[GMCnet] Transmission won't shift from first to second

[Bruce Hislop via Gmclist]

So I've had this issue randomly for over 10 years now. I've had the Murray out for a couple of short runs this year and the transmission shifter fine
every time. Today we left to go for the first weekend trip and right out of the driveway it would not shift on its own from first to second.

I can always shift it manually by putting it into second and then back to Drive it will goto third.

The modulator and governor have been changed along with the tubing and hoses to the modulator. I changed the tranny fluid and filter this spring. I
checked the filter diameter and put 2 o-rings on it.

It may not do this now for the rest of the season or it might do it for the next thousand miles.

Driving me crazy!

Its been doing this on and off since the spring after I bought it rebuilt when I got the engine rebuilt.

Amsoil HD synthetic.
--
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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[Jerry Burt via Gmclist]

I had the same problem. Check the modulator vacuum line for leaks.
--
Patti & Jerry Burt Fresno, CA.
73 Gmc 26' Canyon Lands
77 Palm Beach - Parts Coach - SOLD
Members: FMCA - GMCMI - GMCWS
A truly happy person is one who can enjoy the scenery on a detour.

[Bruce Hislop via Gmclist]

Forgot to add, speedometer works fine, governor gear is fine.

Fluid level is fine.

Plunger valve that the modulator works against moves freely.

[James Hupy via Gmclist]

The modulator steel line is partially plugged up where it narrows down at
the top. What I have successfully done to unplug those crimped lines is to
heat them quite hot, whack them against the work bench to dislodge whatever
is plugging them, and blowing them out with compressed air. Repeat as many
times as necessary until the blockage is cleared. That should fix your on
again/off again problem. Make sure to inspect the lower vacuum line as well.
Jim Hupy
Salem, Oregon

[Jerry Burt via Gmclist]

Mine ended up being an old, hardened lower vacuum line.

James Hupy wrote on Sat, 13 June 2020 13:51

--
Patti & Jerry Burt Fresno, CA.
73 Gmc 26' Canyon Lands
77 Palm Beach - Parts Coach - SOLD
Members: FMCA - GMCMI - GMCWS
A truly happy person is one who can enjoy the scenery on a detour.

[Bruce Hislop via Gmclist]

Sorry guys, more info on repairs I've done.

2 years ago after we aborted a Maritimes holiday because of this issue. When we returned I replaced the upper and lower rubber hoses and the steel
tube. I replaced the steel tube with a 3/16" tube so it would match with the new modulator vacuum tube size. I also added spring type hose clamps to
both ends of the rubber hoses to ensure they are sealed. After that it only gave me issues a couple of times until now.

I'm trying to understand the voodoo that goes on in the transmission that makes it shift gears.

Several times I varied the throttle to change the vacuum level. I thought this might cause the transmission to change gears but no go.

[Terry via Gmclist]

Make sure transmission vacuum line goes to manifold vacuum source. Might try teeing in a gauge to watch that signal when problem occurs. Possible
sticking valve or small piece of trash making one stick once in awhile and it can't escape the valve chamber.
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.

[Ken Burton via Gmclist]

Well, You might r get a cheap vacuum gauge and Tee it in to the modulator to see what happens when it fails.

My other thought is the kick down switch might be sticking. Unplug it at the trans and see if you get any failures. You can always get by without the
kick down (passing Gear) switch. You can always pull the trans down a gear if needed with the gear shift.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[Bruce Hislop via Gmclist]

Kick-down has been disconnected for years.

I'll T into the vacuum line at the modulator and see what I can read there.

Is it supposed to shift at a set vacuum level or how does that work?

[James Hupy via Gmclist]

Have you given any thought to your speedometer cable or governor? Try
lubing that lower cable and see if that helps.
Jim Hupy
Salem, Oregon

[John R. Lebetski via Gmclist]

The lower the vacuum the later the shift. Double check your vacuum hose type. Some is soft and sucks flat especially if curved. Sorry I don’t
have the SAE numbers but I do know that the GM engineering cut sheet for each type vacuum hose is about 4 pages. I know this because my friend is
reproducing GM vacuum hose to be licensed. I also know that soft vacuum hose caused the same issue on my 77 E2
--
John Lebetski
Woodstock, IL
77 Eleganza II

[Ken Burton via Gmclist]

I do not know the shift points. I thought they were variable based on engine or governor speed and vacuum level. What I am hoping is that you will
see what is normal and then see any anomaly / difference when it is failing.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[James Hupy via Gmclist]

The 1-2 shift point is about 14mph with throttle part way open. Quite a bit
faster than that when fully on the floor. Maybe, 25 -30. 2-3 shift normally
occurs around 30 mph. But as high as 50 at wide open throttle. I am talking
about stock transmissions, and stock final drive gearing.
Jim Hupy
Salem, Oregon

[Bruce Hislop via Gmclist]

Jim Hupy, so the shift is dependent on modulator for engine load (light throttle is high vacuum and WOT is low vacuum). And the govenor for road
speed.... That is what I understand. So it could be a govenor issue even though I have changed it and still had issues afterwards.

[James Hupy via Gmclist]

I suggested the speedometer cable lube because that happened to me. My
cable was routed too close to the headers, and the heat carbonized the lube
and rubber covering on the outer cable. My fault. I had changed final drive
gearing, and installed the speedometer reduction gearbox up instead of down
and away from the header heat. When I do one of those today, I insulate the
cable with that bright orange nomex aircraft slip tubing. Good stuff,
spendy, but effective. Never had shifting problems since I did that.
Jim Hupy
Salem, Oregon

[Ken Henderson via Gmclist]

I made an aluminum heat shield to fit 1/2" or so off of the speedometer
cable so that the air flow protected it. Now that I've eliminated that
cable with a digital speedometer, the heat shield protects the Vehicle
Speed Sensor (VSS).

Ken H.

[Ken Burton via Gmclist]

A few years back Dan Gregg had a similar problem. I kept telling him it was modulator associated. He would not listen to me. He replaced the
governor a second time with one rebuilt by Wally and fixed the problem.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[Bruce Hislop via Gmclist]

We just got back from our weekend getaway to the bush. I can type a message now on a real computer and not a silly little smart phone.

On the way here it did not shift right out of the driveway from a cold startup. And today it shifted fine through 5 stop signs in about 10 miles. On
the 6th stop it would not shift and had to manually shift to the balance of the trip (about 20 more miles and 6 more stop signs). So not sure that it
is heat related.

Before we left I checked the hose at the modulator (was new in 2018 with the modulator) and the hose to the manifold also new in 2018. Both seemed to
be sufficiently thick to resist collapse by the vacuum.

I'll T in a hose from the modulator and connect it to a vacuum gauge and see if I can find anything. If I had a spare MAP sensor, then I could log
the vacuum readings on the EBL.

KenB, I'm wondering about the governor. The speedo reads fine, no jumping so the governor must be spinning. I got this one from Al Hamilton who took
it off a spare tranny he had. It did the same thing after changing it so we assumed it was not a governor problem, but it could have been sticky as
well.

[Ken Burton via Gmclist]

RF_Burns wrote on Mon, 15 June 2020 10:14

> We just got back from our weekend getaway to the bush. I can type a message now on a real computer and not a silly little smart phone.
>
> On the way here it did not shift right out of the driveway from a cold startup. And today it shifted fine through 5 stop signs in about 10 miles.
> On the 6th stop it would not shift and had to manually shift to the balance of the trip (about 20 more miles and 6 more stop signs). So not sure
> that it is heat related.
>
> Before we left I checked the hose at the modulator (was new in 2018 with the modulator) and the hose to the manifold also new in 2018. Both seemed
> to be sufficiently thick to resist collapse by the vacuum.
>
> I'll T in a hose from the modulator and connect it to a vacuum gauge and see if I can find anything. If I had a spare MAP sensor, then I could
> log the vacuum readings on the EBL.
>
> KenB, I'm wondering about the governor. The speedo reads fine, no jumping so the governor must be spinning. I got this one from Al Hamilton who
> took it off a spare tranny he had. It did the same thing after changing it so we assumed it was not a governor problem, but it could have been
> sticky as well.

Bruce, I had this same discussion with Dan a few years ago. In the end I was wrong with my diagnosis. I was not there, so I did not see the fix.
Dan just told me that replacing the governor a second time took care of the problem. Unfortunately Dan died a few years back so we can not ask for
more details. Wally, if he is around, might know more.

Ken

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[Johnny Bridges via Gmclist]

Does not the governor push the valve down as the weights spin outward? What this will do if the valve is sticking a bit is slow the shift a bit. Or
in the one we first put in my current coach stick such that nit never shifts. I'd replace it with a rebuild at this point. Smooth speedo operation
says it's rotating correctly, but says nothing about that valve.

--johnny

--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell

[James Hupy via Gmclist]

Reason I mentioned the speedometer cable, is, if it seizes up, one of two
things is going to happen. The nylon gear on the end of the cable strips
out, OR, the pin that locks the governor gear to the shaft shears off, or
that gear strips out, or some combination of all 3 happens. If you have
headers, pulling the governor requires removing the header on that side.
With iron manifolds, the governor will clear. Just saying.
Jim Hupy
Salem, Oregon

[John R. Lebetski via Gmclist]

I’ve fixed 2 coaches with vacuum to modulator issues. Mine a 77 where there was excess hose length forming an S by the modulator and pinching.
Simple cut and reinstall hose in restaurant parking lot. The second one was a 75 that had your shifting issues. The problem was not in either main
line vacuum hose but a hidden 1” piece under the air cleaner that had chafed and rubbed through. Still part of same vacuum circuit and was open to
atmosphere

--
John Lebetski
Woodstock, IL
77 Eleganza II

[Bruce Hislop via Gmclist]

I check my speedo cable. I've removed the non-functioning cruise control module so the cable goes directly from the speedo to the transmission. The
cable spins smoothly and freely so I don't think that is the issue.

I pulled the governor last night. I can see the internal plunger moving up and down. The gear is in perfect shape. Of course there is no way to get
to the weights without cutting the case apart, but I can see they are free to move and nothing seems loose.

I also put my hand vacuum pump on the hose end from the manifold. It pumps up and holds vacuum, so its not a leaky hose. I'm going to T off a hose
at the modulator and bring it back up inside where I can see a vacuum gauge. See if there is any anomalies.

I was reading the TH400/425 troubleshooting manual. In there it shows a screen filter going to the governor, but I can't find any reference to it in
the 425 transmission. Is there such a thing in our transmission? or maybe its just in the TH400.

[John R. Lebetski via Gmclist]

Does the fluid look like new? Wondering if stiction in valve body.

--
John Lebetski
Woodstock, IL
77 Eleganza II

[Bruce Hislop via Gmclist]

Actually fluid is new... Amsoil AFT. I changed the filter and the fluid this spring. First change fluid/filter change in 11 years and about 30K
miles. Old fluid looked good.

Amsoil AFT is what the rebuilder put in after it started doing this the first time out the next spring (2010) after getting the rebuilt transmission
and engine installed in 2009. After that it was Ok for a few years, then started doing it again. Usually the first time out it would do this, then
clear itself for the rest of the season. I had it out a couple of times this spring (short test run, then to town for gas) and had no issues.

If this wasn't a random occurrence, I would have thought that I likely messed something up changing the fluid and filter.

There wasn't much debris in the pan, just a little bit of fine black stuff which I suspect is a bit of worn clutch material.

[Bruce Hislop via Gmclist]

So I T'd a hose off at the modulator and ran it up to a vacuum gauge so I could compare it to my TachVac. Pulled out on the road and it would not
shift with about 12" of vacuum at the modulator. I used the throttle to vary the vacuum from about 8" to 18" and still no shift. No difference in
readings between the two vacuum gauges.

Turned around about 1/2 mile down the road and it shifted fine coming home.

[6cuda6--- via Gmclist]

Bruce few questions:
- Did you back up out of the garage then go or into drive only?
- When you turned around did you reverse then go or just drive only?
- Did you give the trans time warm up before you left?
- Did you put the trans in neutral before you left for a few minutes to allow the pump to circulate fluid threw the trans or did you just drop and
go?

Trying to see if their is a pattern to the problem [so cold or hot, fluid threw trans or not etc ]


RF_Burns wrote on Tue, 16 June 2020 18:28

> So I T'd a hose off at the modulator and ran it up to a vacuum gauge so I could compare it to my TachVac. Pulled out on the road and it would not
> shift with about 12" of vacuum at the modulator. I used the throttle to vary the vacuum from about 8" to 18" and still no shift. No difference in
> readings between the two vacuum gauges.
>
> Turned around about 1/2 mile down the road and it shifted fine coming home.


--

Rich Mondor,

Brockville, ON

77 Hughes 2600

[Ken Burton via Gmclist]

I think that says it is not a vacuum problem. Looks like governor is next.

Another thought. About 8 or 10 years ago Manny was having problems with some of the kick down solenoids causing shift problems due to leakage. He
sent me a replacement and told me to install it even though I was not having problems. You might ask Manny about that. I just did what I was told
and installed it.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[Bruce Hislop via Gmclist]

Rich,
The coach was sitting by the house, so I pulled it over into the shop to get under and add the T'd off vacuum line. Its not a straight run into the
shop so I had to go forward and back several times to line up. run time about 5 minutes.

About an hour later I backed it out of the shop for the road test. Again back and forth several times to maneuver out so it was running for several
minutes before I got on the road. Also in drive and reverse several times to maneuver out of the shop driveway area.

So it was not hot, but was not stone cold either.

When I got on the road and up to about 20mph I knew it was not going to shift, so I varied the throttle position to vary the vacuum as I noted above,
but no shift. Then I manually pulled into second and it shifted fine, then back to drive and it shifted up to 3rd gear. Got up to about 40mph.

I knew then I had vacuum at the modulator so I decided to turn around. There is a turn around where 2 sideroads meet the main road. Left turn onto
sideroad about 200ft the left onto another sideroad for about 300ft then stop at the main road again.

Coming back on the main road from a dead stop, it shifted to second and 3rd on its own just fine. it would have been much warmer then.

Going to the bush on Saturday, just started up and were on the road within one minute and it did not shift right out of the driveway. However 2 miles
down the road in a small town we have to stop twice and it shifted both times, plus at a stop sign in a village another 5 miles further it was fine.
After that for another 20+ miles and a dozen stop signs/lights I believe it may have shifted once automatically.

However coming home from the bush on Monday morning, that was started from stone cold. Put into Drive, no other gears and it shifted fine.
-drove out of bush, up laneway about 1/8 mile (not sure if I went fast enough to shift)
-Pulled onto road (not from dead stop, but close to it) shifted fine.
-drove about 1/2 mile @40mph then stop sign and pulled onto main road, shifted fine.
-Drove 2.5 miles @50mph and stop sign and turned onto another main road, shifted fine.
-Drove 1.25 miles @50mph and stop sign, then turned onto main hyway, shifted fine.
-Drove 1.25 miles @35mph (in a village), then stop light and turned on to another main road, shifted fine.
-Drove 5 miles @50-55mph then stop sign, then turned onto another main road... did not shift automatically

The remaining 20 miles home there were 3 round-abouts, one stop sign and one stop light. Each time I needed to shift manually.

So it doesn't seem to matter whether it is hot or cold, but tends to act up more when hot.

History:
I got this rebuilt transmission from Dick Paterson when he rebuilt my engine in 2009. The next spring on the way to the first rally, it did not shift
out of the driveway on the road. Turned around and went home. I called Dick the next Monday and he put in contact with the guy who rebuilds
transmissions for him. He asked me if I tried manually shifting by pulling into second. I had not thought of this so I tested it and it worked. So I
drove the 175 miles to his transmission guy... sometimes it would shift, sometimes not.

Transmission guy said the valve the modulator was working was sticking and he freed it up and installed a new modulator. He also drained and filled
it with Amsoil ATF synthetic. He said he highly recommended it in HD service like the motorhome. After that it worked pretty good. Sometimes in the
spring it might act up a couple times on the first trip but after that it was good for the rest of the season.

In 2018 we were heading to the Maritimes. When we got into Toronto we found ourselves on a detour through city streets and it started to act up. We
pressed on and stopped at Al Hamilton's near Kingston. We talked with the transmission guy that Al had used. He thought it might be the governor
which Al had a spare. We put Al's spare governor in and I pulled the modulator and checked the valve spool behind it... it seemed to move freely.
Took a test drive and it shifted fine.

So we left the next morning saying if it acted up we would be back. About 20 miles down the road it did not shift again from a stop. We turned around
and went back to Al's. Of course it shifted Ok on the way back.

The next morning we cancelled our trip and went back home. Of course it shifted fine all the way home if I recall.

Back home I put a hand vacuum pump on the line at the manifold and had Paula pump the vacuum up while I went under to see if I could find a leak.
Well I found it... the newer modulators have a larger port so the larger hose did not fit the metal tube coming down. It did fit over a crimp in the
tube which I believe was a stop for the OEM smaller tube. The modulator hose had relaxed over time, and heat would make it worse, but the hose was
now slightly loose on the tube crimp.

So I made a new tube from 1/4" brake line, installed a new modulator (for good luck) and I also installed spring type hose clamps on the vacuum hose.
At some point I put my original governor back on as well. This seem to fix it... the next year we did our maritime trip and I think it acted up maybe
twice.

So now this year.. I changed the transmission fluid and filter. Once again used Amsoil ATF, but my problem is back. BTW I used a Wix filter and
measured the tube diameter 0.75" and installed 2 O-rings.

The vacuum lines are fine to the modulator and I measured the vacuum at the modulator and it is good when it wasn't shifting. Used the hand pump to
confirm no intermittent vacuum leaks.

So I'm thinking the modulator valve must be sticking again... maybe changing the fluid stirred up some crud in the valves. So I'll check that next.

Not today though... Honey-do list first.

Sorry for long post

--
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 Hislop via Gmclist]

Ken,
If the valve test doesn't fix it, I'll call Manny.

I talked to him back in 2018, unfortunately Chemo brain fog, I can't recall much from that conversation.

[John R. Lebetski via Gmclist]

Sounds like you can rule out Amzoil vs Dex/Merc, the modulator and the governor. That puts us at the valve body which could have a burr or chip in
it. Had same thing happen on a friends 57 Chevy with Glide, but never on a THM400/425. I’d say time for a THM expert who should be able to say “
I know what’s wrong” Hopefully bench exam of valve body with trans in coach

--
John Lebetski
Woodstock, IL
77 Eleganza II

[Bruce Hislop via Gmclist]

So I managed to get the Murray inside the shop today.

I pulled the modulator and checked that the spool inside moved freely. It does move freely, as a matter of fact, I would say it moves too freely. I
can feel a bit of side to side movement like either the spool or the valve housing... or both are worn. Its hard to gauge with little room to work
and a hot exhaust right there.

Has anyone heard a worn modulator spool before?

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

[Ken Burton via Gmclist]

No, but transmissions are NOT my forte. I can tell you that my spool valve will come right out with the modulator removed.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[Johnny Bridges via Gmclist]

Time for a Manny chat.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell