Voltage drain when A/C is on - Page 5 - Alfa Romeo Bulletin Board & Forums
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post #61 of 68 (permalink) Old 06-12-2016, 03:25 PM
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Originally Posted by Alfa_Rover View Post
Measurement Points (all in reference to engine block for negative):
A - Main terminal in engine compartment on fender well
B - Back of fuse panel on red wire going to the ignition switch
C - Back of fuse panel on brown wire coming from ignition switch
I am having problems with the A/C compressor shutting off at speed in my S3. I recently installed a BNR 20 amp alternator and replaced the wire from the alternator to the main terminal with 8 AWG. I have also installed a starter relay. Here are my voltage measurements using the reference points above:

Engine------------------------------------A---///--B----///--C---
Running No Load-------------------------14.15 /// 14.00 /// 12.00
Running +AC High (fan only)--------------13.80 /// 13.65 /// 11.10
Running +AC High (fan+compressor)------12.55 /// 12.55 /// 10.40
Running +Heater High--------------------12.45 /// 12.40 /// 10.15
Running +Wipers------------------------12.20 /// 11.85 /// 8.90
Running No Load------------------------11.75 /// 11.30 /// 8.40

Measurement C tracks my voltage gauge. These voltages are much less than others are reporting, especially at point C. Any suggestions on what I need to check?
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Kim - 1987 Spider Quadrifoglio (~170K original owner miles)
Huntsville, AL
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post #62 of 68 (permalink) Old 06-12-2016, 03:38 PM
But Mad North-Northwest
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20 amp alternator? If that's not a typo that's *way* too small and would explain why your "A" values are so low when you load up the system.

You've got a ton of voltage drop across the ignition switch too, which suggests a bad switch. At <10V on the switched circuits I'm amazed your car runs.

Tom

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1974 GTV
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post #63 of 68 (permalink) Old 06-12-2016, 04:13 PM
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I meant 80 amp alternator.
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post #64 of 68 (permalink) Old 06-12-2016, 04:48 PM
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Engine------------------------------------A---///--B----///--C---
Running No Load-------------------------14.15 /// 14.00 /// 12.00
Running +AC High (fan only)--------------13.80 /// 13.65 /// 11.10
Running +AC High (fan+compressor)------12.55 /// 12.55 /// 10.40
Running +Heater High--------------------12.45 /// 12.40 /// 10.15
Running +Wipers------------------------12.20 /// 11.85 /// 8.90
Running +Lights------------------------11.75 /// 11.30 /// 8.40
I had a mistake in the last line which I corrected above. It shows the additional voltage drop when turning on the lights.

Kim - 1987 Spider Quadrifoglio (~170K original owner miles)
Huntsville, AL
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post #65 of 68 (permalink) Old 06-12-2016, 05:48 PM
But Mad North-Northwest
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That sure looks like either a bad ignition switch or a bad connection to the fusebox of the 12V lines going in or out. Probably the former. If you can get to the wires to the ignition switch see what resistance you get across the switch with the key in the "on" position.

(Obviously do this with the battery negative cable disconnected so you don't kill yourself or something)

Tom

1963 Giulia Spider (1750 engine)
1974 GTV
1991 Spider
Former: 1987 Milano Gold
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post #66 of 68 (permalink) Old 06-13-2016, 07:26 AM
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Thanks Tom. I will check the resistance across the switch.

Kim - 1987 Spider Quadrifoglio (~170K original owner miles)
Huntsville, AL
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post #67 of 68 (permalink) Old 06-13-2016, 11:55 AM
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When you measure the resistance of the ignition switch, or anything, remember ohm's law. (volts = current x resistance).

If you measure a tenth ohm resistance, and if there is 10 amps flowing in the circuit you will lose 1 volt across the switch.
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post #68 of 68 (permalink) Old 06-18-2016, 03:03 PM
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I finally set aside a day to do some serious A/C troubleshooting. I decided to first trace the wiring diagram to determine if any wires were loose. I found the nut that secures the ground wire to the compressor was missing. Although the ground wire loop connector was still on the bolt, it wasn't a good connection. I secured the ground wire and the A/C now works great!

I still have low voltage across the ignition switch, so I may install a relay as others have done to improve the blower fan speed.

Kim - 1987 Spider Quadrifoglio (~170K original owner miles)
Huntsville, AL
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