Home » Public Forums » GMCnet » Question for the experts (Transmission shifting problem)
| Question for the experts [message #318392] |
Sat, 03 June 2017 09:58  |
bryant374
Messages: 563
Registered: May 2004
Location: Pleasant Valley, NY 12569
Karma: 1
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Senior Member |
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We had a great weekend at a Connecticut campground last week but had a problem on the road there and back.
The transmissin would not shift into 3rd properly, here are the details.
Starting from a cold start all functions were normal. Several miles down the road the trans would not shift into 3rd.
For the next 100 miles, with precise repeatability this is what happened:
1 to 2 shift normal
2 to 3 shift would NOT occur until 62 MPH was achieved (~3,000 RPM)
Drop below 57 MPH and 3 to 2 shift occured and stayed in 2nd until speed was increased above 62 mph.
As I noted above this was repeated a number of times for the next 100 miles.
And NO, it is not the fan clutch.
About 10 miles from home the problem went away and shifting returned to "normal".
Heading out again in a couple of weeks for Cape Cod, What do you think the problem is, feedback please.
Would rather not repeat this experience :^(
Bill
Bill Bryant
PO 1976~PB (owned 34 years)
1914 Ford (owned 70 years)
1965 Corvette (owned 39 years)
GMC Motorhome History
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| Re: [GMCnet] Question for the experts [message #318393 is a reply to message #318392] |
Sat, 03 June 2017 10:11  |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
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Senior Member |
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Check your vac hose
Sent from my iPhone
> On Jun 3, 2017, at 7:58 AM, Bill Bryant wrote:
>
> We had a great weekend at a Connecticut campground last week but had a problem on the road there and back.
>
> The transmissin would not shift into 3rd properly, here are the details.
>
> Starting from a cold start all functions were normal. Several miles down the road the trans would not shift into 3rd.
>
> For the next 100 miles, with precise repeatability this is what happened:
> 1 to 2 shift normal
> 2 to 3 shift would NOT occur until 62 MPH was achieved (~3,000 RPM)
> Drop below 57 MPH and 3 to 2 shift occured and stayed in 2nd until speed was increased above 62 mph.
>
> As I noted above this was repeated a number of times for the next 100 miles.
> And NO, it is not the fan clutch.
>
> About 10 miles from home the problem went away and shifting returned to "normal".
>
> Heading out again in a couple of weeks for Cape Cod, What do you think the problem is, feedback please.
>
> Would rather not repeat this experience :^(
>
> Bill
>
>
>
>
>
>
>
>
>
> --
> Bill Bryant
> 1976~PB
> 1914 Ford
> 1965 Corvette
> GMC MH History CD & GMC Showroom DVD
> Workprint DVD GMC development 11-70 to 3-71
> http://bdub.net/billbryant/
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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| Re: [GMCnet] Question for the experts [message #318394 is a reply to message #318393] |
Sat, 03 June 2017 10:23 |
Richard Denney
Messages: 920
Registered: April 2010
Karma: 9
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Senior Member |
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Bill, Jim means the vacuum hose to the transmission modulator. If that
looks good, it may be the modulator itself. Unless the hose persuaded me it
was leaking, I'd be tempted to replace the modulator as a matter of
preventive maintenance.
Rick "higher vacuum results in lower shift points" Denney
On Sat, Jun 3, 2017 at 11:12 AM Jim Kanomata wrote:
> Check your vac hose
>
> Sent from my iPhone
>
>> On Jun 3, 2017, at 7:58 AM, Bill Bryant wrote:
>>
>> We had a great weekend at a Connecticut campground last week but had a
> problem on the road there and back.
>>
>> The transmissin would not shift into 3rd properly, here are the details.
>>
>> Starting from a cold start all functions were normal. Several miles
> down the road the trans would not shift into 3rd.
>>
>> For the next 100 miles, with precise repeatability this is what happened:
>> 1 to 2 shift normal
>> 2 to 3 shift would NOT occur until 62 MPH was achieved (~3,000 RPM)
>> Drop below 57 MPH and 3 to 2 shift occured and stayed in 2nd until speed
> was increased above 62 mph.
>>
>> As I noted above this was repeated a number of times for the next 100
> miles.
>> And NO, it is not the fan clutch.
>>
>> About 10 miles from home the problem went away and shifting returned to
> "normal".
>>
>> Heading out again in a couple of weeks for Cape Cod, What do you think
> the problem is, feedback please.
>>
>> Would rather not repeat this experience :^(
>>
>> Bill
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> --
>> Bill Bryant
>> 1976~PB
>> 1914 Ford
>> 1965 Corvette
>> GMC MH History CD & GMC Showroom DVD
>> Workprint DVD GMC development 11-70 to 3-71
>> http://bdub.net/billbryant/
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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--
Rick Denney
73 x-Glacier 230 "Jaws"
Off-list email to rick at rickdenney dot com
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| Re: [GMCnet] Question for the experts [message #318395 is a reply to message #318394] |
Sat, 03 June 2017 10:31 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Driving style may enter into this a bit. If you use full throttle during
later stages of acceleration, but not earlier, you could be suffering from
a lack of vacuum and the transmission may not up shift into third until the
governor forces it to. In addition to checking the hoses, which is
excellent advice, also check the crimped end of the steel line that the
hoses slip over. It can become obstructed with bits of rubber hose and
reduce the vacuum signal as well.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Jun 3, 2017 8:24 AM, "Richard Denney" wrote:
> Bill, Jim means the vacuum hose to the transmission modulator. If that
> looks good, it may be the modulator itself. Unless the hose persuaded me it
> was leaking, I'd be tempted to replace the modulator as a matter of
> preventive maintenance.
>
> Rick "higher vacuum results in lower shift points" Denney
> On Sat, Jun 3, 2017 at 11:12 AM Jim Kanomata
> wrote:
>
>> Check your vac hose
>>
>> Sent from my iPhone
>>
>>> On Jun 3, 2017, at 7:58 AM, Bill Bryant
> wrote:
>>>
>>> We had a great weekend at a Connecticut campground last week but had a
>> problem on the road there and back.
>>>
>>> The transmissin would not shift into 3rd properly, here are the
> details.
>>>
>>> Starting from a cold start all functions were normal. Several miles
>> down the road the trans would not shift into 3rd.
>>>
>>> For the next 100 miles, with precise repeatability this is what
> happened:
>>> 1 to 2 shift normal
>>> 2 to 3 shift would NOT occur until 62 MPH was achieved (~3,000 RPM)
>>> Drop below 57 MPH and 3 to 2 shift occured and stayed in 2nd until
> speed
>> was increased above 62 mph.
>>>
>>> As I noted above this was repeated a number of times for the next 100
>> miles.
>>> And NO, it is not the fan clutch.
>>>
>>> About 10 miles from home the problem went away and shifting returned to
>> "normal".
>>>
>>> Heading out again in a couple of weeks for Cape Cod, What do you think
>> the problem is, feedback please.
>>>
>>> Would rather not repeat this experience :^(
>>>
>>> Bill
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> --
>>> Bill Bryant
>>> 1976~PB
>>> 1914 Ford
>>> 1965 Corvette
>>> GMC MH History CD & GMC Showroom DVD
>>> Workprint DVD GMC development 11-70 to 3-71
>>> http://bdub.net/billbryant/
>>>
>>> _______________________________________________
>>> GMCnet mailing list
>>> Unsubscribe or Change List Options:
>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
> --
> Rick Denney
> 73 x-Glacier 230 "Jaws"
> Off-list email to rick at rickdenney dot com
> _______________________________________________
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| Re: [GMCnet] Question for the experts [message #318400 is a reply to message #318392] |
Sat, 03 June 2017 11:07 |
powerjon
Messages: 2446
Registered: January 2004
Karma: 5
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Senior Member |
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Bill,
As others have already replied the symptoms appear to be a low vacuum or a problem with the rubber and metal line that goes to the modulator or the modulator itself.
As a matter of principal I would replace the rubber portions of the vacuum line that go down to the modulator. I would suspect the rubber line from the manifold to the metal tube that run down to the modulator because of the heat in that area on top of the motor. The bottom piece of rubber could also have failed. The metal that runs down to the modulator is actually a special metal tube as it has a crimp in the line to act as a damper of the vacuum swings that go down to the modulator to shift the transmission. The tube can become blocked or restricted with junk and also cause the problem too. I recommend that the metal tube be flushed out with carb cleaner and blown out with air, you may get some black gunk out of the tube. These are quick and easy fixes and checks you can do yourself.
JR Wright
GMC Great Laker MHC
GMC Eastern States Charter Member
GMCGL Tech Editor
GMCMI
78 GMC Buskirk 30’ Stretch
1975 GMC Avion (Under Reconstruction)
Michigan
> On Jun 3, 2017, at 10:58 AM, Bill Bryant wrote:
>
> We had a great weekend at a Connecticut campground last week but had a problem on the road there and back.
>
> The transmissin would not shift into 3rd properly, here are the details.
>
> Starting from a cold start all functions were normal. Several miles down the road the trans would not shift into 3rd.
>
> For the next 100 miles, with precise repeatability this is what happened:
> 1 to 2 shift normal
> 2 to 3 shift would NOT occur until 62 MPH was achieved (~3,000 RPM)
> Drop below 57 MPH and 3 to 2 shift occured and stayed in 2nd until speed was increased above 62 mph.
>
> As I noted above this was repeated a number of times for the next 100 miles.
> And NO, it is not the fan clutch.
>
> About 10 miles from home the problem went away and shifting returned to "normal".
>
> Heading out again in a couple of weeks for Cape Cod, What do you think the problem is, feedback please.
>
> Would rather not repeat this experience :^(
>
> Bill
>
>
>
>
>
>
>
>
>
> --
> Bill Bryant
> 1976~PB
> 1914 Ford
> 1965 Corvette
> GMC MH History CD & GMC Showroom DVD
> Workprint DVD GMC development 11-70 to 3-71
> http://bdub.net/billbryant/
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
J.R. Wright
GMC GreatLaker
GMC Eastern States
GMCMI
78 30' Buskirk Stretch
75 Avion Under Reconstruction
Michigan
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| Re: [GMCnet] Question for the experts [message #318411 is a reply to message #318392] |
Sat, 03 June 2017 16:48 |
Jeanette
Messages: 118
Registered: May 2013
Karma: 0
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Senior Member |
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Glad to hear u are enjoying your GMC travels. Hope u will come to Elkhart.
Sent from my iPhone
> On Jun 3, 2017, at 9:58 AM, Bill Bryant wrote:
>
> We had a great weekend at a Connecticut campground last week but had a problem on the road there and back.
>
> The transmissin would not shift into 3rd properly, here are the details.
>
> Starting from a cold start all functions were normal. Several miles down the road the trans would not shift into 3rd.
>
> For the next 100 miles, with precise repeatability this is what happened:
> 1 to 2 shift normal
> 2 to 3 shift would NOT occur until 62 MPH was achieved (~3,000 RPM)
> Drop below 57 MPH and 3 to 2 shift occured and stayed in 2nd until speed was increased above 62 mph.
>
> As I noted above this was repeated a number of times for the next 100 miles.
> And NO, it is not the fan clutch.
>
> About 10 miles from home the problem went away and shifting returned to "normal".
>
> Heading out again in a couple of weeks for Cape Cod, What do you think the problem is, feedback please.
>
> Would rather not repeat this experience :^(
>
> Bill
>
>
>
>
>
>
>
>
>
> --
> Bill Bryant
> 1976~PB
> 1914 Ford
> 1965 Corvette
> GMC MH History CD & GMC Showroom DVD
> Workprint DVD GMC development 11-70 to 3-71
> http://bdub.net/billbryant/
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
_______________________________________________
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Unsubscribe or Change List Options:
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| Re: Question for the experts [message #318414 is a reply to message #318392] |
Sat, 03 June 2017 17:53 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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Many vac hoses that look good at a glance are bad under very close inspection. Also if someone used thin wall washer hose it can suck flat when heat softens it. Lastly if it is longer than perfect in say a bit of an "S" it can pinch off under vacuum. Ask me how I know. The shop shall remain nameless and it was a very easy and cheap fix to correct other than the lying on asphalt in a parking lot part---years ago.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] Question for the experts [message #318416 is a reply to message #318399] |
Sat, 03 June 2017 18:06 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Bill,
The steel tube on double trouble had rusted through and was leaking. I cut
it at the hole and reconnected the two pieces with a piece of rubber hose.
Regards,
Rob M.
The Pedantic Mechanic
USAussie - Downunder
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808
-----Original Message-----
From: Gmclist [mailto:gmclist-bounces@list.gmcnet.org] On Behalf Of Billy
Massey
Sent: Sunday, June 4, 2017 1:42 AM
To: gmclist@list.gmcnet.org
Subject: Re: [GMCnet] Question for the experts
Direct your attention to the modulator in the trans or a vacuum hose on
either end of the steel line going to it. Probably a split in one of the
hoses or the steel line.
bdub
-----Original Message-----
From: Gmclist [mailto:gmclist-bounces@list.gmcnet.org] On Behalf Of Bill
Bryant
Sent: Saturday, June 03, 2017 9:58 AM
To: gmclist@list.gmcnet.org
Subject: [GMCnet] Question for the experts
We had a great weekend at a Connecticut campground last week but had a
problem on the road there and back.
The transmissin would not shift into 3rd properly, here are the details.
Starting from a cold start all functions were normal. Several miles down
the road the trans would not shift into 3rd.
For the next 100 miles, with precise repeatability this is what happened:
1 to 2 shift normal
2 to 3 shift would NOT occur until 62 MPH was achieved (~3,000 RPM) Drop
below 57 MPH and 3 to 2 shift occured and stayed in 2nd until speed was
increased above 62 mph.
As I noted above this was repeated a number of times for the next 100 miles.
And NO, it is not the fan clutch.
About 10 miles from home the problem went away and shifting returned to
"normal".
Heading out again in a couple of weeks for Cape Cod, What do you think the
problem is, feedback please.
Would rather not repeat this experience :^(
Bill
--
Bill Bryant
1976~PB
1914 Ford
1965 Corvette
GMC MH History CD & GMC Showroom DVD
Workprint DVD GMC development 11-70 to 3-71 http://bdub.net/billbryant/
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
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_______________________________________________
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: Question for the experts [message #318419 is a reply to message #318392] |
Sat, 03 June 2017 18:31 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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This is pretty much a repeat of what everyone else has said with a little additional information.
There are two vacuum hoses and a steel line that supply vacuum to the modualtor. When the modulator sees no vacuum, it assumes that you are depressing the accelerator all the way down so it delays the upshift. So check or replace the top vacuum hose that is connected to a steel line that is located on top of the intake manifold on the right front of the engine right next to the valve cover. This steel line runs across the top of the intake next to the valve cover and then down the back of the engine. It ends a few inches from the modulator. Connected there is a small piece of vacuum hose that runs from the steel line to the modulator. Check or replace that hose also.
If neither of the above fix your problem, then we need to look at the steel line. You can do this a couple of ways.
1. Disconnect booth hoses and connect air pressure to the BOTTOM END of the line. Then check to see if there is air coming out of the top end of the line. If you have air then the line is good. No air then it is plugged.
If there is no air, then remove the line that is held down with a couple of the valve cover bolts and work on clearing it.
I had one once that would not clear with 100 psi of air pressure attached to it. So I left the air pressure on it and heated the pipe with an acetylene torch. Starting at the top end and working to a past the crimp. It cleared after a minute or two of heat application. I repainted the line gray and reinstalled it about 12 or 13 years ago.
2. You could also attach a vacuum gauge to the vacuum hose on the transmission end. Start the engine and see if you have vacuum. If you do then you probably have a bad / stuck modulator.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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