Fuel Pump Relays

patkeefe · 09-08-2006, 07:05 PM

Maybe I've been staring at my wiring for too long today.

How many fuel pump relays does a 78 SC have? I thought one. So, if that's correct, what is the relay in the engine compartment, on the CDI carrier panel, which is pinned 30-85-86-87-87a. It's the only relay back there. I have a fuel pump relay where Bentley shows it, on the fusebox in the trunk. I don't see this relay on the electrical diagrams. It's been there for as long as I've had the car.

This is a new wrinkle in the fuel pump runs constantly problem.

Thanks!
Pat

Early_S_Man · 09-08-2006, 08:15 PM

Probably your rear window defogger relay ... unless you have a two-position switch for the rear window defogger!

Gunter · Gunter's Garage

One fuel pump relay Bentley SC Repair Manual 970-9.
Pump runs constantly? Check the "Airflow Sensor Contact", it's a switch in the back of the airflow/FD unit. Bentley 970-9.

patkeefe · 09-09-2006, 07:31 AM

I rewired the engine harness while the engine was out. Took out all of the unnecessary stuff which I no longer use. Fixed some questionably overheated wires, made sure the wiring was intact. Now, if I plug in the relay, I get no fuel pump at all, so I have to jump it. The airflow sensor contact checked out ok on voltage and continuity, as did all of the CIS elecrical components before I reinstalled the engine. However, with the airflow switch and the relay plugged in, raising the plate (opening the AF switch) does squat to bring on the pump.

Evidently, according to Bentley, I have a later model SC relay/fuse panel in the left rear engine bay. The only two relays back there are heater blower and rear window defogger. The wiring to these early and late relay/fuse panels are different. It doesn't help that someone had run some extra wires back there for GOK what, which I have yet to shake out. My confusion was that the pins on the window defogger should only have 30-85-86-87, just like the heater fan relay. In the grand scheme of things, there must be a significance to the 30-85-86-87 stuff. Do the numbers designate a control action being performed between terminals?

Additionally, I have been so busy trying to get the turbo set up, and the water injection system wired in, that I have not yet had a chance to dig through the 14 pin connection back to the front of the car. I did check most of the grounds, and they all seem OK. I will investigate further today.

Thanks to you guys for the help!!
Pat

Early_S_Man · 09-09-2006, 09:12 AM

Quote:

My confusion was that the pins on the window defogger should only have 30-85-86-87, just like the heater fan relay. In the grand scheme of things, there must be a significance to the 30-85-86-87 stuff. Do the numbers designate a control action being performed between terminals?

Pat,

Yes, the presence of only four terminals, and #87 being one of them ... means that the relay is only using the NO [normally open, or MAKE contacts] set of contacts and not the #87a NC [normally closed, or BREAK contacts] contacts.

patkeefe · 09-09-2006, 09:53 AM

Thanks Warren
So, at start, the FP relay energizes because of the completed circuit through 85-86 and pulls in the 30-87-FP circuit? And, during run condition, when the AF switch opens, it drops out the 85-86 circuit to go to NO through 87a? OK, now I should just troubleshoot the relay circuits, which should narrow down where the problem is. Then, either the AF switch or the Rev Limiter is still grounded to get no flow through 87? Or something really weird is taking place.

Those EE profs told us ME guys to pay attention! "The world won't be pulleys and gears forever".

Thanks again.
Pat