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Newly purchased 931 - fuel relay
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swj  



Joined: 09 Feb 2004
Posts: 4
Location: Lancashire, UK.

PostPosted: Mon Feb 09, 2004 7:37 am    Post subject: Newly purchased 931 - fuel relay Reply with quote

Hello! I'm new to the board having bought a 931 three weeks ago. It seems to start and go well but I've already used the info I've found here to sort out the alternator and put a new handle on (thanks)!
While getting myself acquainted I noticed a relay in the glove compartment! It turned out to be the fuel pump relay which had been 'replaced' by a wire connected 30 to 87. Might someone do that simply because the relay didn't work? How can I check it? Might someone have done that to deal with a problem that will start again if I replace the relay? Thanks for your help.
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Finbarr  



Joined: 14 Jan 2004
Posts: 85
Location: Worcestershire, UK

PostPosted: Mon Feb 09, 2004 8:56 am    Post subject: Reply with quote

Hi - I'm a recent recruit too. There's been quite a lot of discussion on fuel pumps and relays and the simple answer is yes, that may well be why there's a link. One guy even seemed to have the fuse bridged with a bit of wire as well.

I've had a couple of problems which at first appeared to be the relay, but were actually linked to the boost pressure cutoff switch which causes the relay to open if the pressure coming out of the turbo gets too high. First time, the wire had broken. Second time a bolt had dropped out (or been left out when the head was refitted) and the connection wasn't being made from the cutoff switch to ground. Both easily fixed if you understand the circuit but not at all obvious if you don't - the problem may have been related to this. The relay also has a link to the ignition, the idea being that if the engine stops running (e.g. a crash) the relay shuts off the pump as well.

Using that link in place of the relay prevents both safety mechanisms from working, so it's a very good idea to get to the reason why it was done and put it right.
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swj  



Joined: 09 Feb 2004
Posts: 4
Location: Lancashire, UK.

PostPosted: Tue Feb 10, 2004 3:04 am    Post subject: Reply with quote

Thanks very much Finbar - I think I'll go ahead and change the relay but wish I could test the old one before I buy. BTW, do you have the oxygen sensor on your car? I wondered if that was only for those sold in the US?

Thanks again.
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Finbarr  



Joined: 14 Jan 2004
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Location: Worcestershire, UK

PostPosted: Tue Feb 10, 2004 3:32 am    Post subject: Reply with quote

Nope, no oxygen sensor on mine. I think it's only on US models.

Seriously, I'd check everything you can before replacing the relay as it's not cheap - I actually got a bulb on a length of wire and used that to check the boost pressure cutoff switch while driving to make sure it was OK. That's how I spotted that the switch wasn't making contact properly, and from what I gather they're more likely to go than the relay.
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Smoothie  



Joined: 01 Jan 2003
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Location: DE (the one near MD, PA, NJ)

PostPosted: Tue Feb 10, 2004 9:06 am    Post subject: Reply with quote

There's usually a schematic of the relays' internals printed right on the relay. You should be able to check it with a multimeter and 12v power source, right? apply 12v to the contacts that correspond to the relays' electromagnetic coil and test for continuity between the switch contacts. Is this making sense?
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Finbarr  



Joined: 14 Jan 2004
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Location: Worcestershire, UK

PostPosted: Tue Feb 10, 2004 10:31 am    Post subject: Reply with quote

It makes sense for most relays, but the fuel pump is a more complicated beast as it expects to see a series of regular pulses from the coil in order to make it stay on. To test it, you'd need a running engine or something that can simulate the coil supply of same.
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Smoothie  



Joined: 01 Jan 2003
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PostPosted: Wed Feb 11, 2004 4:30 am    Post subject: Reply with quote

Interesting - I see there's something unusual there between terminals 31 and 31b - looks like it takes the coil pulses there - presumably from 12v coil primary...then a ground at 15? and measure for switch closure between 30 and 87. Maybe the coil primary could be simulated by a 12v AC transformer like might be found on a low voltage outdoor lighting system. It would have to be a transformer with 12v AC out, not DC to simulate the pulses.

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Alex Roy  



Joined: 03 Nov 2002
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PostPosted: Wed Feb 11, 2004 4:59 am    Post subject: Reply with quote

It would have to be AC and rectified to positive only, and that might be too slow anyway since that would only be equal to 120 rpm. Even better would be If somebody had a square wave generator that could output 12v.
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Smoothie  



Joined: 01 Jan 2003
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PostPosted: Wed Feb 11, 2004 5:06 am    Post subject: Reply with quote

Yeah - I was overlooking the fact that a coil just gets on-off positive pulses. Easiest then might be to just hook up 31-31b with some little clamp test leads to coil primary on a car that runs. Obviously could use the 931 with it's relay removed and jumper in place..
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gohim  



Joined: 02 Nov 2002
Posts: 4459
Location: Rialto, CA

PostPosted: Wed Feb 11, 2004 5:55 am    Post subject: Reply with quote

You need a spare fuel pump relay anyway.

Buy a brand new relay, and plug it in. If the engine starts and runs correctly, then I would not be surprised of the relay that you found in the glove box had a problem, but I would still plug it in to find out.

If the relay from the glove box is no good, you should throw it out, and buy another to keep as an emergency spare.

Personally, I keep one of each relay in one of the "saddle bags" in the cargo area along with a realy diagram, so everywhere I go, I will have spare relays, since you can't expect every little town you drive through to have what you may need.
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Finbarr  



Joined: 14 Jan 2004
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Location: Worcestershire, UK

PostPosted: Wed Feb 11, 2004 6:44 am    Post subject: Reply with quote

Interesting - the terminal numbers on the pump relay don't all match the ones on the wiring diagram. The ignition pulses are on circuit 1 on the diagram, which I can only assume goes to 31a on the relay (it also goes to terminal 1 on the tacho). 15 is the ignition supply needed to drive the relay, 30 is the battery live and 87 the output to the pump. 31 is ground, and on the turbo this is connected via the manifold pressure cutoff switch.

Given that the relay is designed to come on for a few seconds when the ignition is turned on anyway, Alex's idea of using rectified ac ought to work OK - the relay probably only needs a pulse every few seconds or so to stay on. I'd be a little nervous of putting a much higher voltage into the ignition pulse input than is coming from the ignition switch, though, as this might overload the input. A 9V transformer might be a better bet.
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swj  



Joined: 09 Feb 2004
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PostPosted: Wed Feb 11, 2004 7:38 am    Post subject: Reply with quote

Thanks for all the help, it really is something the effort people put into this board. My relay diagram doesn't look quite the same as Smoothie's - no 31b and the diagram itself is slightly different - I have a number 1 though! I think I'll check of the boost pressure cutoff switch and if that's okay get a new relay - I don't have the expertise to set up a reliable test. Did notice another job today - air horn is connected to first switch on the console just behind gear stick! I think the steering wheel is from another car and it was too difficult to get the horn working in the normal way so the PO wired it up like that. Had a great ride to work though.
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CMXXXI  



Joined: 05 Nov 2002
Posts: 1939
Location: Vicksburg, MS

PostPosted: Wed Feb 11, 2004 7:40 am    Post subject: Reply with quote

I'm pretty ignorant when it comes to electrical matters. How or why are 924 and 931 fuel pump relays different? This photo shows the 924(L) and the 931(R) fuel pump relay. The only difference I see is that the far right-hand connector is labled "1" instead of "31b". The pin layout is the same, the schematic is the same.

Now, to make matters even more "quizzical", the 433.906.059 relay in the photograph is the actual relay that has been in my 931 ever since I bought it in Germany years ago (never been changed in all the time I've run the car). Does the fact that mine is a 79 Euro without the electronic ignition and lambda and "stuff" make the difference and allow me to use the "924 part number" relay?

(Click photo for larger image)

According to the 924.org Tech Section, there is another 924 fuel pump relay, PN 477.906.059. Is there a difference between it and "the other" 924 relay?
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Finbarr  



Joined: 14 Jan 2004
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Location: Worcestershire, UK

PostPosted: Wed Feb 11, 2004 8:02 am    Post subject: Reply with quote

As far as I can tell, there's no difference at all between them. The parts catalogue shows the same part number for both models - 433 906 059 - and for all model years, so the only difference between these two units must just be the physical case size and the pin numbering.

However, looking at the wiring diagrams for early 924 models (up to 1978) shows a labelling the same as Smoothie's photo - 31 is connected direct to ground and 31a is connected to ground via a thing called an "air flow sensor contact". These early models didn't have a connection to the coil supply, so I'm guessing this air flow sensor is somewhere on the inlet manifold and cuts off the fuel if the engine stops.

I can only guess that the newer type of relay can replace the older one, so the original part with the old markings has been dropped and the new one specified as the replacement for all models.
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Smoothie  



Joined: 01 Jan 2003
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PostPosted: Wed Feb 11, 2004 11:03 am    Post subject: Reply with quote

Finbarr wrote:
looking at the wiring diagrams for early 924 models (up to 1978) ... 31a is connected to ground via a thing called an "air flow sensor contact". These early models didn't have a connection to the coil supply, so I'm guessing this air flow sensor is somewhere on the inlet manifold and cuts off the fuel if the engine stops.

Some models have a switch built into the fuel distributor. That must be the air flow sensor contact they're referring to.

Obviously my guess at those connections needs updating... say 15 to 12v(+)?, then 31 and 31b to the two coil primary terminals? -Or 31b to ground and 31 to one of the coil connections? Whatever - time for a look at the wiring diagrams...
Nice to know that it might be possible to use one of the cheaper relays in place of the $80 931 part.
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