| Alternator resistor [message #273991] |
Wed, 18 March 2015 22:50  |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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This is a pic of the innards of a 100 amp alternator from a Cadillac that I cannibalized the diode trio from to repair the 80 amp alternator in the GMC.
http://www.gmcmhphotos.com/photos/alternator/p57563-100-amp-alternator-innards.html
See the precision wire-wound resistor attached to the regulator assembly? It is grounded on the top screw and the other end attaches to the output of the diode trio which also is the input to the field (which is the rotor in alternators). I think it measured 4.72 ohms. The 80 amp alternator from the GMC did not have the resistor. Is the one in the 100 amp there to take the place of the nichrome resistor wire that the GMC has in the harness? I ask because I cannot get my alternator to sense the battery voltage and raise the output to compensate for the isolator diode and the system barely maintains the battery voltage. Turn on the lights and the battery voltage steadily drops. I have an ammeter in the alternator output that shows when the charge drops off, and this happens when I have 14.3 at the isolator input and 13.6 at the battery. Only losing a tenth of a volt between the alternator and the isolator because of the wire and ammeter, right at the alternator terminal is 14.4V. Jumping the brown wire (without interrupting it) to the battery upped the output voltage of the alternator to just the right level, until I turned the headlights on and blew the diode trio! That's why I had to swipe the one from the Caddy alternator. Really would like to know if that resistor has anything to do with the output voltage. The 100 amp unit is not in my wiring diagrams. Anybody know?
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: Alternator resistor [message #274017 is a reply to message #273991] |
Thu, 19 March 2015 15:36  |
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John Heslinga
Messages: 632
Registered: February 2011
Location: Edmonton, Alberta, Canada
Karma: 4
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Senior Member |
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Terry
The resister is not the same as the nicrome wire in the wiring harness. It's interesting that your 80 Amp did not have that resistor. Mine did. But here is a diagnostic and operational document that I know you will understand.
http://www.gmcmhphotos.com/photos/alternator-repair-27si/p52760-delco-alternator-service-bullitin.html
The Alternators in the GMC is a 27si Delcotron using a Delta wound field (rotor). The rotor is initially charged through the idiot light circuit until enough voltage can be supplied by the diode trio. The output of the diode trio goes Through the rotor windings and back through the regulator which controls the output voltage by controlling the amount of ground. (Full ground equals full output. Some Delcotrons did not have that resistor you are talking about so it might run fine. But it seems strange. It may be that your Alternator was replaced with a one wire style at one time which does not use an external sense wire.
Look throughthroughthe rest of the photo Album. I documented a 80 to 100 amp update using the same Cadillac and GMC SI27 models.
I highly suspect you have a Regulator that needs replacing assuming all the coach wiring is in good shape. The oicture of the alternator you show has horrible corrosion and burning on the output link. Again check out the photo album. You will get it!!
Best regards
John and Cathie Heslinga
1974 Canyonlands 260
455, Manny tranny and 1 ton, 3:70 LS,
Red Seal Journeyman, DTE, BEd. MEd.
Edmonton, Alberta
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| Re: Alternator resistor [message #274028 is a reply to message #274017] |
Thu, 19 March 2015 22:33 |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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Senior Member |
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Thanks John! The alternator in the pics is NOT the one in the GMC, as you correctly noted the melted insulator on the output terminal and the very used condition of the unit, it came from a Caddy with 200K on the odometer. It was still charging but the bearings were dry and it was very noisy, so the owner wanted it replaced. The output terminal got overheated due to resistance at the connection of the charge wire, which was due to corrosion from road salt and an unmaintained engine compartment. I had to replace the last 8 inches of the charge wire when I replaced the alternator. Since the core value was only $11.00, I kept the unit knowing it was worth more that that in parts (to me, anyway). I just gave the customer the core value off of his repair bill. I'm just wondering if that resistor being missing from the GMC alternator is the reason I can't get it to charge at 15 volts so I will have 14.3 at the battery after the isolator. I'll spend some time studying that site you linked before I do any more experiments!
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: Alternator resistor [message #274040 is a reply to message #274028] |
Fri, 20 March 2015 00:23 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
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Senior Member |
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Did you stick a screw driver in the test hole in the rear of the alternator with the engine running? Doing this grounds a circuit in the regulator and the voltage goes higher then normal. If that is what happens you need a new regulator if you can't maintain the normal charge voltage.
Roy Keen
Minden,NV
76 X Glenbrook
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| Re: Alternator resistor [message #274053 is a reply to message #274040] |
Fri, 20 March 2015 09:39 |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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Senior Member |
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Roy,
I didn't full field the alternator through the grounding hole because it was maintaining the proper charge level according to its output. As I indicated, the unit keeps its output voltage at 14.3 and the charge current drops off. I may have partially full fielded it when I applied battery voltage to the brown wire on terminal #1, as this would allow more current to flow than what is usually supplied through the generator light bulb. The light bulb restricts the current through the field circuit, as do the diodes in the diode trio. That is probably why the diode trio smoked.
However, I believe you and John are correct that I need a new regulator in it as the battery voltage was steadily dropping when the headlights were on. When I tested it,the only other device supposed to be drawing current at the time was the ignition system and the charge going to three batteries.
In addition to the engine (chassis) battery, I have two separate extra batteries on different charging circuits. The one that is used for the "house" is back by the generator and is as long as the genny compartment is deep. It powers the lights for a long time before its voltage drops and takes a charge very nicely off the 110V battery charger that I use to charge it when the RV is plugged in to the power grid. I also charge that battery directly from the alternator output through a relay that is only energized (turned on) by the output of the alternator so the battery is disconnected when the engine is not running. The other extra battery (the "boost" battery)is up next to the engine battery and is only charged through the isolator and only connected to the engine battery system through the "battery boost" relay, just like it was from the factory. The engine battery is connected to the chassis wiring just like it was from the factory also, and is only charged through the isolator.
So....
A) The alternator is regulating itself based on the voltage of the "house" battery in the back, even though the remote sensing wire is connected to the engine battery,
or
B) The "boost" battery is influencing the system somehow when the alternator is charging (maybe it is taking too much charge current from the system)
or
C) Leaving the 110 volt battery charger connected to the house battery is influencing the system,
or
D) I need a new regulator
In any case, I need to go modify some parameters of the testing (disconnect the extra batteries)and test certain points again. Since the regulator has both of the terminals I assumed that terminal #2 would provide remote voltage sensing, and this seems to be NOT TRUE according to my previous tests.
So now I will go test it some more...
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: Alternator resistor [message #274152 is a reply to message #274066] |
Sat, 21 March 2015 23:17 |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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Senior Member |
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Thanks Matt, maybe the absence of that resistor in my 80 amp unit is why I smoked the diode trio when I was "experimenting"...
After further investigation of the wiring, I see that I wired the charge wire for the "house" battery from the same output of the isolator as the "auxiliary"(boost) battery. So it is not affecting the output regulation on the alternator. Only the signal for the "house" charge relay and the supply for the HVAC blower relay are connected to the alternator output. The horn relay common positive connection has been moved to the battery junction post. There are some additional circuits I have added, but I can disconnect them at the battery junction post.
Now all I need is the location of the junction of the remote sense wire and the ignition switch/fuse box feed. Only a tenth of an ohm resistance between the battery junction post end and the alternator end, but I wanted to separate all the loads from the sense wire to allow myself more manipulability and serviceability in the system. I want to test the sense wire separately.
Going shopping for some extra regulators now...
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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