| Transmission downshifting [message #308641] |
Wed, 12 October 2016 17:45  |
tgeiger
Messages: 518
Registered: February 2006
Location: kansas city
Karma: -1
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Senior Member |
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Was on a fishing trip last weekend and notice the transmission was downshifting to 2nd as I was coming on slight grades. It seemed to also have a time to shift up to 3rd when I was on a flat road going to a gas station in one of the towns I was traveling through. Was not going anymore than 35 or 40 at that time.
What should I check on this?
Fluid level
Metal vacuum tube blockage (but I thought that was for not down shifting out of 3rd)?
Anything else?
Tom Geiger
76 Eleganza II
KCMO
[Updated on: Wed, 12 October 2016 17:47] Report message to a moderator |
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| Re: Transmission downshifting [message #308643 is a reply to message #308641] |
Wed, 12 October 2016 18:21  |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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I would start by replacing the vacuum modulator and checking the vacuum hose and metal tube to it.
Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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| Re: Transmission downshifting [message #308657 is a reply to message #308641] |
Wed, 12 October 2016 22:18 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Weak / low vacuum level at the modulator. Install a tee and run a hose to the vacuum line at the modulator. Attach a vacuum gauge to that line and see what you have. If it is low, move the vacuum gauge to the other end of that line at the intake manifold. If that is low then look at the vacuum at one of the 0other ports directly attached to the manifold.
If the vacuum level is correct I would replace / adjust the modulator.
Also when you have the hose off of the modulator, check it for presence of trans fluid. It should be dry.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Transmission downshifting [message #308682 is a reply to message #308668] |
Thu, 13 October 2016 12:57 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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James Hupy wrote on Thu, 13 October 2016 08:33The upper end of the metal tube that goes behind the engine block on SOME
coaches has an intentional restriction near the upper end of the tube. It
can easily become blocked by debris and reduce the vacuum signal to the
modulator. I have experienced this several times on these old coaches. Bad
hoses also.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Oct 12, 2016 8:19 PM, "Ken Burton" wrote:
> Weak / low vacuum level at the modulator. Install a tee and run a hose to
> the vacuum line at the modulator. Attach a vacuum gauge to that line and
> see what you have. If it is low, move the vacuum gauge to the other end
> of that line at the intake manifold. If that is low then look at the
> vacuum
> at one of the 0other ports directly attached to the manifold.
>
> If the vacuum level is correct I would replace / adjust the modulator.
>
> Also when you have the hose off of the modulator, check it for presence of
> trans fluid. It should be dry.
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
>
> _______________________________________________
I too, as Jim stated, had that steel line plug. That is why I suggested that you look at the vacuum at the modulator and also after that before the line on the manifold end. In my case the line was plugged so badly that I removed it and attached air pressure to it at around 100 PSI. When that did not clear the obstruction, I left the pressure attached on one end and heated the line with a Mapp torch on the other end until the restriction finally cleared.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Transmission downshifting [message #308719 is a reply to message #308641] |
Fri, 14 October 2016 09:08 |
tgeiger
Messages: 518
Registered: February 2006
Location: kansas city
Karma: -1
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Senior Member |
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Great info guys. Appreciate all the help on this. I will look at this in the coming week. Everything else is working great on the coach and will bet this will correct this issue once I get down through the steps here.
Thanks again,
Tom
76 Eleganza II
455
3:42 TH425
Kansas City, Mo.
Tom Geiger
76 Eleganza II
KCMO
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| Re: Transmission downshifting [message #308736 is a reply to message #308719] |
Fri, 14 October 2016 15:02 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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Another thing you could quickly try is to get about 6 feet of vacuum hose and attach it to the modulator and the intake temporarily replacing what is there.
Be careful with the hose routing if your crossovers are not blocked. The intake can get hot enough to melt the rubber hose. You can tell the hot area by looking at the burned paint on the intake manifold.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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