924Board.org

[Lizard]

Ok, I have the MSD 6a ign system, and I was sick of not having a tachometer, so I decided to try and get the tach adapter, upon making a call they say well you need to get one of these only if it doesnt work. so I took the green wire from the ICM and plugged it right into the tach output of the MSD and it works fine, now the fuel pump relay in these cars (if you still have one ) uses the tach signal for the rev limiter, so as long as you just take the green wire (may have a black one with it) and lplug it right into the MSD 6a tach plug, the factory rev limiter should work with no issues.
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3 928s,

[jpab924]

I haven`t gotten around to installing my msd 6a into the 931 yet Lizard. But when I had it hooked up to my NA 924, I had to bypass the fuel pump relay (to power up my fuel pump) to a seperate switch because I lost tach ref to the relay. So you say I should be able to keep my relay if I get the tach adapter?

On a side note, I didn`t think NA 924`s had the rev limiting relay? I thought that relay was 931 specific by reading some prior threads.

[Lizard]

actually you dont need an adapter for the MSD 6a system,

on one of the ends there is a black plug with a spade connector in it, just take the female spade connector w/ the green wire out of the factory ICM and plug it into this and your tach should work and then your fuel pump relay should still get the ign signal it needs to run.

in regards to the rev limiter fuel pump, you may be correct that it is specific to the 931 however the 924 needs tach signal to keep the fuel pump running as already mentioned. so you shouldnt need to leave the factory ICM in there to keep the tach working, and you shouldnt need to in this instance put a switch for the fuel pump relay.
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3 928s,

[jpab924]

[Min]

I've got a msd 6al on my 80 924 at the moment, the tach output from the msd isn't the correct output to make the fuel relay work, at least not in my car. As far as I understood it the tach input on the relay was not for a rev limiter but so that if the engine stops running for some reason, the fuel pump is turned off, this is a safety feature. You can just install a older style fuel pump relay apparently, but I just went with a slightly more complex custom setup, which uses a switch, only becuase its a slight amount of theft protection. could just wire the small side of the relay to ground and onwithkey power input.

Min

[Lizard]

hmm, I looked throught the wiring diagrams in the haynes and they all show that the wire going to the fuel pump relay for the signal is running right off the tach wire, unless I am missing something that indicates that if the tach works so should the relay.
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3 928s,

[Min]

you are correct, it does run off the tach wire, but, you hook up the msd tach signal, and it does not work. the relay does not function, if you open the relay, you will see that its got a transistor on that part of the circuit.

Min

[Lizard]

so what is so different between the signal the factory ICM produces, and the one the MSD unit produces? the tach doesnt seem to notice that there is a difference.
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3 928s,

[Min]

to answer that question I would have to have access to a oscilloscope.

Min

[Joes924Racer]

yup the signals are not the same so it wont start Ive been using
the green wire for about 2 yrs now though instead of buyin the adapter
I left out the relay and used a switch for the pump. The msd is nicer if you have a p&ped head. At least on a series one, where every little bit helps in the end result of pushin the car faster which is what I like.
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[jpab924]

I was left scratching my head a few years back over this......Different car time to try again.....either that, or just bypass it again...