Home » Public Forums » GMCnet » downshifting on a flat road at 55mph?!?!?!?! (transmission issues)
| downshifting on a flat road at 55mph?!?!?!?! [message #98029] |
Wed, 01 September 2010 07:22  |
Tripp 33
Messages: 99
Registered: October 2009
Karma: 0
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So, I was driving down from Birmingham, MI to Toledo, OH yesterday and for no reason at all my GMC downshifted to a lower gear as I was cruising at 65mph. Now the total elevation change on this 90 mile stretch is about 4' excluding any minor over passes and lone bridge over the Rouge River in the scenic "down river" area of Detroit.
Some things to consider is that it was 90 degress but I had a fresh tank of cool gas, radiator is running cool and I was not pressing hard on the gas. I tried to throw it into neutral several times but when I threw it back into Drive, it went right to the lower gear.
This took place about 5 times and seemed to cycle every ten minutes. The lower gear lasted about 1 minute each but this is certainly concerning me.
I have plenty of transmission fluid and it is only about 3 months old. I have a 403 and it is running so well that it too is scaring me. ha ha.
Any suggesstions, comments or questions are welcome.
Thanks for listening and helping out a newer "GMCer"
Jeff
Toledo, OH
1978 Transmode limo configuration
"WILD ROVER"
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| Re: downshifting on a flat road at 55mph?!?!?!?! [message #98037 is a reply to message #98029] |
Wed, 01 September 2010 08:27  |
roel_b
Messages: 28
Registered: November 2007
Location: The Netherlands
Karma: 0
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Hi Jeff,
Could be that the kickdown solenoid is activated for no apparant reason, I experienced a similar thing a couple of weeks ago.
When we drove off from a parkinglot, the GMC had been there all day, we only had first gear, speedometer worked fine so the governor gear was allright, but no upshifting, added some oil to the transmission but that changed nothing and as it became dark we parked in a parkinglot.
The next morning the same thing and I went to the kickdownswitch at the accelerator pedal, as I had done some work on it before we left for the trip, removed the switch, engaged the ignition and shortcircuited the switch but no clicking at the transmission. After fiddling with the wires coming from the kickdownswitch and following it all the way to the modulator it started clicking again, we drove off and upshifting was OK again.
Would not be driving all the way from Belgium to The Netherlands with only first gear, later on I realized there is a max speed with the kickdown engaged but was not ready to test this at that time...
GMC 230 Painted Desert '73
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| Re: [GMCnet] downshifting on a flat road at 55mph?!?!?!?! [message #98049 is a reply to message #98044] |
Wed, 01 September 2010 10:15 |
kelvin
Messages: 608
Registered: February 2004
Location: Eugene, OR
Karma: 0
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Senior Member |
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It would suck getting stuck somewhere needed a couple simple parts like
that. And they're small enough to carry but not something you're going
to find at your local parts store.
Can we get that governor from a GM dealer? At the least I should have
the P/N written down somewhere. And I assume the Modulator is available
from the same source.
I should probably just get them stowed away in the coach and be done
with it...
Kelvin
'73 23' in Eugene, OR
> Yea, that is my old governor in the second link Larry put up. Wally came down and we finally swapped it out, last resort. Some will recall the thing was broken and fooling the trans into thinking it should be in 2nd gear. What a headache it was trying to get home, 200 miles, with it acting up. Wally took it home and fixed it. I am now looking for a spare gov. to carry with me. So, if it is not the modulator, try another governor, if you can find one.
> Dan
>
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| Re: downshifting on a flat road at 55mph?!?!?!?! [message #98146 is a reply to message #98029] |
Wed, 01 September 2010 22:19 |
Craig Lechowicz
Messages: 541
Registered: October 2006
Location: Waterford, MI
Karma: 0
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| Tripp 33 wrote on Wed, 01 September 2010 08:22 |
So, I was driving down from Birmingham, MI to Toledo, OH yesterday and for no reason at all my GMC downshifted to a lower gear as I was cruising at 65mph.
Some things to consider is that it was 90 degress but I had a fresh tank of cool gas, radiator is running cool and I was not pressing hard on the gas. I tried to throw it into neutral several times but when I threw it back into Drive, it went right to the lower gear.
This took place about 5 times and seemed to cycle every ten minutes. The lower gear lasted about 1 minute each but this is certainly concerning me.
Jeff
Toledo, OH
1978 Transmode limo configuration
"WILD ROVER"
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Jeff,
The 10 minute time interval and the high temperatures outside make me wonder if rather than the transmission downshifting, what you heard was the fan clutch kicking in. If you are not used to sitting on top of the engine like you do in a GMC, it can sound a lot like a downshift of the transmission. Just a thought. Might try driving it on a cooler day. (like this coming Saturday) or hooking a tach to it to see if the engine rpm is really changing.
Craig Lechowicz
'77 Kingsley, Waterford, MI
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| Re: [GMCnet] downshifting on a flat road at 55mph?!?!?!?! [message #98149 is a reply to message #98146] |
Wed, 01 September 2010 22:41 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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In '96, driving a '90 Airstream Land Yacht west through the Texas panhandle,
I slowed from 55 mph and the transmission downshifted. When I'd speed back
up, it would upshift again. Kept that up all through the hot afternoon
until we got to Palo Duro Canyon State Park for the night. When I called my
old USAF pilot friend in Rocky Mount, NC, then an RV dealer, he suggested I
check the radiator fan.
The Airstream had a thermostatically controlled electric pusher booster fan.
I added a manual switch and sure 'nuff, when the transmission downshifted,
flipping that switch on would cause it to "up shift". I soon learned when
to turn the fan on to avoid the "downshift".
The GMC's never fooled me, thanks to that experience. :-)
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI+ & EBL
www.gmcwipersetc.com
On Wed, Sep 1, 2010 at 11:19 PM, Craig Lechowicz <
craig.lechowicz@sbcglobal.net> wrote:
>
>
> Tripp 33 wrote on Wed, 01 September 2010 08:22
> > So, I was driving down from Birmingham, MI to Toledo, OH yesterday and
> for no reason at all my GMC downshifted to a lower gear as I was cruising at
> 65mph.
>
...
> Jeff,
> The 10 minute time interval and the high temperatures outside make me
> wonder if rather than the transmission downshifting, what you heard was the
> fan clutch kicking in.
...
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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: downshifting on a flat road at 55mph?!?!?!?! [message #98161 is a reply to message #98029] |
Thu, 02 September 2010 04:19 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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The modulator controls the trans shift points based on engine vacuum.
1. I am guessing that you have a loss of vacuum to the modulator. Check the hose on the modulator for cracks or maybe it disconnected. That hose just pulls off of the modulator. The hose connects to a small steel pipe that runs up the back side of the engine and across the top of the intake manifold. I had the pipe plug up and cause the same problem. The other end of the pipe is hooked to a hose that goes to the intake manifold. Check or replace that hose also. Again it just pulls off by hand.
2. If the pipe is plugged up the restriction will be on the manifold end about 2 inches from the end. Disconnect the manifold hose and the modulator hose. Then try to blow through the pipe using the hose removed from the manifold.
3. The modulator could be bad. They are available from AutoZone. The old modulator removes from the side of the trans by removing one bolt. If you remove the hose from the modulator and find oil in the hose, replace the modulator because the diaphragm is ruptured.
4. The kick down switch could be engaging. I doubt this is the problem but you can unplug it at the transmission and see if this fixes your problem. It is the only set of wires that plug into the side of the trans. I believe the plug is on the driver's side but could be wrong.
5. Finally, it could be the governor. I argued with Dan over and over when he had a similar problem. I was saying it could not be the governor. Well, I was wrong. Dan and Wally found his problem to be the governor. I would do everything else above before I attacked the governor as the problem.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] downshifting on a flat road at 55mph?!?!?!?! [message #98164 is a reply to message #98146] |
Thu, 02 September 2010 04:52 |
powerjon
Messages: 2446
Registered: January 2004
Karma: 5
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Senior Member |
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I have to agree with Craig, I think that your fan clutch is cycling on
and off especally in the hot temps lately in the Mich/Ohio area this
week. It would sound like your transmission has shifted down. This
has always been a common issue that suprises many new GMC owners and
has been on this talked about many many times in the gmc motorhome
market place magazine, Cinnabars newsletter and other GMC publications.
Do you have a tach and is the actual rpm of the motor really going
higher. Does the coach slow down when you hear the sound and you
haven't changed your gas pedal?
J.R. Wright
GMC GreatLaker
TZE Zone Restorations
77 Eleganza Custom (For Sale)
1975 Avion (Under Reconstruction)
Michigan
>
>
> Tripp 33 wrote on Wed, 01 September 2010 08:22
>> So, I was driving down from Birmingham, MI to Toledo, OH yesterday
>> and for no reason at all my GMC downshifted to a lower gear as I
>> was cruising at 65mph.
>>
>> Some things to consider is that it was 90 degress but I had a fresh
>> tank of cool gas, radiator is running cool and I was not pressing
>> hard on the gas. I tried to throw it into neutral several times
>> but when I threw it back into Drive, it went right to the lower gear.
>> This took place about 5 times and seemed to cycle every ten
>> minutes. The lower gear lasted about 1 minute each but this is
>> certainly concerning me.
>>
>>
>> Jeff
>> Toledo, OH
>> 1978 Transmode limo configuration
>> "WILD ROVER"
>
>
> Jeff,
> The 10 minute time interval and the high temperatures outside make
> me wonder if rather than the transmission downshifting, what you
> heard was the fan clutch kicking in. If you are not used to sitting
> on top of the engine like you do in a GMC, it can sound a lot like a
> downshift of the transmission. Just a thought. Might try driving
> it on a cooler day. (like this coming Saturday) or hooking a tach
> to it to see if the engine rpm is really changing.
> --
> Craig Lechowicz
> '77 Kingsley, Waterford, MI
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J.R. Wright
GMC GreatLaker
GMC Eastern States
GMCMI
78 30' Buskirk Stretch
75 Avion Under Reconstruction
Michigan
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| Re: downshifting on a flat road at 55mph?!?!?!?! [message #98187 is a reply to message #98146] |
Thu, 02 September 2010 08:24 |
shawnee
Messages: 422
Registered: February 2004
Location: NC
Karma: 0
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Senior Member |
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| Craig Lechowicz wrote on Wed, 01 September 2010 23:19 |
| Tripp 33 wrote on Wed, 01 September 2010 08:22 |
So, I was driving down from Birmingham, MI to Toledo, OH yesterday and for no reason at all my GMC downshifted to a lower gear as I was cruising at 65mph.
Some things to consider is that it was 90 degress but I had a fresh tank of cool gas, radiator is running cool and I was not pressing hard on the gas. I tried to throw it into neutral several times but when I threw it back into Drive, it went right to the lower gear.
This took place about 5 times and seemed to cycle every ten minutes. The lower gear lasted about 1 minute each but this is certainly concerning me.
Jeff
Toledo, OH
1978 Transmode limo configuration
"WILD ROVER"
|
Jeff,
The 10 minute time interval and the high temperatures outside make me wonder if rather than the transmission downshifting, what you heard was the fan clutch kicking in. If you are not used to sitting on top of the engine like you do in a GMC, it can sound a lot like a downshift of the transmission. Just a thought. Might try driving it on a cooler day. (like this coming Saturday) or hooking a tach to it to see if the engine rpm is really changing.
|
Jeffd,
I agree, It might be the fan clutch coming on. It is common for one to come on when you let up on the gas. It will occur at the top of a grade or when you let up for an exit, but it will only stay on for maybe 30 seconds. The drag on the engine is enough to possibly make you think the transmission shifted down. What happens is you let up on the gas and the engine rpm reduces. The heat is still in the engine and radiator, but the air flow through the radiator reduces which causes an increase in the air temperature to the fan clutch. This will only happen if you have a strong fan clutch that you can hear when it comes on. The light duty clutches do not make enough noise for us old people to hear.
Gene Dotson
74 Canyonlands
www.bdub.net/Motorhome_Enhancements New Windows and Aluminum Radiators
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| Re: [GMCnet] downshifting on a flat road at 55mph?!?!?!?! [message #98192 is a reply to message #98187] |
Thu, 02 September 2010 08:52 |
Tripp 33
Messages: 99
Registered: October 2009
Karma: 0
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Member |
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Gene dan et al,
I went from 65mph right down to 55 mph. I tried putting it in neutral and then back into drive and it started again.
I am going to try to replace the vacuum modulator/flux capacitor thing first and then see if it help. if not, I will install the tach to see if that tell me anything else.
You guys have been a great help and I really appreciate the responses.
So, my 403 has 82,000 miles and seems to be running great and I was wondering when they stop running so well? I know it all depends on the way it is driven and frequency but I was just curious what others have noticed.
Jeff
> To: gmclist@temp.gmcnet.org
> From: shawnee@charter.net
> Date: Thu, 2 Sep 2010 08:24:00 -0500
> Subject: Re: [GMCnet] downshifting on a flat road at 55mph?!?!?!?!
>
>
>
> Craig Lechowicz wrote on Wed, 01 September 2010 23:19
> > Tripp 33 wrote on Wed, 01 September 2010 08:22
> > > So, I was driving down from Birmingham, MI to Toledo, OH yesterday and for no reason at all my GMC downshifted to a lower gear as I was cruising at 65mph.
> > >
> > > Some things to consider is that it was 90 degress but I had a fresh tank of cool gas, radiator is running cool and I was not pressing hard on the gas. I tried to throw it into neutral several times but when I threw it back into Drive, it went right to the lower gear.
> > > This took place about 5 times and seemed to cycle every ten minutes. The lower gear lasted about 1 minute each but this is certainly concerning me.
> > >
> > >
> > > Jeff
> > > Toledo, OH
> > > 1978 Transmode limo configuration
> > > "WILD ROVER"
> >
> >
> > Jeff,
> > The 10 minute time interval and the high temperatures outside make me wonder if rather than the transmission downshifting, what you heard was the fan clutch kicking in. If you are not used to sitting on top of the engine like you do in a GMC, it can sound a lot like a downshift of the transmission. Just a thought. Might try driving it on a cooler day. (like this coming Saturday) or hooking a tach to it to see if the engine rpm is really changing.
>
>
> Jeffd,
>
> I agree, It might be the fan clutch coming on. It is common for one to come on when you let up on the gas. It will occur at the top of a grade or when you let up for an exit, but it will only stay on for maybe 30 seconds. The drag on the engine is enough to possibly make you think the transmission shifted down. What happens is you let up on the gas and the engine rpm reduces. The heat is still in the engine and radiator, but the air flow through the radiator reduces which causes an increase in the air temperature to the fan clutch. This will only happen if you have a strong fan clutch that you can hear when it comes on. The light duty clutches do not make enough noise for us old people to hear.
> --
> Gene Dotson
> 74 Canyonlands
> www.bdub.net/Motorhome_Enhancements New Windows and Aluminum Radiators
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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| Re: [GMCnet] downshifting on a flat road at 55mph?!?!?!?! [message #98207 is a reply to message #98192] |
Thu, 02 September 2010 11:07 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""You guys have been a great help and I really appreciate the responses.
So, my 403 has 82,000 miles and seems to be running great and I was wondering when they stop running so well? I know it all depends on the way it is driven and frequency but I was just curious what others have noticed.
""
They will easily run well past 100K but your timing chain may be getting a bit loose right about now. Other than that, I don't suspect any problems on the horizon.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] downshifting on a flat road at 55mph?!?!?!?! [message #98210 is a reply to message #98207] |
Thu, 02 September 2010 11:14 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
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Senior Member |
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I always thought the "rebuild section" on Sirum's site was very well written and good information on the live expectancy of a 455/403 ...
http://www.gmcmh.com/New_Folder3/Engine_Rebuild.html
Dennis Sexton
73 PD 230
Germantown, TN 38138-2066
USA
-----Original Message-----
From: Bob de Kruyff <NEXT2POOL@AOL.COM>
To: gmclist@temp.gmcnet.org
Sent: Thu, Sep 2, 2010 11:07 am
Subject: Re: [GMCnet] downshifting on a flat road at 55mph?!?!?!?!
""You guys have been a great help and I really appreciate the responses.
o, my 403 has 82,000 miles and seems to be running great and I was wondering
hen they stop running so well? I know it all depends on the way it is driven
nd frequency but I was just curious what others have noticed.
"
They will easily run well past 100K but your timing chain may be getting a bit
oose right about now. Other than that, I don't suspect any problems on the
orizon.
-
ob de Kruyff
8 Eleganza
handler, AZ
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MCnet mailing list
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Dennis S
73 Painted Desert 230
Memphis TN Metro
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