Home » Public Forums » GMCnet » Alternator 8 volts output.
| Alternator 8 volts output. [message #149482] |
Sat, 12 November 2011 16:15  |
roel_b
Messages: 28
Registered: November 2007
Location: The Netherlands
Karma: 0
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A couple of weeks ago a thing happened which I cannot really explain, I'll try to describe it.
In the morning on the campground got everything ready for departure, started the engine, switched on the compressor and let the engine running idling at about 800 rpm's. Then I thought of making some coffee for on the road and hooked up the electric waterkettle to the invertor (hooked on to the housebattery) for boiling some water, engine was still idling. After the water had boiled and the wife made the coffee noticed that the voltage, measured at the onboard circuit from the starterbattery, was at 12.5 volts and the generator light at the dash was on. Revving up the engine did nothing, voltage stayed the same, stopped the engine and started her again, no change, opened the engine hatch and touched the alternator which was really hot, took the voltmeter and measured 8 volts at the output stud of the alternator, over at the middlepost of the diode also 8 volts and 12.5 at both batteries. As we only had to drive 250 km's to get home during daytime that day thought I'd take the change and go by battery only for the ignition.
But after about 45 mins driving the generatorlight went dim and the voltmeter pointed at 13.8 volts again, don't know if it was a sudden change or gradually going to 13.8 volts. Since then no problems with the alternator whatsoever.
APC cable installed.
What could have happened here, why exactly 8 volts output all the time and no fluctuations for as long as I measured.
Alternator looks as new and is the stock 80 amps, probably never been rebuild.
Any advice appreciated.
Brgds.,
Roel
GMC 230 Painted Desert '73
[Updated on: Sat, 12 November 2011 16:17] Report message to a moderator |
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| Re: [GMCnet] Alternator 8 volts output. [message #149487 is a reply to message #149482] |
Sat, 12 November 2011 17:08  |
Dave Mumert
Messages: 272
Registered: February 2004
Location: Olds, AB, Canada
Karma: 0
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Senior Member |
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Hi Roel
I suspect the alternator overheated. The alternator fan is probably not
moving enough air at idle speeds so a heavy load will build up a lot of
heat. The high temperature may have caused the regulator to shut the system
down. This may be a built in safety feature or the regulator may just fail
when it gets too hot.
If the alternator has never been rebuilt it would be a good idea to have it
serviced to replace the bearings and brushes. You could also replace the
regulator if you feel the high temperature may have damaged it. If you
could touch the alternator I would think it probably still OK.
I bought a kit with brushes and bearings for less than $15. You could do
the job yourself or take it to a local rebuilder, be sure and specify you
want your alternator rebuilt not a replacement unit. I would not buy a
rebuilt from the standard sources, it may be worse than the one you have.
Here is a rebuild kit
http://store.alternatorparts.com/27sialternator.aspx
You could build a new alternator from the parts they sell.
The hardest part of the rebuild is getting the pulley off and pressing out
the little bearing at the end.
I use an impact wrench to get the pulley off and my drill press to change
the bearing.
Dave
>
> A couple of weeks ago a thing happened which I cannot really explain, I'll
try
> to describe it.
> In the morning on the campground got everything ready for departure,
> started the engine switched on the compressor and let the engine idling at
> about 800 rpm's. Then I thought of making some coffee for on the road and
> hooked up the electric waterkettle to the invertor (hooked on to the
> housebattery) for boiling some water, engine was still idling. After the
water
> had boiled and the wife made the coffee noticed that the voltage, measured
> at the onboard circuit from the starterbattery, was at 12.5 volts and the
> generator light at the dash was on. Revving up the engine did nothing,
> voltage stayed the same, stopped the engine and started her again, no
> change, opened the engine hatch and touched the alternator which was
> really hot, took the voltmeter and measured 8 volts at the output stud of
the
> alternator, over at the middlepost of the diode also 8 volts and 12.5 at
both
> batteries. As we only had to drive 250 km's to get home during daytime
that
> day thought I'd take the change a nd go by battery only for the ignition.
> But after about 45 mins driving the generatorlight went dim and the
> voltmeter pointed at 13.8 volts again, don't know if it was a sudden
change or
> gradually going to 13.8 volts. Since then no problems with the alternator
> whatsoever.
>
> APC cable installed.
>
> What could have happened here, why exactly 8 volts output all the time and
> no fluctuations for as long as I measured.
> Alternator looks as new and is the stock 80 amps, probably never been
> rebuild.
>
> Any advice appreciated.
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| Re: Alternator 8 volts output. [message #149496 is a reply to message #149482] |
Sat, 12 November 2011 17:45 |
GeorgeRud
Messages: 1380
Registered: February 2007
Location: Chicago, IL
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Senior Member |
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I'm dealing with the opposite problem! All of a sudden, my alternator seems to be putting out 16 volts (measured by my in-dash voltmeter and also at the center terminal of the isolator with a digital VOM). I'm going to keep track of it for a while as I'm ready to put the coach away for the winter, but I suspect that the voltage regulator is going away.
I did have it rebuilt a couple of years ago and do have an APC cable installed, so I think it should be OK to watch as long as I don't boil out the battery.
George Rudawsky
Chicago, IL
75 Palm Beach
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| Re: [GMCnet] Alternator 8 volts output. [message #149504 is a reply to message #149496] |
Sat, 12 November 2011 18:20 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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George,
Your isolator is probably open on the engine battery side.
With that diode open, the voltage from the alternator is not reaching the
battery so there is no voltage feedback to the alternator's regulator -- it
keeps increasing the output trying to pull the battery up.
Put a jumper wire from the center (alternator) terminal to the engine
battery terminal of the isolator -- the alternator output voltage should
immediately drop to near 13.8 VDC, as required to charge the battery.
If you leave it as it, your engine battery will die and the house battery
will boil.
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI+ & EBL
www.gmcwipersetc.com
On Sat, Nov 12, 2011 at 6:45 PM, George Rudawsky <GeorgeRud@aol.com> wrote:
>
>
> I'm dealing with the opposite problem! All of a sudden, my alternator
> seems to be putting out 16 volts (measured by my in-dash voltmeter and also
> at the center terminal of the isolator with a digital VOM). I'm going to
> keep track of it for a while as I'm ready to put the coach away for the
> winter, but I suspect that the voltage regulator is going away.
>
> I did have it rebuilt a couple of years ago and do have an APC cable
> installed, so I think it should be OK to watch as long as I don't boil out
> the battery.
>
>
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Ken Henderson
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www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: Alternator 8 volts output. [message #149511 is a reply to message #149482] |
Sat, 12 November 2011 20:38 |
GeorgeRud
Messages: 1380
Registered: February 2007
Location: Chicago, IL
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Senior Member |
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Thanks for the tip, I'll check it in the morning. The isolator is only a couple of years old so it shouldn't have failed, but never say never!
George Rudawsky
Chicago, IL
75 Palm Beach
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| Re: [GMCnet] Alternator 8 volts output. [message #149514 is a reply to message #149504] |
Sat, 12 November 2011 20:57 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
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Senior Member |
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Ken how does it feed back thtough the diode? wouldn't the voltage be blocked at this point? The 2 smaller wire connections on the alternator should tell it to function? The mode of operation is not fresh in my memory.
Roy Keen
Minden,NV
76 X Glenbrook
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| Re: [GMCnet] Alternator 8 volts output. [message #149521 is a reply to message #149514] |
Sat, 12 November 2011 21:25 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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Roy,
The feedback's not through the diode -- that's the reason for the problem:
the Sense Line bypasses the diode.
If you check the wiring diagram, you'll find that terminal 2 in the
alternator's plug-in connector is a 12g White wire. That's the Voltage
Sense line. It is spliced, inside the wiring harness, to the 12g Red wire
which connects to the Horn Relay and thence, through the Fusible Link, to
the Battery Pickup Junction Block and the Boost Relay to the Engine Battery
-- a LOT of junctions, but nonetheless, a "direct" connection to the Engine
Battery. And it's where the Alternator Regulator determines whether the
Alternator's output should be increased or decreased.
With a failed Isolator Diode, the Alternator's output does not affect the
Engine Battery's voltage, or, consequently that feedback voltage.
Under that condition, early alternators would literally "run away" -- the
voltage might climb to 100+ volts. Early in GMC production, I've heard,
the regulator was modified to limit output to a maximum of about 18 VDC
(your 16 VDC).
Any time the alternator's output is above about 15 VDC, you can bet that
the isolator's probably failed. There are other possible failures with the
same symptom, such as a break in the Sense Line, any one of those junctions
being loose, or a failed Fusible Link. But the Isolator seems to be by far
the most common.
Ken H.
On Sat, Nov 12, 2011 at 9:57 PM, <roy@gmcnet.org> wrote:
>
>
> Ken how does it feed back thtough the diode? wouldn't the voltage be
> blocked at this point? The 2 smaller wire connections on the alternator
> should tell it to function? The mode of operation is not fresh in my memory.
> --
> Roy Keen
> Minden,NV
> 76 X Glenbrook
> _______________________________________________
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: Alternator 8 volts output. [message #149526 is a reply to message #149482] |
Sat, 12 November 2011 22:25 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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Senior Member |
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| roel_b wrote on Sat, 12 November 2011 17:15 |
A couple of weeks ago a thing happened which I cannot really explain, I'll try to describe it.
In the morning on the campground got everything ready for departure, started the engine, switched on the compressor and let the engine running idling at about 800 rpm's. Then I thought of making some coffee for on the road and hooked up the electric waterkettle to the invertor (hooked on to the housebattery) for boiling some water, engine was still idling. After the water had boiled and the wife made the coffee noticed that the voltage, measured at the onboard circuit from the starterbattery, was at 12.5 volts and the generator light at the dash was on. Revving up the engine did nothing, voltage stayed the same, stopped the engine and started her again, no change, opened the engine hatch and touched the alternator which was really hot, took the voltmeter and measured 8 volts at the output stud of the alternator, over at the middlepost of the diode also 8 volts and 12.5 at both batteries. As we only had to drive 250 km's to get home during daytime that day thought I'd take the change and go by battery only for the ignition.
But after about 45 mins driving the generatorlight went dim and the voltmeter pointed at 13.8 volts again, don't know if it was a sudden change or gradually going to 13.8 volts. Since then no problems with the alternator whatsoever.
APC cable installed.
What could have happened here, why exactly 8 volts output all the time and no fluctuations for as long as I measured.
Alternator looks as new and is the stock 80 amps, probably never been rebuild.
Any advice appreciated.
Brgds.,
Roel
|
Roel,
I am working with a nearly forty year old set of memories here so please bear with me.
Your alternator may be just fine now.
Way back when.... I was running tests for a first tier supplier in a primitive dyno lab. The testing was being done around a Chevy truck big block. There were a ton of electrical accessories and DC powered instruments hung on the test rig. Sometimes the alternator would drop off and the run would go to Hell on us. A replacement behaved the same way.
The client engineer got me in contact with a Delco engineer. When I ran my problem by him, he got kind of quiet. He then admitted that this was a known issue, but as it didn't cost warranty - in fact it was working in warranty's favor, nobody wanted to know what the real problem was (or admit if he did know). He suggest that we go to a new larger Delcotron unit (we call it and alternator). And/or modify it for an external regulator and even gave me the Prestolite number. He told me, but I do not recall whether the armature (stator) or the diode bridge would smoke next.
For the EEs reading this:
He went on to explain that his best guess was that the bias requirement for one of the transistors in the regulator got so far out of parameter when it got hot that it messed up - memory problem here - either the sensing input or the reference - yes opposite directions, but I'm stuck - so that the regulator would reduce the field current.
The test continued with an external regulator and a thermocouple in the alternator. It did eat belts, but they were easy for my tech to change.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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| Re: Alternator 8 volts output. [message #149543 is a reply to message #149482] |
Sun, 13 November 2011 08:51 |
roel_b
Messages: 28
Registered: November 2007
Location: The Netherlands
Karma: 0
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Junior Member |
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Thanks Dave and Matt,
Looks like a not so obvious fault after all, as you both say the overheating of the regulator was the culprit. The combination of a high load and not enough RPM's probably caused this event, still I'm a bit puzzled why it took that long to get back to normal operation if it was only a matter of overheating.
Guess the advice would be not to have a high load on the invertor while the engine is running at idling speed, better would be to have the load on the battery with the engine off and start the engine afterwards or have the engine running at at least 1100 RPM's?
Will try to get the alternator parts kit, looks like a nice project, I'm also not so keen on a replacement unit.
Brgds.,
Roel
GMC 230 Painted Desert '73
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| Re: [GMCnet] Alternator 8 volts output. [message #149555 is a reply to message #149521] |
Sun, 13 November 2011 11:44 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
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Senior Member |
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That is what I was thinking. If the chassis side diode in the isolator were open and the small electrical connector on the alternator had a fair amount of voltage supplied from the chassis battery The voltage at the center terminal of the isolator(alternator feed line) should be higher then 16 volts not lower then 12 volts.
Roy Keen
Minden,NV
76 X Glenbrook
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| Re: Alternator 8 volts output. [message #149581 is a reply to message #149482] |
Sun, 13 November 2011 17:46 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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Ken is correct in that it could be the isolator. It also could be anything between the isolator on the path back to the alternator on the send sense line. I have seen two bad fusible links on two different coaches also cause this problem.
If you install the jumper suggested by Ken it will prove whether your problem is the isolator or something else. If the problem is still there after the jumper install, then look at the fusible link, all wiring back to the alternator sense terminal, and then the voltage regulator in the alternator itself.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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