or vice versa i can't rember excatly, just have a play. Unplugging the AFS switch should also make the FP run (with the relay in) as its the same as breaking the ground contact

In any case i would work on the FP relay before moving on to the RPM limiter.

[ossiblue]

Your original post needs clarification.

You state, " I turned engine over seeing what was happening with the ground. I have continuity to ground (from socket hole #85) before doing anything. When I turn the ignition, the continuity of pin 85 to ground goes away thus the relay will never energize the fuel pump (i.e. pin #30). It stays gone until I turn off ignition."

When I first read your post, I understood it to mean the ground was lost when you were cranking the engine (bold, above), which is exactly what should happen. Later in the OP, you simply use the term "ignition" which implies the ignition switch in the ON or OFF position (not the START position), which would make the loss of ground at 85 a problem.

So, which is it? Does 85 lose ground when the engine is cranking, or does it lose ground as soon as the ignition switch is turned to ON?

Oh yeah, if the ground at 85 is gone with the ignition simply in the ON position, your fuel pump should be running if the relay is inserted. If it isn't, you likely have a fuel pump problem which you can confirm by jumping socket 30/87a with ignition ON--pump should run.

[T77911S]

the relay DOES work opposite of what you think,

when the relay is de-energized the FP runs.
its kind of a reliability thing. if the FP relay fails, the car will still run,


verify you have spark first, the check fuel.
one fuel check you can do is:
with the key on, lift the sensor plate. you should hear a loud squealing coming form the injectors.
then try to start it.
this verifies in one easy step that the FP runs.
it also does the job of the CSV and may get it started.