Wiring challenge and what does this do !?

Sharkbitemarky · 03-01-2019, 06:35 AM

So my 76 body shell is due back from the spray shop any day, so while its been away I have been trying to clean/refurbish some of the first bits that will need to go back in to it, namely the wiring looms. I have tidied up lots of bits and given most of the looms a good clean however, I am stopped from moving forward because of the following:

In the main fuse box I have an original Brown/Red wire that has historically been spliced with a Brown wire (Not original) the Brown wire goes to Ground/Earth via a switch that is mounted in the dash in place of the Headlamp washer switch (Not fitted on my car), I have traced the Brown/Red which just outside the fuse box area continues through a T shaped 2 way connector with a yellow wire where the Brown/Red wire changes to Brown/Black the other side of the connection, I have traced the Brown/Black to the back of the Air Flow sensor assembly where it plugs in the side of a 2 pin connector the other wire being Brown at this connector.

So my questions:

a) What does this connection (See photo) do on the side of the AF sensor ?
b) Why would somebody feel the need to put this switchable grounding capability in the circuit ?
c) Both the Brown/Red and Brown/Black appear to be original however, I can not identify either color/wire on the diagrams I have for a UK/Euro RHD 2.7 911 1976 Coupe SPM. Do I have the wrong diagrams ? I have 911S, USA, Model 76 current flow diagram and 911 T,E & S wiring diagram SL35. If not where can I get the correct diagram/s.

As you can appreciate very difficult making progress here and hoping this forum can save the day again.

Cheers
Mark

T77911S · 03-01-2019, 06:40 AM

sounds like someone has bypassed the "safety" switch in the AFM to turn on the FP.

Sharkbitemarky · 03-01-2019, 07:40 AM

T77911S

So its just a switch in the AFM, is it Closed/Open for flow or no flow ?

Why safety ? are you suggesting the the FP should only run when there is air flow ?

Cheers
Mark T

yelcab1 · 03-01-2019, 07:53 AM

The safety switch turns off the fuel pump when the engine is not turning over. The reason to bypass is to have a manual way to test the fuel pump circuit when the engine is not turning on at all. Although, there are better way of doing that than to provide a bypass ground like that. But, that was a lot of work that person went through to implement the switch so there must be other reasons for it.

Sharkbitemarky · 03-01-2019, 08:24 AM

yelcab1,

Many thanks, could it have been some kind of basic immobilizer ?

I want to remove it and reinstate the original wiring, but worried as the car started first touch of the key and ran very well with it like this !

Thanks
Mark T

T77911S · 03-01-2019, 10:19 AM

that switch lets the FP run with the engine off but key on.
could be it was installed because of hot start issues, cold start issues or the FP was not running at all.
I would test the residual fuel pressure and see if it is good.

kev01 · kev01's Garage

Wiring can certainly test a persons patience. I am also busy with a 1976 911 (930). Does anyone have a wiring diagram that can assist?