GMCforum
For enthusiast of the Classic GMC Motorhome built from 1973 to 1978. A web-based mirror of the GMCnet mailing list.

Home » Public Forums » GMCnet » Motor home feels like it starts in Second sometimes
Motor home feels like it starts in Second sometimes [message #332258] Thu, 17 May 2018 20:39 Go to next message
Duce Apocalypse is currently offline  Duce Apocalypse   United States
Messages: 824
Registered: May 2009
Location: Los angeles
Karma: 0
Senior Member

Just wondering what might be the cause? a bad shift modulator? I checked the linkage and it checks out, but sometimes after stopping at a light it feels sluggish like its rolling in 2nd gear rather than first. tranny fluid is also pink and full so no problems there....

73 Canyon Lands, (a.k.a. The Yellow Submarine) West Los Angeles CA
Re: Motor home feels like it starts in Second sometimes [message #332264 is a reply to message #332258] Fri, 18 May 2018 09:18 Go to previous messageGo to next message
Bullitthead is currently offline  Bullitthead   United States
Messages: 1411
Registered: November 2013
Karma: 5
Senior Member
ATF should be RED, the same color it is new from the can. It turns pink when mixed with water. Compare it to some new fluid to make sure.

Mine with the 455 and a 3.07 FD doesn't want to shift into first automatically unless I make a complete stop. Even after that, it shifts to second within 3 coach lengths unless I really call for power on takeoff. Driving slowly in unknown areas it doesn't bother me or the coach, but if I have to compete with other traffic or traffic controls I have to manually shift it to low at every stop so I can keep it in first gear until about 25 mph. Then the engine is in a better place in the power band at the second gear engagement. If I have to get back to more than 45 after a stop I'll hold off the third gear also because it will shift to third automatically as soon as I allow it with the linkage.

Some modulators can be adjusted with a screw in the vacuum port. I don't remember which way does what, so you'll have to find the instructions, which are probably generic and available online.


Terry Kelpien ASE Master Technician 73 Glacier 260 Smithfield, Va.
Re: Motor home feels like it starts in Second sometimes [message #332269 is a reply to message #332264] Fri, 18 May 2018 12:15 Go to previous messageGo to next message
Larry is currently offline  Larry   United States
Messages: 2875
Registered: January 2004
Location: Menomonie, WI
Karma: 10
Senior Member
Here are the instructions for adjusting the vacuum modulator.

http://www.gmcmhphotos.com/photos/transmission-install/p26806-img-0716.html



Larry Smile
78 Royale w/500 Caddy
Menomonie, WI.
Re: [GMCnet] Motor home feels like it starts in Second sometimes [message #332271 is a reply to message #332269] Fri, 18 May 2018 12:23 Go to previous messageGo to next message
James Hupy is currently offline  James Hupy   United States
Messages: 6806
Registered: May 2010
Karma: -62
Senior Member
In addition to an adjustable modulator diaphragm, you might also check the
metal tube that supplies vacuum to the modulator. It is pinched partly
closed and will sometimes become plugged with hose dust. Check it with
compressed air to see if it has become plugged. If it has, unplug it and
recheck the shifting.
Jim Hupy

On Fri, May 18, 2018, 10:16 AM Larry wrote:

> Here are the instructions for adjusting the vacuum modulator.
>
> http://www.gmcmhphotos.com/photos/transmission-install/p26806-img-0716.html
>
>
> --
> Larry
> 78 Royale w/500 Caddy
> Menomonie, WI.
>
> _______________________________________________
> 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

Re: Motor home feels like it starts in Second sometimes [message #332275 is a reply to message #332258] Fri, 18 May 2018 15:57 Go to previous message
Chris Tyler is currently offline  Chris Tyler   United States
Messages: 458
Registered: September 2013
Location: Odessa FL
Karma: 7
Senior Member
How about when it is in manual low gear?
Any problems Ive encounted withvav to the modulator have resulted in late, not early shifts.
Could also be an accumulator or valve bdy issue.

Try this: Start in manual 1, shift to 2 . Make sure it is actually in 2, as it might stay in first in that position with an unmodified stock valve body. Maually downsfit it into 1 again. If you do that a couple times it may free up.

You didnt recently have a trans "flush" did you? It seems those often create more problems than they solve.


76 Glenbrook
Previous Topic: I'm SO glad I own a GMCMH
Next Topic: [GMCnet] OEM / 1 Ton Front Suspension Comparison
Goto Forum:
  


Current Time: Sat Nov 16 00:01:13 CST 2024

Total time taken to generate the page: 0.00953 seconds