| trans. modulator questions [message #80356] |
Mon, 12 April 2010 17:52  |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
Karma: 0
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Greetings GMC'ers. It's been a long wet miserable winter but it's time to try to get some use from the old girl.
The coach does not shift up.
I followed the lines from the modulator, it goes to two places on the manifold exactly like the GMC except my modulator is considerably larger than the one on the 74 GMC, (which is still here Rich, call me)
Like the GMC, there are wires going into the trans near the modulator. What do they do?
As it was shifting perfectly on the 200 miles drive home, and then stopped shifting after i messed with the manifolds and other stuff on the engine, my troubleshooting leads me to something i touched.
I was hoping to find an obvious disconnected hose, but would smaller vac leaks alone cause it to fail?
Thanks
Dave & Ellen Silva
Hertford, NC
76 Birchaven, 1-ton and other stuff
Currently planning the Great american Road Trip Summer 2021
It's gonna take a lot of Adderall to get this thing right.
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| Re: [GMCnet] trans. modulator questions [message #80360 is a reply to message #80356] |
Mon, 12 April 2010 18:05  |
Mr ERFisher
Messages: 7117
Registered: August 2005
Karma: 2
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did you read here?
http://gmcmotorhome.info/transmission.html#GOV
gene
On Mon, Apr 12, 2010 at 3:52 PM, dave silva <gmc@davesilva.com> wrote:
>
>
> Greetings GMC'ers. It's been a long wet miserable winter but it's time to
> try to get some use from the old girl.
>
> The coach does not shift up.
>
> I followed the lines from the modulator, it goes to two places on the
> manifold exactly like the GMC except my modulator is considerably larger
> than the one on the 74 GMC, (which is still here Rich, call me)
>
> Like the GMC, there are wires going into the trans near the modulator.
> What do they do?
>
>
> As it was shifting perfectly on the 200 miles drive home, and then stopped
> shifting after i messed with the manifolds and other stuff on the engine, my
> troubleshooting leads me to something i touched.
>
> I was hoping to find an obvious disconnected hose, but would smaller vac
> leaks alone cause it to fail?
>
> Thanks
>
>
> --
> Free to good home- the rest of a 1974 GMC. good for extension, or trailer.
> But you gotta' take the whole thing -zip 27944- rear wheels, glass, etc.
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
Gene Fisher -- 74-23,77PB/ore/ca
“Give a man a fish; you have fed him for today --- give him a URL and
-------
http://gmcmotorhome.info/
Alternator Protection Cable
http://gmcmotorhome.info/APC.html
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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| Re: [GMCnet] trans. modulator questions [message #80403 is a reply to message #80360] |
Mon, 12 April 2010 22:14 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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The wires on the trans are the down shift solenoid. They are controlled by a switch mounted under the dash on the top of the gas pedal. You can unplug the wires if you wish to eliminate any question if that is your up shift problem.
The question I have for you is: Does it ever up-shift if you go fast enough? If it does then I suspect a vacuum problem to the modulator or a down shift solenoid switch problem.
If it never upshifts and the speedometer does not work then I would look at the governor drive gear.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] trans. modulator questions [message #80450 is a reply to message #80403] |
Tue, 13 April 2010 09:27 |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
Karma: 0
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Senior Member |
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Good stuff, Ken. Just the kind of insight I was looking for.
Yes, it did shift once or twice when i got the RPMs high enough.
The speedo works but not well. It reads low until it's been running for a while but it never becomes accurate, off by about 20%
What's the connection between the speedo and the shifting?
Thanks
Dave & Ellen Silva
Hertford, NC
76 Birchaven, 1-ton and other stuff
Currently planning the Great american Road Trip Summer 2021
It's gonna take a lot of Adderall to get this thing right.
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| Re: trans. modulator questions [message #80505 is a reply to message #80356] |
Tue, 13 April 2010 22:22 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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Check the easy stuff first. Really check the vac lines carefully as it is Rubber--steel---rubber. They can be cracked where you can't see or if too long pinched (asd\k me how I know) or if they have been exposed to chemicals can age "soft" instead of cripy and suck flat. Veeeerrryy cheap fix if that's it.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] trans. modulator questions [message #80507 is a reply to message #80450] |
Tue, 13 April 2010 23:41 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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| hertfordnc wrote on Tue, 13 April 2010 09:27 |
Good stuff, Ken. Just the kind of insight I was looking for.
Yes, it did shift once or twice when i got the RPMs high enough.
The speedo works but not well. It reads low until it's been running for a while but it never becomes accurate, off by about 20%
What's the connection between the speedo and the shifting?
Thanks
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OK Emery gave you most of it but ..
The reason I asked about the speedo is if it did not work at all that tells me the problem was the governor gear.
I suspect the modulator or the vacuum to the modulator is your problem. Emery gave you one way to check it. If you pluu the hose off of the modulator and find trans fluid in the hose then the modulator has a bad diaphragm.
If you have a vacuum gauge and a long piece of hose you can insert a tee at the modulator and run the hose through the passenger window and watch the gauge.
I had your failure exactly on mine. It was caused by a clogged steel vacuum line on top of the engine. There line that runs from the front of the engine to the rear of the engine across the right side of the intake manifold and down the rear to the modulator. The steel line was so badly clogged that I could not open it up. I finally removed the line and attached 120 PSI air pressure to the bottom end it. Then I heated the front end where the crimp is with an acetylene torch until it finally opened up. After that I bead blasted the line, painted it gray and reinstalled it. Several other people have had this same problem so it is not unique to my coach.
I would check the vacuum at the modulator at idle and see what you have there.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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