Excessively high charging system voltage
#2784194
06/11/20 04:14 PM
06/11/20 04:14 PM
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Joined: May 2019
Posts: 17 Chicago area
Magnum GT
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Chicago area
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I'm needing assistance with a charging system problem. Vehicle is a 1979 Magnum, factory 100 amp alternator, external voltage regulator (1970 & later electronic type/ triangle connector). The Magnum uses the shunt wire to bypass the amp gauge so charging current does not pass thru the bulkhead or the ammeter. There have been no charging system problems for years and everything was fine when I parked the car last fall for the winter season. Over the winter I replaced the ignition switch (bad mechanically, not electrically) and the starter relay (just because it was original). This past Saturday, I replaced the battery because it was 12 years old and did not turn over the engine. Sunday, I fired up the car for the first time this season. My voltmeter (Auto Meter) spiked to 16+ volts on start up but after a few seconds came down to the 14-15V range. I consider 14-15V at idle (1000 rpm) normal. After a few minutes running, the voltmeter spiked to 16+V again and remained. I connected my digital multi meter to the alternator output post and read almost 18V. I checked the voltage regulator ground and it is clean-I also use a braided grounding strap from one of the VR mounting bolts to the engine. I had a spare VR (not new but know good) and swapped that on but it did not change things. There is a ground wire from the alternator (alt. ground post) to the left cylinder head where the negative battery cable connects-that is good. I let things sit for a day then started some checks. Battery voltage, key off-13.1V. Blue wire at VR connector, key on-12.9V. Blue field wire at alternator, key on-12.9V. I checked continuity of the blue and green field wires, each has about 0.5 ohms resistance, and no continuity between the two field wires. I have not yet pulled the bulkhead apart but my readings appear to be normal. I next bench tested the alternator according to the FSM-"Rotor Field Coil Current Draw Test". The result was 4.2-4.3 amps. According to the FSM, "Field coil draw should be 4.75-6.0 amps at 12V. A low rotor coil draw is an indication of high resistance in field coil circuit. (Brushes, slip rings or rotor coil)." This is where I'm stumped. Is this the cause of the high voltage output of the alternator? To double check the alternator, I took it to a local parts store for testing and they said the rectifier is bad. I don't want to throw parts at this, but rather learn. I have a handle on electrical basics but that's it. Is the problem solely the alternator or am I missing something? Sorry for the long post but wanted to provide as much info as possible. Thank you. Steve
Steve
1979 Magnum GT 2011 1500
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Re: Excessively high charging system voltage
[Re: NITROUSN]
#2784249
06/11/20 06:33 PM
06/11/20 06:33 PM
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Joined: Jan 2003
Posts: 4,292 Morrow, OH
markz528
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master
Joined: Jan 2003
Posts: 4,292
Morrow, OH
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You may have an issue with feedback voltage to the regulator. If the feedback voltage is low to the regulator, then the regulator will want to kick the voltage up to compensate. That is why grounds are first suspect, but it does not need to be the ground - could be the feed.
Do you have a schematic for the circuit? I would measure the voltages at the voltage regulator. It could be ignition switch related.
67 Coronet 500 9.610 @ 139.20 mph 67 Coronet 500 (street car) 14.82 @ 94 mph 69 GTX (clone) - build in progress......
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Re: Excessively high charging system voltage
[Re: markz528]
#2784251
06/11/20 06:38 PM
06/11/20 06:38 PM
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Joined: Jan 2003
Posts: 4,292 Morrow, OH
markz528
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Morrow, OH
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Looks like the voltage regulator is fed from the ballast resistor. Make sure you have full battery voltage on the ignition side of the ballast resistor.
67 Coronet 500 9.610 @ 139.20 mph 67 Coronet 500 (street car) 14.82 @ 94 mph 69 GTX (clone) - build in progress......
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Re: Excessively high charging system voltage
[Re: Andrewh]
#2784334
06/11/20 10:11 PM
06/11/20 10:11 PM
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Joined: May 2019
Posts: 17 Chicago area
Magnum GT
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Mark & Andrew, I do not have a ballast resistor in the system. I run a MSD6AL and Blaster 2 coil-these have been on the car for years. When I checked the sense line voltage at the VR, I removed the VR plug, positive side of the meter directly into the plug female connector and negative side of the meter to the VR bolt that also holds the ground strap to the engine. The VR plug connects quite tightly to the VR terminals-it requires some effort to remove the plug.
Steve
1979 Magnum GT 2011 1500
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Re: Excessively high charging system voltage
[Re: Magnum GT]
#2784358
06/11/20 11:04 PM
06/11/20 11:04 PM
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Joined: Jan 2003
Posts: 4,292 Morrow, OH
markz528
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I would try to measure the ignition side of the voltage regulator when it is running. A bad connection in the ignition switch can cause a voltage drop under load.
You might need to trace the wire to the bulkhead (assume a 79 Magnum has a bulkhead?) to get access to the wire. Then measure from positive to the voltage regulator case - my suspicion s that you will see some drop and it may be inconsistent.
67 Coronet 500 9.610 @ 139.20 mph 67 Coronet 500 (street car) 14.82 @ 94 mph 69 GTX (clone) - build in progress......
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Re: Excessively high charging system voltage
[Re: Magnum GT]
#2784495
06/12/20 09:13 AM
06/12/20 09:13 AM
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Joined: Feb 2003
Posts: 8,720
Andrewh
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Mark & Andrew, I do not have a ballast resistor in the system. I run a MSD6AL and Blaster 2 coil-these have been on the car for years. When I checked the sense line voltage at the VR, I removed the VR plug, positive side of the meter directly into the plug female connector and negative side of the meter to the VR bolt that also holds the ground strap to the engine. The VR plug connects quite tightly to the VR terminals-it requires some effort to remove the plug. might not make a difference, but don't use the bolt head for ground. use the vr case body. just in case. then see if it is different if you use battery ground. again, last test would be to run a pos line direct from the battery to the feed to the vr. I think you might be able to tap into the msd feed, but not familar enough with that wiring. That is to say without cutting and splicing into a line.
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Re: Excessively high charging system voltage
[Re: Magnum GT]
#2784591
06/12/20 12:31 PM
06/12/20 12:31 PM
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Joined: Jan 2003
Posts: 26,037 Rio Linda, CA
John_Kunkel
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Rio Linda, CA
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Since it all started after a new battery was installed, try another battery. New battery could have a shorted cell.
The INTERNET, the MISinformation superhighway
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Re: Excessively high charging system voltage
[Re: John_Kunkel]
#2784789
06/12/20 09:59 PM
06/12/20 09:59 PM
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Joined: May 2019
Posts: 17 Chicago area
Magnum GT
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Thanks everyone for the responses so far. I'll be checking things this weekend.
Steve
1979 Magnum GT 2011 1500
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