924Board.org

John Brown · Joined: 07 Nov 2002 Posts: 903 Location: Leesburg VA

FWIW, I have posted pics of the two relays. You can see that the 924 is much simpler consisting of a single pcb featuring the venerable 555 timer ic; while the 931 is 2 pcbs with 2 1455 ics. Why the extra comlexity is anybodies guess. I always wondered if the cheaper part would work.

http://users.adelphia.net/~johnbrown3/parts10.htm

http://www.meridianelectronics.ca/gadgets/mc1455.htm
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John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

Smoothie · Posted: Thu Feb 12, 2004 3:39 am Post subject:

Clearly I got carried away there overthinking the testing procedure. While it might be interesting to know how the thing works, all that "what goes where" isn't needed to test it. Obviously, if the car runs with 30-87 jumpered and it doesn't run with the relay installed (and it's the correct relay for the car), somethings wrong with the relay. Just remove the jumper and plug the relay in - that's the test.

Well...except for the fact that the relay can be good, but the wiring to terminals 15, 31 and/or 31b(1) might be bad.

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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Lizard · Posted: Thu Feb 12, 2004 3:47 am Post subject:

smoothie are you trying to confuse everyone?
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3 928s,

Smoothie · Posted: Thu Feb 12, 2004 8:15 am Post subject:

Hey! I got a sudden burst of energy..

For a 931, relay wiring:
31b (or 1) - to coil primary terminal #1 primary(-)
31 - to overboost pressure switch, then to ground (ground for testing off the car)
15 - to 12v(+)
30 - to battery 12v(+)
87 - to fuel pump fuse 25A, then on to both fuel pumps

For '78 na:
31b - to airflow sensor switch
31? - to ground (no terminal number given - guessing 31)
50 - to 12v(+)
15 - to 12v(+)
30 - to fuse, then on to 12v(+)
87 - to single fuel pump

For '79, '80, '81, '82 na:
1 - to coil terminal #1 primary(-)
31 - to ground
15 - to 12v(+)
30 - to 12v(+)
87 - to 25A or 16A fuse, then on to fuel pumps

Connections go to the same places for '79-'82 na and 931's, so it still looks like the cheaper na relay could work in a 931.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

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

The plot thickens! I've just checked my 931 and it's got the cheaper relay in (and always has as far as I know) - 433 906 059.

Inside it, almost to spite the words of John Brown, is a single PCB with a 1455 IC instead of the old 555.

The merest mention of the 555 has brought back so many memories of electronic projects from years gone by (sighs wistfully) that I'm going to make it my mission to trace the circuit and figure out how it works.

Somebody stop me if it's been done already!
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1980 931 (with the odd electrical quirk...)

Smoothie · Posted: Thu Feb 12, 2004 9:39 am Post subject:

Very nice. At $21.64, I think I can afford to carry a spare.
433.906.059 is available from www.thepartsbin.com for $21.64 as part number P2037-20302.
-And if that price is too good, you can get it from www.NAPAonline.com for $34.99 as part ECHAR292.

The gold plated 931 relay 931.615.113.00 made of unobtainium is at thepartsbin for $96.90.

The -'78 na relay 477.906.059 goes for $36.01 as P2037-20749 at thepartsbin.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

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

Ah ha! So that's why the one I found in my glove compartment is the cheaper version too - thanks Finbarr.
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1980 931

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

This "revealation" that the relays do in fact interchange should be good news to all us 931 owners. But curiosity still makes me ask "why are they different inside"? As John wrote: "Why the extra comlexity?" (I'm just glad that there are those of you out there that can make sense of all the electrical stuff. I can deal with the car's wiring diagrams in the manuals, but much beyond that, and it's all voodoo).
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'79 Eurospec 931

Paul · Posted: Fri Feb 13, 2004 9:51 am Post subject:

Maybe the more expensive one has a rev limiter. My car shuts down slightly above red line (80 931). I'm still using the factory installed original fuel pump relay.

Smoothie · Posted: Fri Feb 13, 2004 10:31 am Post subject:

Wow

, the plot just thickened again.
But what about NA's - don't they have rev limiters?
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Lizard · Posted: Fri Feb 13, 2004 10:55 am Post subject:

the 931 rev limiter is the fuel pump relay,
the 924 NA doesnt have have a rev limiter, yah shyadup my peddle got stuck down once.
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3 928s,

numbbers · Posted: Fri Feb 13, 2004 11:15 am Post subject:

No, Lizard, thr '80 931 rev limiter is in the rotor in the distributer. It has a spring and steel ball contactor under the epoxy filled part. At approximately 6300 revs, the centrifical force causes the steel ball to overcome the spring tension, and open the spark circuit.
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1980 924 Turbo

Paul · Posted: Fri Feb 13, 2004 11:15 am Post subject:

Smoothie · Posted: Fri Feb 13, 2004 2:22 pm Post subject:

Well, the jury's still out (for me at least) concerning where the rev limiting takes place. Not too sure about the "spring theory" - it's entirely possible, but the rotor for my car has something encased in epoxy, but it's a resistor for radio noise suppression. Is the spring and ball mechanism visible? -or is it encased in the epoxy? I know some earlier, pre-924 Porsches used a rev limiting rotor, but that would cut off spark on the coil secondary. I believe you'd want to affect the coil primary circuit which in turn would deactivate the fuel pump relay and cut off the fuel supply. According to a Bosch book I have here, fuel is cut off to protect catalytic converters nowadays, while in the past cutting spark and allowing unburned fuel into the exhaust was ok.
As for the limiter being in the relay - that's possible too, but I just don't want to believe it's so.

-And after a little reading, I couldn't find anything definative, but this excerpt from "Porsche 924 928 944" by Jerry Sloniger talking about the Siemens-Hartig (DITC) suggests rev limiting is handled there (by the DITC): ", more sophistication in this case came with lower maintenance while the 1981 rev-limiter operated electronically."

In any case, it still makes sense to carry a 433.__ relay as an emergency backup in a 931.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

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

Here's another "data point"...
I looked on the PET CD, and see that for ROW 931, only the 433.906.059 relay is listed for the 931 model years '79-85. No listing at all of the "931" numbered part. For US models '80-82, the "931" part number is called for in the 931, and the "433" part is called for in the 924. This suggests that there is something peculiar with the US market setup.
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'79 Eurospec 931