How did my 914 run?

chroma22 · 04-06-2005, 10:06 PM

Ok, here's a weird one...
I have a '74 2.0 914, and I have no idea of the history.
The PO installed a momentary switch (hold it down, it's on) in the dash which connects directly to the fuel pump. So the fuel pump only gets juice when the switch is engaged.
I just figured out this wiring detail a few days ago.
There are no carbs on the engine, and I drove this car home 2 years ago.
It's been parked ever since.
So the question is...how did I get home?
Doesn't the fuel pump need to run at least a little to keep the gas moving, even in a carb'ed system?
There are no other leads to the fuel pump.
One possibility I thought of is that the momentary switch was permanently stuck to 'on', but then the fuel pump would run all the time (even without a key in the ignition). I just don't know.

Could the FI be totally bypassed, and will the car still run (without power to the pump)?
My goal is to get the FI system working again, but if anyone could provide insight as to what may have been the logic behind the switch, that would be fantastic.

I bought a serious plate of spaghetti, so any help unraveling it is greatly appreciated.
Thanks,
bri

ps. Fuel pump and tank are out of the car, but the pump has been bench-tested and works.
It's an after-market Piersburg 1-in, 1-out pump.

Dave at Pelican Parts · Dave at Pelican Parts's Garage

It could be that the momentary switch is an override. The pump may also run when the key is on or...

Umm, you drove the car with no carbs and no fuel injection system on it? I don't think that's possible, unless the trip home was all downhill.

I don't think I understand your post. "Can the FI be bypassed"? Not really, unless you start removing the FI parts from the car. After all, air and fuel both have to get into the engine somehow.

--DD

chroma22 · 04-07-2005, 11:20 AM

Sorry Dave, I wasn't very clear.

I guess the question is, since the car does not have carbs, then the fuel injection must have been working at least to some extent, right?

I wasn't suggesting that I would like to bypass the FI, just trying to figure out what the PO may have done, why, and possible ways he may have rigged the system to get the car to run (like the switch).

I may just have to start tracing the FI wiring harness and see where everything goes, that may be the only way to really figure this one out.
The car appears to have much creative electrical work.
Thanks,
bri

Dave at Pelican Parts · Dave at Pelican Parts's Garage

It doesn't have carbs, OK. Does it actually have the fuel injection on it?

The momentary switch could be an override for the pump. As in, "it works normally, but when I want to force it to turn on I press this button". Which is one way of dealing with vapor lock, I suppose.

--DD

chroma22 · 04-07-2005, 12:06 PM

Yeah, it has the ECU and all, seems that the Decel is missing though.

I can understand that use for the switch, but there is only one positive lead to the pump, i.e. the switch is the only way to turn the pump on (and only temporarily at that). If the 'it works normally' premise holds, then where would the power for the pump be coming from?
That's the most confusing part to me...

wynsurf · 04-07-2005, 01:06 PM

I think you will find the one wire to the pump has two sources
that is further up the line 2 wires are spliced onto the one wire
going to the pump. One wire will goto the blue wire comming from the alternator regulator, so the pump will shut off when the
engine shuts off ( safety) . The other wire will go to the momentary contact switch. I've seen this in a schematic, but I'm
not sure the reason for it, to force the pump on for priming
when the motor is shut off, I guess. To prove you could
ohm out from the pump, to the blue wire on the volage regulator.

Dave at Pelican Parts · Dave at Pelican Parts's Garage

Sorry, Wynsurf, but you have a little bit of misinformation about the way the D-jetronic FI powers up the fuel pump.

One of the relays on the relay board tells the pump when to run. This relay gets half of the "pump now" signal from hooking into the circuit that sends power to the coil. It gets the other half of the "pump now" signal from the ECU, or the "brain" of the fuel injection system.

It is possible to jumper the relay to force the pump to run, even if the ignition is off. It is also possible, as you said, to splice into the fuel pump power circuit somewhere with a lead that provides power. This would let someone override the ECU's control over the pump, forcing it to run.

--DD

wynsurf · 04-07-2005, 03:48 PM

Oops, your right. I was thinking of an engine that had been
converted to carbs, and had the fuel pump rewired without
the ecu, sorry for the misinformation.

chroma22 · 04-08-2005, 01:24 PM

I am looking at the schematics for the car now, and it looks like the only lead (on a normal system) to the fuel pump should be directly from the Fuel Pump relay, right?

I'm a little confused by the annotations on the schematic, though. What do the little numbers mean at the pins of the individual relays? (86,30,85,87)

How is the 'Current Track' section of the schematic used?

The T's with a number above and below?

And finally the numbers directly on the wires?
Thanks, you guys are teaching me a lot.

Dave at Pelican Parts · Dave at Pelican Parts's Garage

See my reply to this question on the other thread.

--DD