fuel pump with the key on

[18Fiatsandcounting]

I agree with you Steiny, and if you look back at my first response, the first question I had was whether it was an electric or mechanical fuel pump. I thought the electric fuel pump didn't come until later, perhaps late 1978 or 1979 or even 1980, but the only basis I have for this is my recollection of the various model years that I've owned or worked on.

The OP did verify that it was an electric pump, so my guess at this point is that this isn't original. Stranger things have happened, but yes, it could be that the DPO put in an electric pump and the wiring has gone goofy.

-Bryan

[aj81spider]

I'm not sure of anything relative to the fuel pumps. However my 1974 has an electric pump in the trunk. The wiring looks like factory wiring, and the wiring diagram I have for a 1974 shows a fuel pump powered through a fuel pump relay.

[18Fiatsandcounting]

Maybe you're on to something A.J. Wouldn't be the first time I've been wrong. Anyway, I have the wiring diagram for the '75 spider, and it definitely shows an electric pump (with a green-black wire feeding it) and a pump relay that does have a connection to the oil pressure sensor (gray-yellow wire). Perhaps the '74 was similar.

-Bryan

[fimrite]

The electric pump in the trunk definitely seems to be set up as if it was standard from the factory but I could be wrong. I thought I'd once seen a diagram of a electric pump as well. I'll be looking into this issue probably this weekend as I have to go birthday shopping for my kids tonight (procrastinated too long).
I appreciate all your inputs!

[SteinOnkel]

Brilliant! Another case of they installed whatever they had on the shelf that day*. How is anyone supposed to work on these cars?

*This type of nonsense cost companies like Anton Schlüter München their neck, by the way.

[18Fiatsandcounting]

Steiny, maybe your '78 is really supposed to have an electric fuel pump, and the esteemed DPO has struck once again...?

-Bryan

[spider2081]

Probably someone on Mirafiori.com would know what years the Carburetor Fiat Spiders used electric pumps and the logic for them. I think it was 1974 through 1976. They might have used them to get fuel to the carburetor faster as many Spiders were not daily drivers. Also Fiat was attempting to satisfy the U. S. market where driving conditions were different than what it was use to in Europe.

[rridge]

Electric fuel pumps on carbed cars are a fix for high underhood temps and vapor lock. The mechanical pump sucks from the engine compartment, lowering inline pressure between tank and pump and reducing vaporization temp. Once the fluid turns to vapor in the line sucking no longer moves fuel. The electric pump pushes from the trunk, increasing line pressure and raising the boiling point.

Mechanical pumps stop when the engine stops. Electric pumps just keep pumping. If the float valve sticks open and the carb floods the engine, or if the fuel line separates in the engine compartment the fuel just keeps flowing until the driver shuts off the ignition. It's a safety issue. Years ago Fiat set up factory electric pumps to cut off when oil pressure dropped to zero, even if the ignition remained on. Find the wiring diagram for your model year and trace the power back to the safety cut off and be prepared to deal with a stuck relay or a plugged oil pressure sender.

[SteinOnkel]

Steiny, maybe your '78 is really supposed to have an electric fuel pump, and the esteemed DPO has struck once again...?

-Bryan

I doubt it. He did kindly throw away the rear trunk liner. There's wiring for the sender and that's it. Also, no relay.

What does yours have, Bryan?

@rridge

Oh, don't get me wrong, the advantages of an electric fuel pump are there. But, don't fix it if it ain't broke. Even in 110* weather my car doesn't vapor lock, so I'm sticking with the ol' diaphragm pusher.

[spider2081]

Electric fuel pumps on carbed cars are a fix for high underhood temps and vapor lock.

Ahh Now that make sense thanks rridge

[18Fiatsandcounting]

Steiny, maybe your '78 is really supposed to have an electric fuel pump, and the esteemed DPO has struck once again...?

-Bryan

I doubt it. He did kindly throw away the rear trunk liner. There's wiring for the sender and that's it. Also, no relay.

What does yours have, Bryan?

My '69 and '71 both have mechanical fuel pumps as you would expect. My other Fiats, mostly in the early to mid 1970s range, also had mechanical fuel pumps. I had a '78 spider and it was also mechanical from the factory. I had only one 1980 and it was electric.

So, I really can't give you a definitive answer. Seems hit or miss to me. But, I can't argue with the wiring diagrams that show an electric fuel pump and a relay. But, at the same time, these same diagrams also show an electric radiator fan, whereas my '69 definitely had the electromagnetic relay on the end of the water pump shaft for the fan. About the only wiring diagram that I have found that is truly accurate for my '69 (and '71) is the one in the original owner's manual. All other diagrams out there on the WWW have issues.

-Bryan

[kmead]

1975 Spiders were the first year Spider to have an electric fuel pump. 1976 went back to the mechanical and it remained that way until 1980 when the fuel injected cars came to California and then the rest of the US.

Yes it was an odd one year only anomaly.

It was a well engineered solution that used, as pointed out, the oil pressure sensor to ground the run relay so if the engine stopped generating oil pressure the fuel pump would stop. This was done so that if the car was in an accident and the engine stopped running the fuel supply would stop.

Fuel injected cars used an input from the ignition coil to effectively do the same thing with the dual relay.

I suspect that fixing what ails the oil pressure light not coming on will resolve the fuel pump not working prior to starting.

Karl

[18Fiatsandcounting]

1975 Spiders were the first year Spider to have an electric fuel pump. 1976 went back to the mechanical and it remained that way until 1980 when the fuel injected cars came to California and then the rest of the US.

Yes it was an odd one year only anomaly.

It was a well engineered solution that used, as pointed out, the oil pressure sensor to ground the run relay so if the engine stopped generating oil pressure the fuel pump would stop. This was done so that if the car was in an accident and the engine stopped running the fuel supply would stop.

Karl, thanks for that info, and 1975 may be the one model year that I have not owned, which would explain my almost zero exposure to electric fuel pumps on spiders.

A question: I thought with the electric fuel pumps, they would turn on as soon as you turned the ignition key to "run", and quickly pump fuel up to the carb. However, before you start the car, there is no oil pressure, so wouldn't the electric fuel pump not run until the engine started? This would seem to present a problem as, if the car has been sitting for a long time (fuel drained back to the tank and float bowl empty), and you have weak oil pressure to begin with (old engine), there would be no fuel to the carb, and the electric fuel pump would not come on as there isn't enough oil pressure to trigger the relay. So, you would just sit there and crank and crank and nothing. Or is there enough oil pressure developed during cranking to eventually turn the fuel pump on?

Or am I missing something?

-Bryan

[kmead]

A friend of mine has a ‘75.

The system has two circuits, cranking and running which uses a relay for the oil pressure and connections to the ignition relay to make each one work.

Under cranking the pump will run briefly, then switches to the run side when there is oil pressure and ignition.

A rather painful wiring diagram of the system is available on MiraFiori if you are really curious.

Karl

[Kryger]

Just for the record, I have a 1974 CS1 with a electrical pump.