Question about sharing computer circuits?
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11 Second Truck Club
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From: Missouri
Question about sharing computer circuits? I am curious if I can use the wire coming out of the PCM that grounds the fuel pressure relay to ground another relay also. Or would this place too much load on the circuit and fry the PCM? I am thinking it would be ok since it is a ground circuit but I don't wanna fry anything. I am talking about just "T ing" into the wire and letting it control the fuel pressure relay and an add on relay. Any thoughts on this?
10 Second Club
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From: Berkeley, California
Stop! Depending on which relay you select you could damage your PCM. The output circuits are designed to safely switch some maximum current. Adding another identical relay will double the operating current which would probably exceed the safety margin. At the very least, this should not be done without more investigation.
Joined: Nov 2001
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From: Anderson, Indiana
Your off to a bad start! The fuel PUMP relay isn't grounded by the PCM, it is powered. So don't wire up another relay to be grounded; it ain't gonna work.
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91 Z28 LS2 408CI, LS9 Supercharger, LPE GT7 cam, Yank3000, 3450 raceweight.
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91 Z28 LS2 408CI, LS9 Supercharger, LPE GT7 cam, Yank3000, 3450 raceweight.
Latest numbers: 9.71 ET, 141.42 MPH, 1.40 60' , 610 RWHP Mustang Dyno
www.speartech.com
Thread Starter
11 Second Truck Club
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From: Missouri
Ok, thanks for the heads up-I do NOT want to burn up the pcm !! I do need to have a circuit that is only on when the engine is running. How about using the wire powering the fuel pump itself to trigger the new relay? Are there any un-used circuits in the pcm, such as AIR (just an example) that I could hook up to then program to come on when I want with edit?
Joined: Nov 2001
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From: Anderson, Indiana
As long as you wire it up correctly you should have no problem driving two relays. Just use GM relays or at least measure the coil resistance of the relay you are adding so that it is not too low causing high current draw on the PCM.
I used two fuel pump relays for a long time when I was using dual fuel pumps; no problems at all.
That's probably easier than trying to configure an unused PCM circuit to work the way you want it to.
I used two fuel pump relays for a long time when I was using dual fuel pumps; no problems at all.
That's probably easier than trying to configure an unused PCM circuit to work the way you want it to.
__________________
91 Z28 LS2 408CI, LS9 Supercharger, LPE GT7 cam, Yank3000, 3450 raceweight.
Latest numbers: 9.71 ET, 141.42 MPH, 1.40 60' , 610 RWHP Mustang Dyno
www.speartech.com
91 Z28 LS2 408CI, LS9 Supercharger, LPE GT7 cam, Yank3000, 3450 raceweight.
Latest numbers: 9.71 ET, 141.42 MPH, 1.40 60' , 610 RWHP Mustang Dyno
www.speartech.com
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On The Tree
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be careful relays have a bad habit of creating a current spike when shut off. If you want a techinical description I can give you one (electrical engineer) but for now just be really careful and use a very good relay with coil suppresion that will be compatable with the pcm. your best bet would be to tap at some point after a drive relay that would isolate the pcm from and crap that your relay would create on the line.
like the case where the 2 fuel pumps were installed I would have attached the input to the 2nd relay to the output of the first relay and run that in parellel with the original pump circuit thus avoiding and feedback to the PCM.
the stock pcm to relay interfaces are well worked out and tested with some very special circuits to protect to solid state electronics in the PCM.
More Than Zero
ps: this stuff is not a problem when you are just dealing with relays, it only comes into play when you throw solid state devices into the mix.
like the case where the 2 fuel pumps were installed I would have attached the input to the 2nd relay to the output of the first relay and run that in parellel with the original pump circuit thus avoiding and feedback to the PCM.
the stock pcm to relay interfaces are well worked out and tested with some very special circuits to protect to solid state electronics in the PCM.
More Than Zero
ps: this stuff is not a problem when you are just dealing with relays, it only comes into play when you throw solid state devices into the mix.
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