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Old 10-17-2012, 07:24 PM   #1 (permalink)
JLanzino
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94 6.5 TD Alternator Overcharging!!!!

Hi All,

Went to fire up my 6.5TD tonight and noticed she was charging at almost 18 Volts, battery light lit up and everything.

Is my regulator inside the alternator toast??

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1991 GMC Sierra 1500 SLE 1/2 Ton Extended Cab, Long Box, 6.2L C-Code Detroit Diesel, 700R4 Transmission, 3:73, 4X4, 326`000KM
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Old 10-17-2012, 07:50 PM   #2 (permalink)
Big Bertha
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what volts are you getting at the batteries while running? my gauge always sits at 16 but a volt meter shows 13.6 to 14 at the batteries while running aand yes it is possible that the regulator is bad.

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Old 10-17-2012, 08:13 PM   #3 (permalink)
FloridaHorseman
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You may want to check the 10 ga red wire from the ALT to the battery (passenger side I think) and make sure it's not chaffed and grounding to the engine. Also check battery connections and battery condition. I had a 1 month old 770 CCA battery (part of a matched set) short itself down to SIXTY amps max load! The alternator went way high on the gauge constantly trying to charge a passenger side battery that was for all intents and purposes... dead.

If everything checks out OK then X2 on the internal regulator. ~FH
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Old 10-17-2012, 08:14 PM   #4 (permalink)
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where you located? i have a spare
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Old 10-17-2012, 08:17 PM   #5 (permalink)
tigert
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The internal regulator looks at Batt. voltage then adds the voltage to charge the batt. It can fail but most times in the other direction IE alt no output. At 18 volts you should see steam out of the battery cell caps and acid blowing out the terminals. 1: Clean all the battery connections, positive and negitive both ends of all cables. 2: Clean the alt connections and the alt mount bolts as well, ie remove clean and replace. 3: x2 above measure at the batts terms. 4: Be carefull if what you say is true, you have hydrogen gassing in the engine/battery compartment, one spark and it could be over, 2 batts exploding and you being burned and covered in acid while being puntured with lead and batterycase shrapnel. Do the above and it won't happen. Far more likely is a poor connection to the gauge, that being said be safe!
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Old 10-17-2012, 08:44 PM   #6 (permalink)
JLanzino
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The 18 volts i got from the volt meter on the battery, I will clean all conections & grounds & mounts, if that doesnt solve any problems I will be replacing alternator. I am North of Toronto Ontario
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1991 GMC Sierra 1500 SLE 1/2 Ton Extended Cab, Long Box, 6.2L C-Code Detroit Diesel, 700R4 Transmission, 3:73, 4X4, 326`000KM
1991 GMC Sierra 2500 SL 3/4 Ton Regular Cab, Long Box, 6.2L J-Code Detroit Diesel, 4L80E Transmission, 3:73, 4X4, 570`000KM
1994 GMC Sierra 3500 SLE 1 Ton Crew Cab, Long Box, Dually, 6.5L F-Code Detroit Turbo Diesel, 4L80E Transmission, 4:10, 4X4, 432`000KM, Dailey Driver
1988 GMC Sierra 1500 SL 1/2 Ton Extended Cab, Long Box, 5.0L H Code, TH400 (3L80) Transmission, RWD, 3:42, 306`000Km & Counting
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Old 10-17-2012, 10:52 PM   #7 (permalink)
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Quote:
Originally Posted by FloridaHorseman View Post
...the 10 ga red wire from the ALT to the battery...
Just FYI...

The Red charge wire is actually 13sq.mm (~6ga) or 19sq.mm (~4ga). Only the Orange fusible link wire at the battery is 10ga (always ~4AWG smaller than the wire it protects).

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Old 10-18-2012, 09:51 AM   #8 (permalink)
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I would make sure that the voltmeter is accurate, use it to measure something that you know the voltage.
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Old 10-18-2012, 07:11 PM   #9 (permalink)
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Hey Guys I cleaned up all my connections & mounts, Started the old girl up and on Idle it was charging Normal 14.50 (Checked with Volt Meter), But once i increased the RPM up went the voltage at 1500 RPM it was charging at 15.60 and the higher the RPM went the higher the voltage went. Is the regulator toast?, I loaded tested my batteries today they passed, they hold 12.80 volts sitting with everything off, thats why im asking about the regulator?
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1991 GMC Sierra 1500 SLE 1/2 Ton Extended Cab, Long Box, 6.2L C-Code Detroit Diesel, 700R4 Transmission, 3:73, 4X4, 326`000KM
1991 GMC Sierra 2500 SL 3/4 Ton Regular Cab, Long Box, 6.2L J-Code Detroit Diesel, 4L80E Transmission, 3:73, 4X4, 570`000KM
1994 GMC Sierra 3500 SLE 1 Ton Crew Cab, Long Box, Dually, 6.5L F-Code Detroit Turbo Diesel, 4L80E Transmission, 4:10, 4X4, 432`000KM, Dailey Driver
1988 GMC Sierra 1500 SL 1/2 Ton Extended Cab, Long Box, 5.0L H Code, TH400 (3L80) Transmission, RWD, 3:42, 306`000Km & Counting
1991 GMC Sierra 1500 SL 1/2 Ton Regular Cab, Long Box, 5.7L, 700R4 Transmission, RWD, 3:08, 260`000KM
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Old 10-18-2012, 07:41 PM   #10 (permalink)
racer55
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Sure sounds like it can no longer control the voltage.

If/when you replace the alternator make sure you keep or get the same diameter pulley as the old alternator-otherwise the tach will read wrong.

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Last edited by racer55; 10-18-2012 at 07:42 PM.
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