TQ: Fuel relay failure + more

[PaulHere]

Spoke to member Roger Hinchcliffe (who raced cars in the seventies) a day or two ago and had a great chat about his car.

(I'll paraphrase (is that the right expression) the key points at first and add more to this post when I get more time!)

Bought his 1983 2.0 in July 05
Unused for 8 years!
Spent time and money recommissioning the car.
Pirelli P3000 tyres transformed the handling

Has problem with fuel pump relay blowing every 2-3 months, has anyone got an idea why this should be so?
Friend says relay is a "universal type 555" and can be found in millions of things all round the world and is the simplest of relays.
Friend wants to know more about what signals does the unit send and receive?

 

[924nutter]

I am prepared to be corrcted, but I can't see how a universal relay is going to function correctly as a fuel pump relay. I have expounded the following theory before, and the more I think about it the more sense it makes. The fuel system in a 924 has to have a method of confirming legitimate demand for fuel, so as A) not to flood the engine, and B) not to continue to pump fuel from a ruptured fuel pipe in, for example, the case of an accident, where the engine stalls and the ignition switch continues to suppply the fuel pump. On the earliest of the K-Jetronic (incidentally this is a misnomer because there is very little "electronic" about the system) an earth connection was provided on the body of the metering unit. When the metering plate was raised by by airflow through the engine, the earth was broken and the fuel pump relay became energised. In this case a universal relay might work for a while but I dont think it would be rated at at least 20 amps continuous, which would be the order of rating for a fuel pump protected by a 16 amp fuse; Sooner or later it will die.
Unfortunately if the car was inverted in an accident the plate would also lift and so a different method of establishing demand was needed, and later models have a connection to terminal 1 of the coil, which form part of an electronic circuit that monitors applied coil voltage to determine if the ignition module is at least switching suggesting the engine is being told to start. I very much doubt if the universal realy has this facility either, but as I say, I am prepared to be corrected.

 

[rhitchcott]

My relay kept going too. In fact I've had quite a number of problems I've solved but can't remember the solutions. Let's see,um, the fuel pump fuse kept going too, and that's been solved.

I know the fuse contacts needed cleaning/sanding and I had a new fuel pump. I think the old pump was drawing too much current.

Of course there's the statistical solution - random events tend to happen close together (I remember seeing the proof of this once in a maths lecture but of course I can't remember the reasoning) so if a relay randomly goes it's more randomly likely that the next one will go too rather than last. I've found this a lot with Porsche bits. Now Porsche would no doubt say it just verifies the Maths lecture but I have another theory which says what many have said about Porsche bits at some time or other..

 

[924nutter]

Somewhere in the FAQ, or in previuos posts there is an answer by myself to a previous post as this has come up before. When the fuel pump is really on its last legs, drivers may experience misfire under acceleration due to the pump's inabilty to provide full fuel pressure. You are only days away from abject failure of the fuel pump, and the current being drawn melts the insulation on the pump wires at the fuse box, and the area around the terminal where the relay plugs in. I ended up using two 30 amp teminal blocks, to connect in new cable, and replacing the relay plug which in fact just clips to the top of the fusebox.

 

[Fred Hindle]

I did here that at one point in the 924 production Porsche had to use an inferior relay for the fuel pump. Replacing the relay with the correct one cured the problem.
Don't ask me part numbers, I'm like a deer with no eyes!

Cheers,