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morghen · Joined: 21 Jan 2005 Posts: 8884 Location: Romania

The relay gets its command from the IGNITION (key) and from the COIL(spark) so when the ignition is ON the pump should run for a few seconds and if you dont crank the engine the Beta 555 opens up the circuit and the pump stops.

My question is: what kind of signal should i be expecting from the COIL (terminal 1) ? when i switch the key on, the signal at terminal 1 is 12V constant but when i crank the signal gets variable.

The signal at terminal 15 (ignition) is 12V constant when i turn on the key and remains 12V constant when i crank.

I did not find a replacement relay so i'm trying to repair the old one...for now i've changed the Beta 555 bug in it and it still does not work....the problem seems to be one of the transistors....probably the one comanded trough port 15(ignition key)
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morghen · Joined: 21 Jan 2005 Posts: 8884 Location: Romania

anyway..i guess that while the coil generates some signal..any signal...the beta555 wont open the circuit so it wont cut the pump off.

oh i hate electronics !!
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morghen · Joined: 21 Jan 2005 Posts: 8884 Location: Romania

i could not find a replacement relay so i'm going to let this go.
I'm going to mount a relay controled only by the switch ignition and thats it.
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!tom · Posted: Thu Jul 19, 2007 7:33 am Post subject:

I would guess you'd get 2 pulses per revolution on your signal wire. It seems obvious the intent of this circuit is to kill the fuel pump if the engine speed goes below a certain RPM.

I'm not exactly sure what your beta555 is, but the 555 IC is a very popular 8-legged component that is a timer circuit. It was apparently among the first IC's to be commercialized. For the IC to work, it relies on an external capacitor and resistor to set the time constant. I'm expecting your beta555 is this same component.

I'm guessing the output of the 555 timer is driving a transistor, which in turn energizes the relay.

You should find lots of examples of how to use the 555 timer with searches -- I'm sure the datasheet will have some simple examples too.

So, either the signal input to the 555 timer no longer works, the r/c circuit is fried, or your transistor is cooked that energizes the relay. My bet is it's the drive transistor -- it's possible the flyback diode (which protects the transistor from the back-EMF the relay generates) has failed, which in turn exposed the transistor to failure.

Since I have an older car, and hence a different operation for the fuel pump relay, I'm just guessing at how your part might work. I've never seen the type of fuel pump relay your car is supposed to use.
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78 924 NA
5-lug

morghen · Joined: 21 Jan 2005 Posts: 8884 Location: Romania

yep...its exactly the way you described it.

I cant find the damn transistors for sale....they probably are the problem here...i think they got spiked or something.

i'll try to find the transistors tomorow but if i cant i'll just use another relay and only use the signal from the ignition key to switch the relay with no switch-off circuit.
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!tom · Posted: Thu Jul 19, 2007 8:21 am Post subject:

Seems to me like you should be able to find a transistor that should work fine quite easily. There's no requirement that you use the exact same type. Just make sure you drive it properly and it can handle the current. Pretty much anything should do. You could even replace it with a FET I'm srue.
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78 924 NA
5-lug

Druman · Joined: 19 Mar 2007 Posts: 419 Location: Middleport, Ohio

Don't forget to use a PNP if it's a PNP in the car or a NPN if it's a NPN, otherwise it won't work.

7osh
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85' 944 "early" NA (Red) 130K miles

PORSCHE pisses excellence...

!tom · Posted: Thu Jul 19, 2007 8:51 am Post subject:

"Just make sure you drive it properly" was intended to encompass that, among various other issues.

The missing pulse detector sounds like it's pretty similar to the fuel pump relay in the later 924's.
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78 924 NA
5-lug

PORSCHEV · Posted: Thu Jul 19, 2007 9:19 am Post subject:

What does the relay look like..I have a few spares.
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1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.

1978-#53 "D" track racer.

RC · Joined: 25 Mar 2007 Posts: 2636 Location: Australia

Never having any experience with these relays myself I can only state that from the description given that !Tom`s info is accurate and he knows what he is talking about.

I can offer some common substitute transistors if you are able to provide the numbers on the transistor, as I have databases of specs of all registered semiconductors. However most reputible electronics shops do also and could supply one immediately. The exception here is unregistered, Asian, or specific manufacturers unique numbers, but a reasonable overspec substitute could be estimated.

If you at least have a meter with a diode (forward V) function, suggest checking the transistor first anyway.

The easiest and possibly cheapest foolproof option is to merely replace the whole relay.

Roger

Paul · Posted: Thu Jul 19, 2007 2:20 pm Post subject:

Lots of info here:

http://www.924board.org/viewtopic.php?t=10360&highlight=rpm+limiter
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Silver 98 986 3.6l 320 HP "Frank N Stein"
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!tom · Posted: Thu Jul 19, 2007 4:03 pm Post subject:

That's great, but what happened to Smoothie's pics?
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78 924 NA
5-lug

RC · Joined: 25 Mar 2007 Posts: 2636 Location: Australia

Good lead there Paul.
Really useful info there when you need it, makes it easy when someone has done the hard work and layed out the circuit.

The relay driver transistor BC337 is a relatively common one. It lists at 25c trade here. Should be available in most electronic shops but will have literally thousands of suitable alternatives.

Basic specs are:
NPN silicon
50V, 0.8A, 0.625W, 100Mhz

If your local electronics store cant give you a suitable replacement for it they shouldnt be in business!

Also suggest replacing the 24V zener diode, that clamps any spikes across the coil. Simply Zener diode 24volt and go for a 1 watt although 400mW will do. eg. 1N4749

Roger

PORSCHEV · Posted: Thu Jul 19, 2007 9:13 pm Post subject:

Reading threads like this one just make me feel stupid...how do you guys know this shit???

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1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.

1978-#53 "D" track racer.

Smoothie · Posted: Fri Jul 20, 2007 2:20 am Post subject:

Here's the predecesor post to the one that Paul linked to - http://www.924board.org/viewtopic.php?t=10331
-A bunch more back and forth there to figure out what was going on with the fuel pump relays. -But to sum it up, there were basically 3 versions - a 12Amp version for the early 924NA that had only one fuel pump, a 16Amp for use with the later two pump setup, and a 16Amp with built-in rev limiting for catalytic converter equipped 931s. They're all interchangable (obviously excepting use of the 12A relay in a car that has 2 fuel pumps - which would be unwise)(and of course using an non-rev-limiting relay in a 931 that needs the rev-limiting feature which would leave you without rev-limiting - again unwise). The 16A 924NA fp relay is available from www.thepartsbin.com for approx. $25 new and the 931 rev-limiting version is over $100.
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