924Board.org

[Eliza]

Hi all!

I am looking for somebody who lights up inside at the mention of "NE555"

Reason: I have been searching on and off for years about our tachymetric fuel pump relay and its priming feature at ignition-on.

Not much info available on its OEM operation, also on all other forums dealing with K-Jetronic/CIS-equipped cars with hall sender ignition.

HOWEVER, I have found on the interweb a thread that seems to suggest that the FPR priming feature ( or in fact, the initial duty cycle an NE555 performs at power-up ) can be forced to occur every time by slightly changing the layout of the existing relay board, allegedly by adding an extra diode and a resistor to forcibly discharge the timing capacitor during a cycle and thus consistently getting trigger 2 to LO and output 3 to HI at power-up.

Whenever my fuel pump relay does prime the fuel pump, the car starts so much faster that I really want to make that happen every time I start her, and now that it seems possible by tweaking the relay, I'd love to talk to someone with in-depth knowledge of and experience with that IC.

Anyone?

Eliza
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1985 924 NA RoW
1981 931 S2 RoW

[peterld]

Ooooh Eliza, 'NE555'and tachymetric fuel pump''. I just love it when you talk dirty!
Lot's of old school people on the Board, but sadly very few wizards amongst us.
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80/81 932/8 ROW

[Eliza]

Surely that's not true - Morghen constructing a supercharger, people installing intercoolers all the time, suspension upgrades and 5-lug setups - all I'm asking for is help with an IC that's been on the market since 1971!

Should have asked this a few years sooner. There was a guy nicknamed McGyver here who would know. He's a 914 guy, anyone know how to reach him?

Come on, guys, help me out!



Eliza
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1985 924 NA RoW
1981 931 S2 RoW

[morghen]

Hey Elisabeth,

The beta 555 is a simple back counting clock, it gets a signal, probably primes up a capacitor and then somehow reads when that capacitor is empty and then performs an output. That output shuts off the fuel pump.
The signal from the tachometer keeps the 555 capacitor always full and the fuel pump is not shut off as long as that voltage pulse from the tachometer comes.

As i understand you would like to keep the fuel pump running longer before starting the car for better fuel priming.

For that there are several options:

1. turn the key, wait until the pump turns off, then turn back the key and do it again before cranking the engine, this one is free and easy, just makes you look like your car is broken.

2. have a look at the solid state DME relay that some guy posted around here...its a modern relay that simulates the function of the old but if i recall correctly there is a simple way to adjust the time the pump stays on without the tacho signal.

3. dive into the 555 workings and find which resistor it takes to modify the time that capacitor takes to discharge...i would suspect the capacitor discharges trough a resistor and if you'd change that it would alter the time it takes to shut off the pump.

4. do as i did, hack the original relay: bring a wire from the pin that powers the relay command circuit to the pin that normally comes from the tacho, this way...if the key is on, the fuel pump runs. The only bad aspect is the fact that if you have an accident and you are knocked out..the pump will not shut off...and thats actually the main function of this relay. When i had my crash in 2018 i immediatly turned off the ignition as one of the fuel lines got cut as well as the headlight wiring and the wiring was shorting on itself creating sparks when i looked. So i cant recommend this..but if you're willing to take the risk its the easiest and cheapest way to make your car start better and faster every time...you just turn the key and wait for a few seconds then crank.
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Supercharger and EFI kits
https://www.the924.com

[924RACR]

LOL... well, the Megasquirt/Microsquirt conversion primes the pump every time! I suspect those of us who are knowledgeable enough to dive that deep into the electronics (and no, that's not me, I'm a Mechanical Engineer working on software, the electrons are beyond my abilities) are perfectly ready to just dive right into the conversion outright, as it's simpler than actually designing/making boards yourself!

Though it has been noted here as well, and I have experienced it myself for sure, that the stock ignition/wiring will prime the fuel pump every time too, unless there's some ground issues... so could be worth simply checking/cleaning grounds behind the dash, if you haven't already...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[D Hook]

My '80 924 NA had a quirk that primed the fuel pump by moving the adjusting joystick of the driver's side rear view mirror. Worked every time. I've heard of others with the same quirk. Thought about trying to fix it but it came in handy several times for just testing to see if the pump was working ok. YMMV.
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'80 924 n/a SOLD

[Eliza]

[924RACR]

[Eliza]

Oh ok - that sounds interesting. Ready to spill some more? I admit, I haven't driven my turbo a lot yet, and I use her as a street car not a racer of course, but I have to say the Siemens-Harting ignition paired with CIS really makes me smile. What a brilliant system, still, fourty years after it was invented.

I had two Fiat barchetta's before I bought the turbo, so I know what "modern" fuel injections feel like. I very much prefer the old school stuff. Oh of course, apart from an ECU induced fuel priming at power up

I'd love to hear more about your EFI experience though!

Eliza
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1985 924 NA RoW
1981 931 S2 RoW

[Eliza]

[morghen]

Euro 924 turbo S2’s are very smooth and feel very modern compared to the 924NA or the 924 turbo S1. From driveability point if view in didnt feel the need for efi on an S2.

Here is a link to a relay that fits the 2.5 engine, it may be that it will work on the 2L or can be made to work on the 2L by re-aranging the terminals in the relay panel of the car.

https://www.ftech9.com/new-products/993-ssr-pp

Here is another maybe interesting link:
https://www.924board.org/viewtopic.php?p=188463&sid=8b16fa541b0f2f16042f9439cdf45cc9


I remember that there is a solid state relay that fits directly on the 2L…but cant remember where..
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Supercharger and EFI kits
https://www.the924.com

[Eliza]

I've been doing almost everything in the last few weeks you did in 2007, just to get the NE555 to produce that power-on automatic cycle... to no avail . The relay works fine otherwise though.

If you do remember any info about a solid state FPR for the 924, I'd love to hear that!

Eliza
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1985 924 NA RoW
1981 931 S2 RoW

[Eliza]

Just to wrap this up:

Thanks to the invaluable help from somebody over at Rennlist, I now have a priming fuel pump relay in my n/a!

This gentleman owns a 928 S4 and built his own priming relay to speed up starting. He was so kind as to send me pictures of his relay but also pointed out that the early, CIS 928s had a priming relay from factory. He also supplied the part number.

So, if anyone with a thyristor ignition wants their car to start from cold at a touch of the key :
KAE 3.308.200 is the part number. Pinout is an exact match to the 3.300.300 ( OE 433.906.059 ) for our cars. Plug and play, in other words.

Love it!

Eliza
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1985 924 NA RoW
1981 931 S2 RoW

[morghen]

Excellent value for the comunity!
Thanks for sharing!
_________________
Supercharger and EFI kits
https://www.the924.com