Modulator for Transmission

T

tom geiger

New member
Dec 31, 2006
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Just got my motorhome back from my mechanic. I gave him the symptoms on the Transmission. That it shifts 1st to 2nd fine, its just 2nd to 3rd, it
jumps back to 2nd gear from 3rd and then it stays in 2nd even when I'm doing 50-60 mph. I cleaned out the tubes and checked the hoses. They suggested
I take it off to a Transmission shop because they said the modulator was fine. So I guess I'm a bit lost on this.
 
Makes me wonder if the kick-down switch is shorting. Try unplugging it from
where it plugs in adjacent to the modulator and see if that fixes it.

Rick "don't need a kick-down anyway" Denney

> Just got my motorhome back from my mechanic. I gave him the symptoms on
> the Transmission. That it shifts 1st to 2nd fine, its just 2nd to 3rd, it
> jumps back to 2nd gear from 3rd and then it stays in 2nd even when I'm
> doing 50-60 mph. I cleaned out the tubes and checked the hoses. They
> suggested
> I take it off to a Transmission shop because they said the modulator was
> fine. So I guess I'm a bit lost on this.
>
> _______________________________________________
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> Unsubscribe or Change List Options:
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>

--
'73 X-Glacier 230 "Jaws"
Northern Virginia
Offlist email: rick at rickdenney dot com
 
I would be absolutely sure that the trans shift linkage is correctly adjusted so when it is displaying "D" in that cab, that the linkage at the trans
is in the same detented position at the trans.

I like Rick's suggestion to unplug the down shift solenoid.

I would insert a "Tee" in the vacuum line at the modulator. Run a temporary hose inside the cab. Now when it down shift to second or sticks in
second, read the actual vacuum at the modulator to be absolutely sure the you are not having a vacuum blockage or leak in the line to the modulator.
Your problem really sounds like a modulator problem.

That is my two ideas.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Manny is Your Friend. Give him a shout.

--johnny
--
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" - ol Andy, paraphrased
 
G'day,

The line to the modulator on Double Trouble had rusted badly and broke when I R&R'd the cast iron manifold with an aluminum one. To
fix it I cut the rusty part out (about 1/2" or so) and where it broke was located in an area that wasn't hot I joined the pieces
with rubber tube. That was 5 or six years ago and it still works fine.

Also I vaguely remember reading something about a restrictor in that line that can get plugged. Finally I think someone noted you
could drill it out.

Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of Ken Burton
Sent: Thursday, August 3, 2017 5:57 AM
To: gmclist
Subject: Re: [GMCnet] Modulator for Transmission

I would be absolutely sure that the trans shift linkage is correctly adjusted so when it is displaying "D" in that cab, that the
linkage at the trans is in the same detented position at the trans.

I like Rick's suggestion to unplug the down shift solenoid.

I would insert a "Tee" in the vacuum line at the modulator. Run a temporary hose inside the cab. Now when it down shift to second
or sticks in second, read the actual vacuum at the modulator to be absolutely sure the you are not having a vacuum blockage or leak
in the line to the modulator.
Your problem really sounds like a modulator problem.

That is my two ideas.

Ken
 
> G'day,
>
> The line to the modulator on Double Trouble had rusted badly and broke when I R&R'd the cast iron manifold with an aluminum one. To
> fix it I cut the rusty part out (about 1/2" or so) and where it broke was located in an area that wasn't hot I joined the pieces
> with rubber tube. That was 5 or six years ago and it still works fine.
>
> Also I vaguely remember reading something about a restrictor in that line that can get plugged. Finally I think someone noted you
> could drill it out.
>
> Regards,
> Rob M.
> Sydney, Australia
> AUS '75 Avion - The Blue Streak TZE365V100428
> USA '75 Avion - Double Trouble TZE365V100426
> USA '77 Kingsley - TZE 267V100808
>
> -----Original Message-----
> From: Gmclist [mailto:gmclist-bounces] On Behalf Of Ken Burton
> Sent: Thursday, August 3, 2017 5:57 AM
> To: gmclist
> Subject: Re: [GMCnet] Modulator for Transmission
>
> I would be absolutely sure that the trans shift linkage is correctly adjusted so when it is displaying "D" in that cab, that the
> linkage at the trans is in the same detented position at the trans.
>
> I like Rick's suggestion to unplug the down shift solenoid.
>
> I would insert a "Tee" in the vacuum line at the modulator. Run a temporary hose inside the cab. Now when it down shift to second
> or sticks in second, read the actual vacuum at the modulator to be absolutely sure the you are not having a vacuum blockage or leak
> in the line to the modulator.
> Your problem really sounds like a modulator problem.
>
> That is my two ideas.
>
> Ken

I had that restrictor plug once. I was not going to tell him about it until he stuck a vacuum gauge on it to see it vacuum was really the problem.

My fix was to remove the line, put about 100 psi on the non-restrictor end, and heat the restrictor end. The blockage cleared in less that one
minute. A cracked hose will do the same thing. It does not take much of a crack because the very small steel line is not designed to move much air.
It only moves a minute amount to activate the modulator.

Let's first see if he has a vacuum problem before we give him all of the possible fixes.



--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Ok guys, alot of good stuff here. I will start with the vacuum test down at the modulator to make vacuum is good. Had done the flush with carb cleaner
and blew it out with high pressure air. Had a friend down below confirm tube was clear. Did have a funny feeling modulator might still be the culprit,
but will check vacuum first and if it checks I'll check linkage position then disconect kick down switch. If none of that works, modulator will get
replaced. And we'll see after that.

Thanks again guys,
Tom
 
Vacuum will vary with throttle position, from over 20" Hg on coast-down
(foot off the pedal) to zero on full acceleration (foot to the floor).
That's how the modulator knows to delay shifting on full acceleration. On
gentler acceleration, vacuum will increase as the RPMs climb, and that will
trigger a shift. Look for values above 10" Hg. You should see that full
range of vacuum over those various conditions. At 800 RPM idle in Park, it
will be 16-20, depending on altitude, the cam in your engine, and
condition. At 650 RPM in gear, it will be a bit less.

Rick "who looks at his vac gauge a lot while driving" Denney

> Mmmm, what is the vacuum pressure number I'm looking for when I take it
> for a test drive?
>
> _______________________________________________
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> Unsubscribe or Change List Options:
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>
--
Rick Denney
73 x-Glacier 230 "Jaws"
Off-list email to rick at rickdenney dot com
 
Ok, something s bit different in the vacuum hose setup that I noticed.
I have got a new modulator on, along with a tee and a temp hose running up to a guage inside the coach.
Went. Ack up on top to check over things and noticed the vacuum hose for the modulator is setup different than what someone sent to me. Mine comes off
a 3 connection tree on the right front side of the engine. Comes off that and then hits a tee. The right leg of the tee connects to a hose that goes
to the small tube, the left side of the tee goes to a hose that connects to a port on the bottom left front of the quadrojet carb. The vacuum port to
the left front of the carb has a hose that runs back to the cruise control and the other port is capped. Does this sound right?
 
Ok, thought this question posted before, but I'll try again.

Got the modulator replaced. Put the tee in the hose connection at modulator and ran a independent line to a vacuum gauge I've set as a test.

Went back on top and noticed my vacuum hose setup looks different than a example given to me. Just want to make sure this setup is ok.

Mine is setup off a 3 port tree on the right front of the Quad carb and the line that feeds the tube to the modulator also tees and goes to a port on
the bottom left of carb. The the 2 port tree to the left of the carb feeds the cruise control and the the other port is capped. Is this a correct
setup on vacuum?
 
Sounds like a governor problem to me. The governor (on the left side under the exhaust manifold) might be worn or sticking. The same thing happened
to Dan Gregg a few years back with slightly different symptoms. He replaced the governor with a rebuilt one from Wally Anderson. End of problem.

http://appliedgmc.com/prod.itml/icOid/1024
--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member
 
Ok, so this shifting issue is still happening. Put the new modulator on, tested the vacuum at modulator and it appear vacuum is good. 16# at idle, 20
coasting, near zero at exceleration. Shifted very smothly when first warming up and then after it ran for awhile the issue between 2nd and 3rd started
up. It kept popping out of 3rd down to 2nd and would hold on second till I manually shifted up to 3rd. Got back to the house and unplugged the kick
down at right side of modulator and symptom still continued. So not sure whats next. Is it time to call Danny Dunn, or take it to a local transmission
shop?
 
Tom,
Can't believe nobody has chimed in here on the "tree". That beautiful piece of motordom is usually removed - not used. Then the vac hoses are run to
the carb, etc. Don't ask me, as I don't know all the specifics, but someone on here will tell us.

Shot
--
John Shotwell
Ridgeville Corners, OH
78 Royale Center Kitchen
Web Site: GMCmhRegistry.com
Email: john at gmcmhregistry dot com
 
Those are good vacuum numbers. Have you replaced the modulator?

The purpose of that three-port vacuum device (called the TVS, or thermal
vacuum switch) is to switch the vacuum advance from the distributor from
ported vacuum (taken from the carburetor) to manifold vacuum when the
engine starts getting too hot. This increases idle speed and the more
advanced idle runs cooler. It should have a hose from the carb, another
hose from a fitting on the manifold, and a hose to the vacuum advance on
the distributor.

The modulator should get vacuum directly from the manifold. It could be
teed into the vacuum line from the manifold to the TVS.

But the readings you report look correct to me, so as long as they aren't
changing when the engine gets hot, I don't think that's the issue. It's
worth me saying it that way, though, since you report that the problem gets
worse as the engine warms up. You could always connect the modulator
directly to a fitting on the manifold and be sure that isn't the problem.

Rick "fickle finger of fate pointing to the modulator" Denney

> Ok, so this shifting issue is still happening. Put the new modulator on,
> tested the vacuum at modulator and it appear vacuum is good. 16# at idle, 20
> coasting, near zero at exceleration. Shifted very smothly when first
> warming up and then after it ran for awhile the issue between 2nd and 3rd
> started
> up. It kept popping out of 3rd down to 2nd and would hold on second till I
> manually shifted up to 3rd. Got back to the house and unplugged the kick
> down at right side of modulator and symptom still continued. So not sure
> whats next. Is it time to call Danny Dunn, or take it to a local
> transmission
> shop?
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
--
Rick Denney
73 x-Glacier 230 "Jaws"
Off-list email to rick at rickdenney dot com
 
I thought the governor was responsible for the shift into direct drive (transmission's point of view) and I'm surprised Carl is the only one to
mention it so far, so I may be wrong. I do know that the modulator is responsible for shift timing information on ALL shifts, not just 2>>3.

That said, I had a Turbo 400 in my Firebird that would only shift to third under a load, and only stay in third if it was under acceleration until I
got to 80 MPH. If I let off the throttle before then, it would drop out of third back to second and freewheel until I increased the engine RPM to
increase the road speed. No engine braking unless I left the shift lever in the second gear position and never shifted to third at all. I replaced the
modulator too, but I knew in the back of my mind that was not the problem because it shifted normally under load at exactly the right load factor 1,
2, 3, just perfect and then let go of third as soon as the load was gone. A transmission guru told me it was the rear pump, but that was the THIRD
transmission and SECOND engine in that car in a year's time and then the engine broke so I had to transfer the onus to another entity :lol:
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.
 
> Tom,
> Can't believe nobody has chimed in here on the "tree". That beautiful piece of motordom is usually removed - not used. Then the vac hoses are run
> to the carb, etc. Don't ask me, as I don't know all the specifics, but someone on here will tell us.
>
> Shot

I believe we are confusing the word "tee" and "tree". Either way the modulator line does NOT connect to a tee or a tree (other than the temporary one
you inserted for a temporary vacuum gauge).

You did not state what the vacuum was reading when the trans down shifted unexpectedly to 2nd. That is what we are concerned with. If the vacuum
goes low then the modulator will cause a down shift.

I am looking for a vacuum diagram on line so we can show you exactly where the modulator vacuum line plugs on the front of the carb. It sounds like
you have the modulator vacuum line attached to the wrong place on the engine.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Ok, I give up. I know that vacuum house routing picture is somewhere on the photo site but I can not find it.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
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