924Board.org

[Minhiriath]

I'm been working on a electrical problem I have, I've narrowed it down to the fuel pump circuit, and after staring at the eletrical schematic in my haynes manual for awhile, I've decided to ask for some help, is there anyone out there who has a good understanding of the fuel pump circuit that can explain how it works (from how the relay knows to turn on etc..) what I'm really looking for is how the circuit is activated.

[Minhiriath]

additional info, the car is a 1980 n/a
there are 5 wires going to the fuelpump relay, a black and red one, a blue and read one, a white and green one,a black one and a brown one(ground) the red and black one goes through the fuse 2, 25 A on the additional fuse board, the blue and red one goes to the battery. the green and white one goes to the what would have been the coil, and also the tachometer. and the black one goes to C15 (c is the white plug on the back of the fuse box). when I have the key in the "on" position. the red and blue wire has 12 volts, the black wire has power 12 volts. the green and white one has 15 millivolts. and the ground is a ground. in a properly working circuit. what should the green and white wire have for a voltage reading?. (btw when I bypassed the whole relay the fuel pump came on ... and stayed on which is unusual. normally when I turn the key on the fuel pump comes on for about 2 seconds then goes off) any ideas?...

thanks

[924RACR]

Of course it stayed on - you jumped the relay!

Now for more useful help.

Buy a new fuel pump relay, and be glad you don't have a turbo - they're double the cost (or more) of a 924 fuel pump relay. The circuits sound fine, and you've verified that the wiring to the pump (and the pump itself) are in working order. All you need now is a working relay.

_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Minhiriath]

the assumption I made is that the pump would turn off once the desired fuel pressure was reached.

[924RACR]

It won't.

_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Minhiriath]

how is the fuel pressure controlled? .... I always assumed it was that way becuase when I turned my key into the on position I heard the fuel pump come on for 2 seconds then shut off ....

[ltgland]

the pump (some cars have 2, 1 in tank, 1 exterior), runs all the time while the engine is running, if it stops, no fuel to engine, no moving....

The pressure is controlled by a pressure accumulator. Some cars have 2.
On a single car its a 40cc (I think)
On a double, 2 20's

The Haynes manual should tell you all of this, and if you can get under your car, its easy to see.

tank -> fuel pre-pump --> exterior fuel pump --> accumulator --> engine. (in mine).

My independant specialist informs me that the accumulators are quite robust (no rush to replace them).

With an old fuel system a check valve leaks(dont know where, either pump or accumulator), this lowers the pressure, and makes "hot starting" a problem. You either need to crank a lot (1 minute). I have a manual override switch for my fuel pump. When it struggles to start (keeps turning over but not starting), I run the pump for about 40 seconds to get the pressure up. Usually starts after about 10 seconds then.

[924RACR]

Sorry; close, but not quite right, as you did omit a few parts. After the pump, fuel goes to the accumulator, then up front to the fuel filter, then the fuel distributor (through the pressure regulator) and the control pressure regulator. Operating (system) pressure is controlled by the pressure regulator in the fuel distributor; control system pressure is established by the control pressure regulator (bolted to the back of the intake manifold). Excess fuel is then retuned to the tank.

The Haynes is oversimplified.

Accumulators are replaced if externally leaking; otherwise if a hot-start problem exists and the check valve in the external pump is good (or non-existent in earlier cars) and the fuel injectors are not leaking. You are correct in the source of the hot-start problem, in that pressure is not maintained at the engine end of the system.
Minhiriath - get a relay. If you don't believe me... well, I did write the FAQ...

_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Minhiriath]

heh, thank you for all your advice so far vaughan and others (btw vaughan, I was simply trying to get a better understanding of the system. and once you said relay, I went about trying to get another) ... I've now put this new(from my other 924, not a brand new one) relay in ... and the car starts. and the fuel pump comes on ... but I have some weirdness going on now the relay seems to be coming on .. then going off ... the coming on .. and going off again about once every 2-3 seconds ... I don't recall hearing all the ticking before .. any ideas on this one ?

[escholte]

Thanks for putting this up in the forum, we're struggling too.
After doing some reading here and in the FAQ, i think we too need to put a bypass switch on the fuel system.
Since our '82 924 is not used daily, we too suffer from starting problems after the car is not used for a few days. (from about threedays and on) We have already changed both the fuel pump and the relay (that even twice) and now have the situation as described by topic starter. The pump runs for two seconds but that is not enough to get the fuel presure up as i read in 924racr's replies.
We also thought that there might be some safety in the circuit that might stop the pump when no pressure is build up in the system fast enough. This might be for the fuel pump to stop when the fuel system is leaking, and thus not building up pressure (wich it should do in the previous mentioned two seconds) If the pressure drops during long stays, and if this (switch?) device is defective that can be the source of the problem. I understand such an electric device is not in the car.
Please note that our car is originaly German, and might differ from the American version. My assumption now is that both regulators are mechanical and do not control the fuel pump in any way. Am i right 924racr?

How does the cold start valve in any way add to my problem? And should the fuel pump run when cranking the engine?

[Minhiriath]

My car runs fine .... at idle at anyrate .. buts its kind weird having the relay constantly clicking ... and I'm sure that once its starts needing more fuel it'll start being unhappy. so I was kinda looking for vaughins opinion on the issue. if all else fails I'll just run switched power to the relay but I don't want to splice into the electrical if I don't have to.

[escholte]

From what you tell, it seems your car is getting all the fuel it needs. That's why there is a return line in the fuel system. Maybe checking the behaviour of the cold start valve will shine som new light on the subject? (For me that is)

_________________
Still thinking of a good sig...

[ This Message was edited by: escholte on 2002-07-18 18:51 ]

[924RACR]

The relay from the other car... hmmm, it may be flaky or for a different year; the lead from the tach shuts off the pump when the distributor is not turning to prevent fuel spills in case of accident. Earlier cars had different systems for the same purpose, such as a switch in the air flow meter.

I've had a similar result (relay switching rapidly) when trying a standard relay in the fuel pump relay socket for diagnostics; the relay also heats up very quickly.

I've put in a dash switch in the racecar for the fuel pump rather than have to crawl under the rollcage to get to the relay, but I don't recommend it for a street car.

Minhiriath, your car should be fine when you get the right relay.

escholte - the cold start valve operation seems to be your problem, since what you're describing is a cold-start problem. It should inject a pulse of fuel into a cold engine. Either the wiring to it and the thermo-time switch (which triggers it) is bad, or perhaps the thermo-time switch is bad. The thermo-time switch is located in the back of the head under the heater hoses -very hard to get your hands on!

Pull the connectors off that and the cold start valve and check them, with a multi-meter, make sure they're wired together as per the wiring diagram. From memory, 1 wire on the CS valve should be grounded, the other should be connected to the thermo-time switch. Then the other lead on the thermo-time switch should be +12VDC switched to ignition.

If the thermo-time switch is working correctly, you should have a short burst (<10sec) of power to the cold start valve on a cold engine (length of time based on temp).

_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[ltgland]

Vaughan said

"Sorry; close, but not quite right, as you did omit a few parts. After the pump, fuel goes to the accumulator...blah, blah, blah... Excess fuel is then retuned to the tank."

Dude, for the mechanically declined like me, those bits in the engine bay qualify as the engine . Anything besides VERY basic that happens in there is something I stay well away from.

[Joes924]

MM what can I say Vaughen was there again.