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TOPIC: Voltage discrepancy

Voltage discrepancy 06 Dec 2009 09:51 #49


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I am troubleshooting my 1978 928 ignition system and have reached the point where I am checking and comparing the voltage at the ignition control unit plug Terminal 15 with the battery voltage. Shop manual trouble shooting says these voltages should be the same. My battery reads 12.03 volts while the ignition cotrol unit plug 15 reads 11.83 volts is this significant enough to warrant futher readings back to the ignition switch to determine where the voltage drop is occurring? Manual says they shoul be the SAME not an acceptable range. Any comments are greatly appreciated. Best, Greg
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Voltage discrepancy 07 Dec 2009 20:52 #51

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I don't have the answer, trying to find someone who can help.
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Voltage discrepancy 08 Dec 2009 13:25 #52


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Just to clarify, are we talking about a total ignition failure that you're troubleshooting for? If so, I don't believe your findings would explain a "no spark" fault because your ecu is being energized.
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Voltage discrepancy 09 Dec 2009 15:28 #53


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KilnRed,
Thanks for your reply. I thought 11.8 volts should be sufficient but wasn't absolutely sure because I've had an intermittent no spark/no start problem for awhile. I continue to work my way through it and the car started again as I troubleshooted it again. It's a real pain in the neck and too difficult to explain in writing --lots of troubleshooting over the years without uncovering the problem only to have the car start and run again for 6mos or so and then not start again. I'd like to discuss on the telephone if possible or convenient for you if you have some experience regarding the ignition system. If you don't have the time or interest that's fine. My tel # is 212 360 1295 if you call and don't reach me please leave a convenient date/ time for me to call back. Thanks again for your helpful response. Best regards, Greg
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Voltage discrepancy 09 Dec 2009 17:04 #54


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The ignition on your car is identical to mine, though, it is an '84. The most common fault for a no spark condition on our cars is the infamous "green wire." Have you inspected it or are you aware of the wire that I'm referencing?
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Voltage discrepancy 09 Dec 2009 21:22 #55


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Yes I have addressed that--replaced green wire more than once only to have problem return again months later. I have ordered from 928 intl an ignition switch . My problem seems to have the ignition switch as the common denominator. Hopefully that will solve it.
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Voltage discrepancy 11 Dec 2009 17:14 #58


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So you're saying that replacing the green wire fixed the problem before? The ignition switch most likely isn't you're issue judging from what you've said here in this thread. Like I said, your ecu is being energized.

More often that not, an ignition switch failure is very obvious because most or all circuits it's responsible for supplying power to are effected. A good example of this would be having a working radio while the vehicle is started, but no radio power when the ignition key is turned to the auxiliary position.

Have you had your coil checked? Perhaps you have a bad connection to it or your flywheel position sensor? Any number of things could be creating your intermittent spark condition. My advice would be to continue investigating step-by-step with the help of your workshop manual's troubleshooting guide. It's a far more efficient and cost effective way to fix your car.
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Voltage discrepancy 22 Dec 2009 15:19 #59


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vze2jshn;50 wrote:
I am troubleshooting my 1978 928 ignition system and have reached the point where I am checking and comparing the voltage at the ignition control unit plug Terminal 15 with the battery voltage. Shop manual trouble shooting says these voltages should be the same. My battery reads 12.03 volts while the ignition cotrol unit plug 15 reads 11.83 volts is this significant enough to warrant futher readings back to the ignition switch to determine where the voltage drop is occurring? Manual says they shoul be the SAME not an acceptable range. Any comments are greatly appreciated. Best, Greg
Well I would say 11.83v is absolutley sufficient to start the car... if its really there...

I say this because its only when you are trying to crank that matters - not just with the ignition on. 12.03v is very low for the battery anyway - I suspect you need to charge it.

20mV is also a big drop on that wire... I'd wonder what that voltage is when cranking - if its below 7-8V you have a problem - maybe there is excess resistance in the circuit.

so:

1. Charge or replace the battery until you get 12.6 or more steady state
2. Check the final stage voltage while cranking

Alan
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