I am putting an electric fuel pump into an older car & want to use a fuel pump relay that will sense when the engine quits and shut off the pump. I know that many vehicles had this type of relay.
I'll probably get something out of a wrecking yard - I would prefer to get a relay AND socket. Any suggestions as to what vehicles to steal this from? Reliability & availability are the two obvious issues that come to mind.
Again, I'm not looking for a normal relay, but a "fuel pump relay" that remains active only as long as it is receiving pulses from the ignition tachometer line.
Are you building a relay control circuit that responds to the tach pulses or do you think that a relay can literally be run from the coil negative? The latter isn't going to work. Running thru an oil pressure switch would work much better.
I don't like the oil pressure switch idea simply because there may be situations where you make the educated decision to fry your engine to get out of harm's way. but that's just me.
The VW relay works like this; when power is applied to it (i.e. you turn the key on) it will run for 5 seconds and then shut itself off - this pressurizes the fuel rail for initial startup. Then when it starts receiving pulses from the coil, it will latch in until it no longer receives a signal from the coil.
Let's say you spin a bearing in the hammer lane of the freeway, or you run over some unavoidable debris that holes your oil pan. Oil pressure suddenly drops to zero. However, to get over to the shoulder, you still need power to merge through three other lanes of traffic... Also, with the oil pressure switch, you won't have immediate fuel pressure for lightning-fast starts unless you also incorporate some kind of timer running off the rising state of the ignition switch; the engine will have to crank over and build oil pressure before the pump will start feeding the engine.
If you or another poster wants scans of the factory diagrams showing how to wire it up, I can try to accomodate... I am actually contemplating just this kind of thing for my '55 Stude if I ever go with an electric fuel pump. (the stock mechanical pump is LOUD and I don't know why; I've tried several different pumps including one NOS. Works fine, but it sounds like a rod knocking at idle, which is bad.) So I've already taken a stab at puzzling out the factory diagrams (I have several Bentley manuals for various VW's I've owned) but haven't gotten as far as sketching a simple "retrofit" diagram. I think you probably just need switched power, ground, battery power, coil, and output to fuel pump, but I can't swear to it. i'd probably throw an "ice cube" at it to take the load off for a heavy-draw racing pump.
i'd be willing to bet that other mfgrs. use a similar fuel pump control scheme; I just happen to be *familiar* with VW's so that is what I suggested.
When the OP stated that this installation was in an older car, I assumed* it was a carbureted vehicle. (* TMOAFU's)
I'll take a look thru Mitchell on Demand and see if I can pull up a schematic.
Similar in that a tach signal is needed, but they usually rely on the engine computer to recognize it and thusly control the FP relay. That VW has this ability self contained in the relay is intriguing. A pre computer era example would be the Chevy Vega (spit) and its variants, carbureted engine, electric in tank fuel pump controlled by an oil pressure switch. A post computer GM car would have the FP relay and the oil pressure switch in parallel
So was I, honestly... my only carbureted car tends to sit for weeks at a time, so it takes a bit of cranking to fire it. That's a selling point to me of an electric fuel pump - no worries as to whether the battery is going to run down before it lights off.
Yup, it's all in the little relay. The downside is that the socket is generally contained in the main fuse block, so the OP's idea of using a pre-made socket would probably involve a hacksaw, unless a generic Bosch relay socket has the right holes in it.
It is a carbureted engine. I still don't want to use oil pressure (at all) as the means of detecting engine operation. Beyond that is the need, as Nate pointed out, for running the pump initially to refill the carb after sitting for a month or two.
My recolection was that when they were first used, the socket for these relays in some cars was an add-on - not integral with the fuse box. My trips to the wrecking yard so far have been futile - virtually all of the relays are being stripped out of these cars (& sold on ebay?). I suspect that they may be leaving the wrecking yard in jacket pockets.
I do now have a fuel pump relay Socket out of a Volvo, but no relay. From what I've seen, there are a number of different relays that Volvo used - I suspect that the later ones do not have the start timer built in. Anybody know which Volvo relay(s) do have the start timer & ignition sense built in?
I think it's more like every owner of a VW of "a certain age" wants a spare fuel pump relay to leave in the glovebox. Generally they can be disassembled and made to work again, but it's not something you want to do at the side of the road when it's raining and your car just shat the bed. So if you do go that route, I'd get two.
I thought the add ons were the repair/replacement. Ifin I remember, the Audi's, Rabbits, Jetta's and the like had a problem from the factory with water from the windshield/cowl, leaking on the fuse box and rusting the connections at the fuel pump relay. To get the pump to run, I think it was terminal #'s 83 and 57 you jumped. The factories recognized the problem and had a kit that included a new relay with a pigtail socket to relocate the relay to the fuse box, but on top. This was back in the 80's, so my 'memberin might be off a bit!
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