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Fuel Pump Relay circuit diagram
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Finbarr  



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

PostPosted: Thu Feb 12, 2004 11:09 pm    Post subject: Fuel Pump Relay circuit diagram Reply with quote

As promised last night, curiousity and nostalgia got the better of me and I traced the circuit diagram of the cheaper (supposedly 924 only) fuel pump relay (433 906 059):

Basically, it's a 555 timer IC set to turn the relay off after a certain delay, but which is being continually reset by the electronic equivalent of the opening of the points. When first switched on, the relay should energise for the time delay period (set by the 620K resistor and 1u5 capacitor to around one second) - then switch off again until the engine starts running. As long as a pulse from the points/coil is received at least once a second (corresponding to only half a revolution per minute!) the relay will stay closed.

By taking contact 31 to ground via the manifold pressure cutoff switch on the 931, power to the relay circuits is removed when pressure exceeds the safety limit and the fuel pump is shut off.

To continue with the discussion on another thread, it should be perfectly possible to test the thing using the rectified output of a small transformer fed into terminal 1. Alternatively, a pushbutton switch to +12V (terminal 15) will do the job as long as you can push it more than once a second.

Two things now make me curious - what does the more expensive part (931 615 113 00) do that is so much more complicated, and what exactly goes wrong with them when they fail? None of the parts are hard to find apart from the relay mechanism itself, so they should be easy enough to fix - although at the cheaper price it's hardly worth the effort.
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Finbarr  



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

PostPosted: Fri Feb 13, 2004 1:09 am    Post subject: Reply with quote

Duh. Just found some answers to my questions in the 924 FAQ (Technical) section:

"One special note for the 931: the 931 uses a special fuel pump relay, not common with VW's, and not surprisingly much more expensive. Due to the expense, the relay will often get replaced with the cheaper relay - which will work, for a while. However, the key difference is that the 924 relay is only rated for 12A, whereas the 931 relay is rated for 16A! So, if the incorrect relay is used, it will continue to blow. Time to bite the bullet and spend the extra money for the right part. Think of it as an investment in reliability."

I suppose it depends on how long the cheap one lasts, and whether you carry a spare...
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Smoothie  



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

PostPosted: Fri Feb 13, 2004 2:47 am    Post subject: Reply with quote

There's a misprint somewhere then, unless the FAQ is refering to the 477.__ relay being 12A.
CMXXXI's photo shows the 433.__ and 931.__ relays both rated at 16A:
http://www.924board.org/viewtopic.php?t=10331&start=12

'77-'78na cars have one fuel pump and use 477.__ (so 12A ?)
'79-up na's have 2 fuel pumps and use 433.__ (so 16A makes sense)
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'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox
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CMXXXI  



Joined: 05 Nov 2002
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Location: Vicksburg, MS

PostPosted: Fri Feb 13, 2004 7:07 am    Post subject: Reply with quote

Man, y'all are too much!! Hats off to all of you. I sure am glad that someone understands this stuff.
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Finbarr  



Joined: 14 Jan 2004
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PostPosted: Sat Feb 14, 2004 3:14 am    Post subject: Reply with quote

Actually, the photo shows the 931 part number as 16A, it doesn't show a current rating for the other. Mine (the 433 part) just says 12V on it with no current rating.

I wonder if the 12A relay they're talking about is the really old 477 part?

I'd be intrigued to take apart the 931 part and draw it's circuit diagram too to see what the difference is. Next time anybody blows one, send it to me! It would be interesting to see if it really does have a rev limiter inside as speculated elsewhere - with two chips inside, it's a possibility.
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CMXXXI  



Joined: 05 Nov 2002
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PostPosted: Sat Feb 14, 2004 3:29 am    Post subject: Reply with quote

No, the photo does show "12v 16A ind" on both relays
(click for larger image)
Also, as another "data point", I looked at the PET CD and note that only the US market 931, '80-82 calls for the "931" numbered relay. All model years of the ROW version call for the "433" part (no listing at all of the "931" number). This suggests that there is something peculiar with the US market version. I don't have a clue as to why they should differ when it comes to this relay.
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Lizard  



Joined: 03 Nov 2002
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PostPosted: Sat Feb 14, 2004 3:35 am    Post subject: Reply with quote

Finbarr I can tell you the 931 does have a rev limiter built into it as I bounded off it once with the relay in there and when I took it out and put a switch in there, there was no longer any rev limiter
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Finbarr  



Joined: 14 Jan 2004
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PostPosted: Sat Feb 14, 2004 3:57 am    Post subject: Reply with quote

Well, so it does. I had to up the brightness on the picture to see it, but it's there. Curiously, my 433 part doesn't have the markings printed on the side in white like that one - they're embossed into the end, and it just says 12V, like so:



OK, so now I'm convinced that the 433 part is good enough for ANY 931, and maybe the only difference is the rev limiter. Is there some legislation about them in the US? I'd still be keen to get my grubby mits on one and disect it. Seems daft for you guys to be paying more for a feature you probably don't want anyway!
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Lizard  



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PostPosted: Sat Feb 14, 2004 4:01 am    Post subject: Reply with quote

there is no law I am aware of on rev limiters anywhere
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AGD931  



Joined: 17 Mar 2003
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PostPosted: Sat Feb 14, 2004 4:08 am    Post subject: Reply with quote

finbarr,

sorry to throw a spanner in the works, but my 1981 931 (177PS) is fitted with 931.615.113.00

I thought I had a problem with the relay when the car suddenly died whilst driving, after a LOT of searching, it was traced to the rear of the fuse box where the spade connector for pin 87 had come adrift from the fuse box!!! All Ok now.

Arthur
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Finbarr  



Joined: 14 Jan 2004
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Location: Worcestershire, UK

PostPosted: Sat Feb 14, 2004 4:24 am    Post subject: Reply with quote

Hmmm... I still say the 433 will work fine in your car, it just won't have any rev limiter. Mine may have had it originally, of course, and had it replaced before I got hold of it.

I've got a similar problem with the rear window demister relay - the contacts have just about fallen apart, and it's going to take some careful surgery to put them back together. I'll do it the same time as I fit the extra fuses in the auxiliary fuse panel for the alam, stereo and electric aerial and tidy things up in general.

And you've backed up the statement I made in the original 931 Technical thread - there are a lot of things that can go wrong with that system ratther than the relay itself...
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Smoothie  



Joined: 01 Jan 2003
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PostPosted: Sat Feb 14, 2004 5:54 am    Post subject: Reply with quote

I popped the cover off my 931.__ relay and took some pics. As far as ic's, there are 2 - they're both the same Texas Instruments printed with "NE555P F8101B".

Click on links for larger pics -
http://www.geocities.com/smoothies931pics/HPIM2288.txt
http://www.geocities.com/smoothies931pics/HPIM2289.txt
http://www.geocities.com/smoothies931pics/HPIM2290.txt
http://www.geocities.com/smoothies931pics/HPIM2293.txt
http://www.geocities.com/smoothies931pics/HPIM2295.txt
http://www.geocities.com/smoothies931pics/HPIM2297.txt
http://www.geocities.com/smoothies931pics/HPIM2299.txt
http://www.geocities.com/smoothies931pics/HPIM2309.txt
http://www.geocities.com/smoothies931pics/HPIM2306.txt
http://www.geocities.com/smoothies931pics/HPIM2302.txt
http://www.geocities.com/smoothies931pics/HPIM2303.txt
http://www.geocities.com/smoothies931pics/HPIM2304.txt

Other than the IC's, there are 3 transistors, the relay, and your usual assortment of resistors, diodes, capacitors and 2 rectangular gold colored components with 2 connectors each that I'm guessing are also capacitors. Would there have to be another type of IC in there to count rpm's? -or can the 555 timers perform that function? Also, there is an adjustable component visible in the first pic (variable resistor?). If it does do rev limiting, I'd say that adjuster is used to set the cut-off rpm.
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Finbarr  



Joined: 14 Jan 2004
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PostPosted: Sat Feb 14, 2004 10:43 am    Post subject: Reply with quote

Great camera work! We could almost pick up your fingerprints from that...

Yes, the second 555 could function as a rev limiter. Basically, you trigger a time delay from the coil, and if another pulse comes in before the time delay has expired, you shut off the relay. Both the timers have to be happy with what they see in order for the relay to be activated and turn on the fuel pump.

If the preset has to be adjusted by hand, and two separate boards have to be assembled and soldered together, it would certainly explain the much higher cost for this part. And yes, those gold coloured things are capacitors too.
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dpw928  



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PostPosted: Sat Feb 14, 2004 12:28 pm    Post subject: Reply with quote

Not sure if this applies to the 931 but on the early US 928's the rev limiter is in the ignition module which cuts the signal to the relay at 6200 rpm. On the euro the rev limiter is part of the rotor which has a centrifugal spring switch cut off. Are the ignition modules the same for both the US and ROW 931's?

Dennis
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924 turbo  



Joined: 02 Nov 2002
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PostPosted: Sat Feb 14, 2004 12:32 pm    Post subject: Reply with quote

The '81 model supplement refers to the relay as "Fuel pump relay with rpm limiter".

Strangely, I remember my limiter still working with an early 924 fuel pump relay installed (the one with the fuse attached). Maybe I had the limiting rotor?
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