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| [GMCnet] Just a Short Trip [message #166365] |
Sat, 14 April 2012 23:44  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Yesterday I got a call from Jerry Holloway. He and Audrey were taking a
short weekend trip from Griffin, GA to Warwick, GA to attend the "Grits
Festival" (another reason to visit us, JimK). He was seeing higher than
usual engine oil temperature, 250*F coming out of the oil pump. While sort
of high for a 70*F day on mostly flat roads, I assured him that it wasn't
anything to be overly concerned about.
Last night I got another call: "Everything's falling apart!: The
refigerator's quit. The generator runs but doesn't put out 120 vac. My
turn signals don't work. Can I come by there on our way home tomorrow?"
"Sure -- plan to spend the night here." "We'll be there about 4:30."
Just after noon today I got another call: "We'll be there in an hour or
so. Oh yeah, I just stopped for gas and had to use the boost switch to
start the engine. I turned on the dash air blower to Hi and it works, so
the alternator's putting out."
About 2 PM they pulled in behind my engine-less GMC and Jerry and I started
troubleshooting.
Sure 'nuff, the refrigerator wasn't working. He'd recently replaced the
control board with one of the great "Dinosaur" boards. It had a blown DC
supply fuse, which he'd replaced. I couldn't find anything wrong, so
advised him to exercise the 3 year warranty.
The generator, probably the cleanest 6kW Power Drawer I've ever seen,
started right up but wouldn't power anything. I checked the CB and it was
OK. We decided the generator wasn't essential to their trip home. I
pointed out to Jerry that the house battery in the rear compartment was
leaking and corroding the surrounding aluminum. When he touched it, he
found the case swelled and hot! Hmmm...
So we moved on to the starting problem. The engine wouldn't start. I
checked and found the battery at 10.5 VDC, and noticed the sticker on the
WalMart battery said "10/06". With no further discussion, Jerry said
"Let's go to WalMart", and we did. After he installed the new battery, the
engine fired right up. When I checked the voltage on the alternator
terminal of the isolator, it was 27.5 VDC so I immediately told him to shut
the engine down. He raised the hatch and reported that the alternator was
VERY hot. Assuming that the regulator had failed, we pulled the
alternator, an AutoZone 100A version installed 02/07 IIRC. They tested it
and despite good readings, immediately shut it down because of excessive
noise and vibration. Surprisingly they had one in stock.
After we got the alternator back in, I monitored its output while Jerry
started the engine. OOoops! 31 VDC!!! SHUT IT DOWN!
After disconnecting the leads to the alternator, I checked the diodes.
Sure 'nuff, the chassis battery side's diode was open. That deprived the
alternator of feedback voltage and allowed it to run "open loop". What I
do NOT understand is why the voltage went so high. It has always been my
understanding that shortly after GMC MH production began, they began to use
regulators with an internal voltage limit of about 18 VDC. I've never
before seen an alternator output voltage that high.
After re-attaching the isolator leads, with the chassis battery wire moved
to the alternator terminal, we again checked voltages with the engine
running: Chassis side @ 14.6 VDC and House side @ 13.7 VDC -- Exactly what
I expected.
Now we know pretty well what happened: The isolator failed some time over
the past year while they haven't used the coach. During the 100 mile trip
down I-75, the chassis battery was receiving no charge current and the
alternator was busy boiling the house battery with 27+ VDC. It also
damaged the refrigerator's Dinosaur board before its fuse blew. And the
oil temperature read high, tracking the high applied voltage. The miracle
is that more electronic components weren't damaged (there may be some we
haven't yet found).
If I'd checked the isolator first off (which was my first inclination)
Jerry could probably have gotten a little more use out of the 6 year old
chassis battery. And we might not have replaced the alternator (even
though AutoZone did so willingly). The house battery's only a year or so
old and will undoubtedly require warranty replacement soon, though it's
holding full+ charge at the moment.
We don't know what happened to the generator -- I suspect the bridge
rectifier blew all on its own. We'll check that tomorrow -- they're
sleeping in their coach tonight with full hookups 1/2 under my RV shelter.
SO, I learned something new today (as during each of the previous 27,454):
Not all alternators are internally limited to 18 VDC!!! BAD NEWS!
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI+ & EBL
www.gmcwipersetc.com
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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: [GMCnet] Just a Short Trip [message #166373 is a reply to message #166367] |
Sun, 15 April 2012 04:48  |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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| Bob de Kruyff wrote on Sat, 14 April 2012 23:55 |
Could you imagine if the poor guy had a Cad 500?
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Are you picking on Colonel Ken?
He has had enough bad luck for while so these days I'm picking on Dan instead.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Just a Short Trip [message #166375 is a reply to message #166367] |
Sun, 15 April 2012 06:13 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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He doesn't know it yet, but I'm gonna keep finding things wrong 'til the
Cad's ready for him to help re-install it! :-)
Seriously, he was my victim when I went to TN all those years ago to get
the X-PB with the rebuilt front clip. He's the one who went along and
helped install my old clip in that hulk. He probably will be back for the
Cad re-installation.
He's and ex-USAF/Delta retired aircraft mechanic who's had twice as many
GMC's as I have.
Ken H.
On Sun, Apr 15, 2012 at 12:55 AM, Bob de Kruyff wrote:
>
>
> Could you imagine if the poor guy had a Cad 500?
> --
>
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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: [GMCnet] Just a Short Trip [message #166376 is a reply to message #166372] |
Sun, 15 April 2012 06:15 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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Yep. I've told him to order a combiner instead of a replacement isolator.
He did have an APC. I'm not sure how much it helped since everything DC
in the coach was exposed to the high voltage in the "forward direction".
Ken H.
On Sun, Apr 15, 2012 at 5:41 AM, gene Fisher wrote:
> so a combiner might have averted this problem with the open isolator?
> and
> I hope he had an APC ;>)
>
>
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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: [GMCnet] Just a Short Trip [message #166377 is a reply to message #166373] |
Sun, 15 April 2012 06:19 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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And here I thought you were mad at me for "outing" you to the world!
Ken H.
On Sun, Apr 15, 2012 at 5:48 AM, Ken Burton wrote:
>
>
> Bob de Kruyff wrote on Sat, 14 April 2012 23:55
> > Could you imagine if the poor guy had a Cad 500?
>
>
> Are you picking on Colonel Ken?
>
> He has had enough bad luck for while so these days I'm picking on Dan
> instead.
>
>
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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: [GMCnet] Just a Short Trip [message #166423 is a reply to message #166376] |
Sun, 15 April 2012 14:48 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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The APC is useless in protecting the House side when there is no reference voltage to the alternator from the engine side. The APC was not designed for the function. Remember back when we were discussing various fixes for the over voltage problem. This potential failure was an unresolved issue. At the time we were more concerned in burning up the nichrome wire than anything else.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Just a Short Trip [message #166437 is a reply to message #166365] |
Sun, 15 April 2012 17:00 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Yes it's fun to assemble the facts once you solve it and see what happened. My guess at what started the failure train is the relatively old chassis battery may have had a shorted cell that took out the old isolator, Then the rest of the events exactly how you listed them. Old batteries that don't want to charge anymore can really put a load (short) on the charging system and present a low impedance to the charge circuit. I had a dead Delco 75 a few months ago and I put a 50A charger on it (that was the lowest setting on the only unit available) and came back to find the charger breaker cycling (fan on and speed slowing each time the load came on!) The battery had smoke blowing out of the vents and the clamps were so hot you could not touch from current as was the battery... too hot to handle. Good thing I didn't leave that one unattended for long. It would have eventually melted to China.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] Just a Short Trip [message #166446 is a reply to message #166437] |
Sun, 15 April 2012 18:30 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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John,
It's certainly possible that the weak chassis battery did have dead cell
and drew too much current, causing the isolator to fail. But I doubt it.
Under that condition I'd expect the continuous current to overheat the
diode, eventually causing it to short. Shorting is the normal failure mode
for solid state diodes in most applications. But in an isolator that's not
a common failure mode -- they usually open. And that's a mechanical
failure. The thermal cycling of the isolator's potting compound eventually
breaks the weld of the diodes to the plate to which they're both welded.
Which is unfortunate: if the diode shorted, the chassis battery would see
full alternator output and would feed it back so the voltage would not
rise. The only detrimental effect would be the house battery undercharged
by 0.7 VDC.
If I were building an isolator, I'd make sure the diodes were surrounded
with a flexible material before the potting compound was applied so they'd
be unaffected by thermal cycling. Or use a flexible potting compound.
Since I'm not, I'll just keep using Yandina combiners & not worry about
isolator failures or consequent high alternator output.
I still don't understand anyone building an alternator without internal
voltage limiting.
Ken H.
On Sun, Apr 15, 2012 at 6:00 PM, John R. Lebetski <gransport@aol.com> wrote:
>
>
> Yes it's fun to assemble the facts once you solve it and see what
> happened. My guess at what started the failure train is the relatively old
> chassis battery may have had a shorted cell that took out the old isolator,
> Then the rest of the events exactly how you listed them. Old batteries
> that don't want to charge anymore can really put a load (short) on the
> charging system and present a low impedance to the charge circuit...
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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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