Still not consistently shifting from first to second

RF_Burns

RF_Burns

Super Moderator
Staff member
Sep 7, 2008
5,166
1,363
113
Ontario Canada
So I thought I had found my vacuum leak to the transmission modulator. I did some short trips and it shifted every time. Yesterday we headed out for
a 3 hour trip. After the 3rd full stop, it did not shift again. If I pull it from D to S it will shift to second, then returning to drive it will
shift to third without issues. About 1/3 of the time it will shift properly, but the rest of the time I must shift manually.

Previously it seemed to act up only when it was hot as in stop'n go traffic or city with lots of stopping. This time it happened on open country
roads. Tranny temperature reading under 160F, likely about 140F.

Any ideas where to look next.

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
Bruce,
Perhaps I missed it in a previous post.

How many miles on this transmission, and when was the pan last off for inspection/oil change?

I’m wondering if there might be some debris that has found its way into the valve body, causing some minor scoring and sticking of the valves.

I have seen clutch material and small slivers of rubber seal material get stuck in valvebodies. This has occurred mainly on fresh rebuilds, but can also occur on older transmissions.

Sometimes, a hard acceleration will blow the crud out and the valve body returns to normal function. More often, the crud is a sign that something was incorrectly assembled, or is failing and requires removal. It all depends on what you find in the pan/valvebody during removal.

Have you tried calling Manny for suggestions?

Les Burt
Montreal
'75 Eleganza 26'

>
> So I thought I had found my vacuum leak to the transmission modulator. I did some short trips and it shifted every time. Yesterday we headed out for
> a 3 hour trip. After the 3rd full stop, it did not shift again. If I pull it from D to S it will shift to second, then returning to drive it will
> shift to third without issues. About 1/3 of the time it will shift properly, but the rest of the time I must shift manually.
>
> Previously it seemed to act up only when it was hot as in stop'n go traffic or city with lots of stopping. This time it happened on open country
> roads. Tranny temperature reading under 160F, likely about 140F.
>
> Any ideas where to look next.
>
>
> --
> Bruce Hislop
> ON Canada
> 77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
> http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
> My Staff says I never listen to them, or something like that
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Replace the steel line along with the rubber.
The teel line is known to rust through, rubber can split even when it is
rather new.

> Bruce,
> Perhaps I missed it in a previous post.
>
> How many miles on this transmission, and when was the pan last off for
> inspection/oil change?
>
> I’m wondering if there might be some debris that has found its way into
> the valve body, causing some minor scoring and sticking of the valves.
>
> I have seen clutch material and small slivers of rubber seal material get
> stuck in valvebodies. This has occurred mainly on fresh rebuilds, but can
> also occur on older transmissions.
>
> Sometimes, a hard acceleration will blow the crud out and the valve body
> returns to normal function. More often, the crud is a sign that something
> was incorrectly assembled, or is failing and requires removal. It all
> depends on what you find in the pan/valvebody during removal.
>
> Have you tried calling Manny for suggestions?
>
> Les Burt
> Montreal
> '75 Eleganza 26'
>
>

> >
> > So I thought I had found my vacuum leak to the transmission modulator. I
> did some short trips and it shifted every time. Yesterday we headed out for
> > a 3 hour trip. After the 3rd full stop, it did not shift again. If I
> pull it from D to S it will shift to second, then returning to drive it will
> > shift to third without issues. About 1/3 of the time it will shift
> properly, but the rest of the time I must shift manually.
> >
> > Previously it seemed to act up only when it was hot as in stop'n go
> traffic or city with lots of stopping. This time it happened on open
> country
> > roads. Tranny temperature reading under 160F, likely about 140F.
> >
> > Any ideas where to look next.
> >
> >
> > --
> > Bruce Hislop
> > ON Canada
> > 77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
> > http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
> > My Staff says I never listen to them, or something like that
> >
> > _______________________________________________
> > 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
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
You do not need the restriction.

> Is that steel vacuum line with the restriction still available?
> --
> Bruce Hislop
> ON Canada
> 77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
> http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
> My Staff says I never listen to them, or something like that
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
I'm going to make the same suggestion that I made before. Run the entire line from intake to modulator in new vacuum hose. If that fixes your
problem, you can fabricate a new steel line at some later date after you are sure that really was your problem.

If you think the steel line is leaking, remove it, attach air pressure to one end and block the other end. Then go over it with spray soap or gas
leak detector looking for bubbles.

When I had mine out I bead blasted it and painted it with Rustoleum.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
And I would also recommend the reverse -- cap one end of the steel line
and apply suction with a handheld vacuum pump and look for leak down.
I've seen several lines that only leaked "in one direction" (vacuum or
pressure).

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of Ken
Burton
Sent: Thursday, August 16, 2018 9:03 AM
To: gmclist
Subject: Re: [GMCnet] Still not consistently shifting from first to
second

I'm going to make the same suggestion that I made before. Run the
entire line from intake to modulator in new vacuum hose. If that fixes
your
problem, you can fabricate a new steel line at some later date after you
are sure that really was your problem.

If you think the steel line is leaking, remove it, attach air pressure
to one end and block the other end. Then go over it with spray soap or
gas
leak detector looking for bubbles.

When I had mine out I bead blasted it and painted it with Rustoleum.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

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GMCnet mailing list
Unsubscribe or Change List Options:
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That will also work. Use what ever is easier for you.

I only suggested pressure because you will probably hear or feel the leak and can way over pressure what the line usually runs.

Ken B.

> And I would also recommend the reverse -- cap one end of the steel line
> and apply suction with a handheld vacuum pump and look for leak down.
> I've seen several lines that only leaked "in one direction" (vacuum or
> pressure).
>
> -----Original Message-----
> From: Gmclist [mailto:gmclist-bounces] On Behalf Of Ken
> Burton
> Sent: Thursday, August 16, 2018 9:03 AM
> To: gmclist
> Subject: Re: [GMCnet] Still not consistently shifting from first to
> second
>
> I'm going to make the same suggestion that I made before. Run the
> entire line from intake to modulator in new vacuum hose. If that fixes
> your
> problem, you can fabricate a new steel line at some later date after you
> are sure that really was your problem.
>
> If you think the steel line is leaking, remove it, attach air pressure
> to one end and block the other end. Then go over it with spray soap or
> gas
> leak detector looking for bubbles.
>
> When I had mine out I bead blasted it and painted it with Rustoleum.
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
>
> _______________________________________________
> 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

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Try different grade of vac hose. Not sure how they categorize it but get the heavier wall stuff. Also run it as straight as possible point A to B.
Mine was randomly sucking flat down by the modulator and was slightly too long in a sweeping S.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
The modulator has a bigger vacuum inlet than the OEM and larger than the metal tube coming down from the manifold. . I had originally found the rubber
hose to the modulator was leaking was leaking at the metal tube. I replaced it with a smaller tube and all seemed well.

I went under to check the tube and found even with a spring clamp on it, it was no longer snug on the tube. So I added a couple of tie-wraps to act as
an additional clamp. I also did this on the top end.

On the way home it shifted fine every time. It was also cooler and raining all the way home VS hot and muggy on the way down.

I'm going to check the metal tube for leaks again. I'll likely replace it for the cost, but I like to troubleshoot to the exact cause.

Without the restriction, what are the ramifications? Always have had a mushy shift.

Thanks

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
>
> I'm going to check the metal tube for leaks again. I'll likely replace it for the cost, but I like to troubleshoot to the exact cause.
>
> Without the restriction, what are the ramifications? Always have had a mushy shift.
>
> Thanks

Bruce,

The restriction is a left over from passcar service. It is to prevent "shuttle shifting" and I very much doubt we need it.

(I am going to have to turn this upside down in my head an I hope I get that translation right.)
Shuttle shifting is caused when the manifold vacuum goes low (big throttle) so this causes the transmission to downshift, and when it does that the
higher crankshaft speed makes the manifold vacuum increase. If the set points are just right, this can cause the transmission to want to upshift
again....

That is not something that most people like.

Worse still, is if this is coupled with another "Kickdown" system (such as most of us still have in the coach) the system could get unstable. The
mass of any coach pretty much prevents that.

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
GM used a high heat red/orange silicone heavier wall vac tube to go from the steel line to the vacuum controled heat riser valve on mid-late 70s
Pontiac cars and others. I need some for my 76 Grand LeMans as it rubbed a hole through. This would be great in that high heat GMC environment if I
could find a source. Rubber hose won't survive so close to exh man.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
We had a similar problem that turned out to be the governor sticking, though I think it might have been the 2-3 shift.
The governor had just enough sludge inside it to not move reliably. Took it apart, cleaned it up really well, reassembled. Problem gone.
If you do take it apart, be sure that stake pin is secure; it can come out part way and make it darned near impossible to get the governor back out.
Manny told me once that he routinely takes governors apart to clean and that usually fixes them. I also carry a spare that I got at a junkyard from
an Eldorado;
cleaned it up the same way, and it works great. Easier to swap one out on the road than take one apart.

Karen
1975 26'
 
Matt,
Thanks for clarifying the purpose of the restriction.

Sounds like I can replace the existing tubing with a new tube that matches the size of the modulator inlet (0.300"). Then I should be able to use an
appropriate size hose that matches. The existing tube is rusted at a metal mounting clip so it maybe rusted there as well.

Thank
--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
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