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Old 01-14-2010, 06:44 PM   #16
schwank
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This change would not affect the factory DME harness. The DME harness remains untouched. This actually would make it much more obvious to inspect for cheaters as there would be few wires to inspect and trace. I believe that both Cup and Spec make provisions for removing all unused wiring in the car and this falls under that provision in my opinion. It is purely a reliability move and would have no affect on performance except maybe to pull a little weight out.

I've seen several Cup cars with aftermarket fuse boxes.
Good deal... just making sure you don't go too far! I have removed a few wires too but have not gone so far as to build any new simplified harnesses... though I don't doubt a fair amount of weight could be lost under the dash.
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Old 01-14-2010, 06:45 PM   #17
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I'm also thinking of changine the alternator output so that it only sends power to the battery, i.e. the car won't run without the battery in the circuit. Anyone see an issue with that?
If you start a car, you can remove the battery and the car will still run. The battery acts like a large filter (capacitor) to the alternator and filters out all the spikes the alternator puts out. Most new cars have a warning label about removing the battery since it can damage the electronics and computers in a car.

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Old 01-14-2010, 08:12 PM   #18
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Brian, all very good points, thank you.

I see what you mean by using the DME relay ground though I wouldn't re-use the stock DME relay, I could duplicate that safety feature. Basically, you are saying that when the DME realizes the engine is no longer running, it would cut power to the fuel pump in the case of an accident.

I didn't mean to imply that I would necessarily use the DME pin 4 to carry load to the starter, just that the start wire needs to feed back to the DME.

I missed the idle controller in my research but I would have gotten it by default when I put power to T21 Pin 2 for the injectors out of dumb luck.

How is the alternator regulator separately powered? I see connector T24 with two terminals which look like they go to the regulator (I'm assuming these cars don't have a separate regulator). T24 Pin 1 goes to the ignition switch and starter solenoid and would get powered from my momentary switch. I assume that is to "flash the field" to get the alternator to generate electricity. What is T24 Pin 2? Does constant voltage go to the alternator via Pin 2. I'm a little confused here.
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Old 01-14-2010, 08:18 PM   #19
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If you start a car, you can remove the battery and the car will still run. The battery acts like a large filter (capacitor) to the alternator and filters out all the spikes the alternator puts out. Most new cars have a warning label about removing the battery since it can damage the electronics and computers in a car.
Yes, on further thought you are correct. If there is power at the positive battery terminal from the alternator, it won't matter if the battery is in the circuit or not. Guess that's why the kill switch has to be wired to the alternator as well.
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Old 01-14-2010, 09:02 PM   #20
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How is the alternator regulator separately powered? I see connector T24 with two terminals which look like they go to the regulator (I'm assuming these cars don't have a separate regulator). T24 Pin 1 goes to the ignition switch and starter solenoid and would get powered from my momentary switch. I assume that is to "flash the field" to get the alternator to generate electricity. What is T24 Pin 2? Does constant voltage go to the alternator via Pin 2. I'm a little confused here.
I think I confused the issue so let me state it a little more clearly. The wire that goes to the alternator, D+/61 (from T24 pin 2) comes from the alternator light on the dash and the alarm module. If it is held at ground (the alarm module does this when triggered) the alternator will not generate voltage. The alternator dash light has one side connected to +12VDC and this kick starts the field of the alternator. So, you really don't directly supply power to the D+ pin of the alternator, you apply voltage to the alternator light in the dash which passes power to the alternator to kick starts the field. Once the field starts, it maintains itself and the voltage on D+ goes to 14VDC and the light goes out. Another point, on this year, there is a resistor in parallel with the alternator light so that if the light burns out, the alternator will still work.
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Old 01-14-2010, 09:16 PM   #21
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Brian,

You are a true wealth of information and I sincerely thank you!

Sorry to belabor this subject but would you supply T24 Pin 2 with battery voltage at all times when the car is running? Also, what is the function of T24 Pin 1 if it is Pin 2 that is flashing the field?

Thank you again and I owe you a beer (or two)!
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Old 01-14-2010, 09:57 PM   #22
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You don't have to do anything to T24 pin 2 if you are still using the stock dash with the alternator light in it. If you were to remove the stock dash and use an aftermarket dash, you would connect a 68 ohm, 5 watt resistor (or a small light bulb) to 12V on one side and the alternator D+ (T24 pin 2) on the other. Once the car is running, you can leave the D+ pin open because it will generate its own voltage that maintains the field in the alternator. Most auto circuits just keep 12V ( after it goes through a light bulb) on the D+ pin. The connection to the D+ pin should be current limited by using a light bulb or a resistor. When the alternator isn't moving/running, the D+ pin has a very low resistance to ground and it can pull a lot of current so it needs to be current limited. The low resistance to ground is what turns on the alternator light until the alternator starts to generate voltage.

T24 pin 1 connects to the starter solenoid, pin 50. That needs to go to your push to start push-button switch. You really don't need a relay on that push to start switch because a little drop in voltage due to the long wires will not effect the starter solenoid much. Pin 30 of the starter connects directly to the battery.

Last edited by Bri Bro; 01-14-2010 at 10:17 PM.
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Old 01-15-2010, 02:00 PM   #23
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If you use a after-market dash with an LED indicator, you might need to add in this circuit to stop the LED from flickering when the alternator is working. The light bulb reacts slowly since it has to heat up to emit light, the LED reacts very fast in comparison. I did this circuit for a rennlister some time ago and it solve the problem of flickering.
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Old 01-19-2010, 09:11 PM   #24
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OK, today I sketched up what I think the wiring will look like for my race car using a Painless Wiring 8 circuit race car harness. This harness comes with 8 single pole relays and all circuits are fused.

Note that I didn't show the fuses in the diagram.

I didn't draw the lines from the power side of the relays to the non-engine vital circuits as they are pretty self explanatory and would only complicate the diagram.

The DME side of T21 remains stock.

I'm using all aftermarket gauges so there is no need for any of the gauge wiring.

The one part I'm still not fully understanding is the resistor in line with the D+ pole on the alternator but I did draw it in. Is it OK to have this come off the ignition circuit? What happens if you don't have that resistor?

Thanks again for the help!

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Old 01-19-2010, 09:46 PM   #25
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At first glance, looks good. Are you going to keep the factory dash and instrument cluster?
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Old 01-20-2010, 12:36 AM   #26
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Nope, all instruments and dash are gone. I'm using mechanical instruments with dummy lights. There are going to be 7 toggle switches and a momentary switch per the diagram.

Still a little confused on the purpose of the resistor in the alternator circuit. If I used a light bulb in this place, would the bulb go out when the alternator generates? I think what you are saying is that the alternator acts like ground until it generates and then ground goes away and therefore the light goes out when generating. Is that right? I never knew that was how the light worked if that is the case.

Thanks again for the help!
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Old 01-20-2010, 12:36 AM   #27
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Im going to be doing all the same thing in the next week or 2, ill be sure to send pics on how i do it, it might help, if not it will help someone who is not technically inclined.
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Old 01-20-2010, 12:38 AM   #28
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Oh and if anyone needs wiring diagrams let me know ive got a whole parts room full of every porsche ever made, except the super cars.
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Old 01-20-2010, 01:10 AM   #29
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Nope, all instruments and dash are gone. I'm using mechanical instruments with dummy lights. There are going to be 7 toggle switches and a momentary switch per the diagram.

Still a little confused on the purpose of the resistor in the alternator circuit. If I used a light bulb in this place, would the bulb go out when the alternator generates? I think what you are saying is that the alternator acts like ground until it generates and then ground goes away and therefore the light goes out when generating. Is that right? I never knew that was how the light worked if that is the case.

Thanks again for the help!
Yes, the light from the bulb go out when the alternator is generating power. The bulb should go in parallel to the resistor. You want the bulb (when it lights up) to tell you that the alternator is NOT generating power. The purpose of the resistor is to insure that the alternator works if the bulb burns out. In older 944s, if the dash bulb for the alternator burned out, you lost the alternator and the battery soon drained out and you were dead by the side of the road.

The picture below is from the dash, bulb 18 is the alt light bulb and the resistor is marked 68/5 and is in parellel with the light bulb.
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Last edited by Bri Bro; 01-20-2010 at 01:58 AM.
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Old 04-10-2010, 03:42 PM   #30
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I still have my dash and would like to replace my dead ignition switch with a toggle switch and a push button using the wires at the back of the ignition switch. I want to keep my dash. What should I know to keep the fuel pump off when motor not running function?
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Old 04-10-2010, 03:42 PM
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12v, 85b, 944, batteries, battery, diagram, ecu, honda, ignition, lighlt, racecar, relay, simple, switch, weight, wiring

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