Fuel pump relay with delay wiring question

[manoa matt]

I've got a 78 spider with a mechanical fuel pump. I want to install an electric pump with the oil pressure switch as the ground. I was going to graft in the wiring from a 76 that had an electric pump and relay, but I don't think it will do what I want.

I'd like to have it similar to the FI cars. When the key is in the on position the pump runs for a few seconds then turns off. When the car is started the pump runs constantly. I'm thinking I can tap into the seat belt circuit,either the seat belt relay, or the seat belt delay switch.

I know the relays can be triggered by positive or negative. The standard bosch relay is a single pole double throw design. Can this be achieved with a standard bosch relay, or will I need a double pole double throw relay?

How would this be wired up?

[So Cal Mark]

actually, Fiat is one of the few FI cars that don't run the pump for a few seconds when the key is turned on. The engine has to be cranking so that the switch in the AFM opens for the pump to run. If your carb has a return line, running the pump all of the time with key on isn't a big deal.

[lanciahf]

Matt,

I have done this in the past, What you need is a VW Fuel Pump relay, part # 321 906 059 C.

Below is information found from a VW site. Fuel Pump gets its signal from the coil and only runs when the car is running. The relay also "primes" the pump for a few seconds when the key is turned on.

Fuel Pump Relay with Connections to Coil

While converting a type IV motor to run in the '60 Double Cab, I found that I need to add a relay to control the fuel pump. I have found numerous solutions on the net, but decided to use the following circuit. I pulled two fuel pump relays at the local junkyard, one from an Audi and the other from a VW Rabbit. Both of these cars had Bosch CIS fuel injection. It turns out that both of these relays have the same VW/Audi part number. The Audi relay was made in Austria and the VW was made in Germany. Both relays were made by Siemens.

Part number: 321 906 059 C

I checked at the local parts house and a new relay costs around $45. I picked up both of these relays for $1.00/ea and with all the taxes was out of the junkyard with a total cost of $3.06

I opened both relays up and looked inside and found that the basis of the circuit is a NE555 timer chip operating in the One-Shot Mode. As long as input pulses are received by the relay faster than the RC time constant, the relay will stay on.

Calculating the time constant based upon the resistor and capacitor values results in a time of 0.52 Seconds. This allows the slow rotating distributor during startup to cause the fuel pump relay to turn on.

One interesting note that I found out while experimenting with the relay, is that the circuit requires that the coil be in the circuit. Just applying 12V to the trigger input is not enough. The circuit relies on the higher voltage pulse that is generated by the coil when the points open.

I hooked up a coil in the circuit and closed the coil circuit by hand (simulated points) and the relay operates exactly as expected.



As you can see by the connections, this is really quite a simple circuit to use. When the motor stops, the relay will open 0.5 seconds later and the fuel pump will turn off.

[manoa matt]

Lanciahf: So it stays on .5 seconds after the engine turns off? How long does it stay on with the key in the ignition in the on position when it's in the "priming" stage, if its only when the distributor is turning then that will not be enough time. Also, no pick and pulls out here.

I appreciate the responses, but there is a certain way I want it wired.

[Newell33]

I hate to hijack, but I've got a similar question. My '76 came with an electric pump. The previous owner had some things cobbled together. I noticed my electric pump is always on when the key is on. It doesn't shut off. What kind of system did the '76 have to prime the fuel system when the key was turned on? It really should turn off within a few seconds right? If so, what relay or components control that? Is it tied into the oil pressure circuit? All I have is a Haynes manual at this point and it doesn't mention anything about the fuel pump circuit. Does anyone have any input on how it's supposed to be wired from the factory?

Thanks,
Josh

[manoa matt]

Josh,

Go to this site : http://www.spidercenter.com/ and go to the "hot downloads" tab on the left column. the 76 wiring diagram is in there.

If your car was wired correctly, I'd ask if you pump primes, but I guess I can't. I think Ga, Craig also has a 76, but he may be in the same boat with the miss wired relay.

[lanciahf]

Matt,

Pump stay on for .5-1 second after ignition is switch on not after. The part is easily available new at a VW parts shop. I have bought new through Autozone and GermanAutoParts.com.

[manoa matt]

.5-1 second is no where near enough time for the system to prime. If the relay primed the system for minimum 5 seconds then I'd consider it. I want it to cut off with the oil pressure for safety reasons.

Tons of VW shops around, but I'm not paying $50 when I already have a $5 bosch relay that should work.

[lanciahf]

Sorry Matt my relay was only 20.00 new.

Good luck

[lanciahf]

Matt,

Using the seat belt/brake warning light timer is a good idea for a "prime" function. Do you know which wire you would you use?

[manoa matt]

I got it hooked up with the relay very similar to how the diagram shows. I had it hooked into the delay switch and it was working.

It would prime the system for as long as the seat belt lights were on (about 5-7 seconds) I had one wire going out from the relay to the oil pressure switch. Once the delay switch went off I went out and pulled the wire and touched to ground and the pump came on again as it should when the car would be running.

The problem was that when I spliced that wire in with the existing oil pessure light the pump would run constantly even when hooked into the delay switch.

I made a wiring diagram of how I had it wired, but its at home right now. I was getting frustrated the other day (sorry Lanciahf) because I kept blowing fuses and switching wires around. I was fortunate enough not to melt my entire harness.

I'm ok with simple wiring, but switching wires around on a relay is a good way to "let the smoke out of the wires"

[racydave]

Matt, I got kinda interested in this, did some investigatin. Normally the ECU will control pump prime. You may need to install some diodes to make it work. I still have some old GM manuals...

[lanciahf]

Matt,

Maybe I'll use a combination of my relay and the seat belt timer as a safety relay and longer primer.

[manoa matt]

I was just temporarly cliped into the dark blue wire with red stripe that is for the light bulb for the "Fasten seat belts" light on the dash.

[lanciahf]

Good to know,

Thanks