Home » Public Forums » GMCnet » Modulator for Transmission
| Modulator for Transmission [message #321504] |
Tue, 01 August 2017 17:20  |
tgeiger
Messages: 518
Registered: February 2006
Location: kansas city
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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.
Tom Geiger
76 Eleganza II
KCMO
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| Re: Modulator for Transmission [message #321535 is a reply to message #321533] |
Wed, 02 August 2017 14:57  |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
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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
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| Re: Modulator for Transmission [message #321539 is a reply to message #321504] |
Wed, 02 August 2017 15:22 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Manny is Your Friend. Give him a shout.
--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
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| Re: [GMCnet] Modulator for Transmission [message #321541 is a reply to message #321535] |
Wed, 02 August 2017 15:35 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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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@list.gmcnet.org] On Behalf Of Ken Burton
Sent: Thursday, August 3, 2017 5:57 AM
To: gmclist@list.gmcnet.org
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
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] Modulator for Transmission [message #321546 is a reply to message #321541] |
Wed, 02 August 2017 17:21 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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USAussie wrote on Wed, 02 August 2017 15:35G'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@list.gmcnet.org] On Behalf Of Ken Burton
Sent: Thursday, August 3, 2017 5:57 AM
To: gmclist@list.gmcnet.org
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
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| Re: Modulator for Transmission [message #321610 is a reply to message #321504] |
Thu, 03 August 2017 20:58 |
tgeiger
Messages: 518
Registered: February 2006
Location: kansas city
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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
Tom Geiger
76 Eleganza II
KCMO
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| Re: [GMCnet] Modulator for Transmission [message #321641 is a reply to message #321640] |
Sat, 05 August 2017 06:36 |
Richard Denney
Messages: 920
Registered: April 2010
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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
On Sat, Aug 5, 2017 at 6:40 AM tom geiger wrote:
> Mmmm, what is the vacuum pressure number I'm looking for when I take it
> for a test drive?
>
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Rick Denney
73 x-Glacier 230 "Jaws"
Off-list email to rick at rickdenney dot com
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| Re: Modulator for Transmission [message #321704 is a reply to message #321504] |
Sun, 06 August 2017 13:57 |
tgeiger
Messages: 518
Registered: February 2006
Location: kansas city
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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 back 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 tree 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?
Tom Geiger
76 Eleganza II
KCMO
[Updated on: Sun, 06 August 2017 13:59] Report message to a moderator |
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| Re: Modulator for Transmission [message #321705 is a reply to message #321504] |
Sun, 06 August 2017 14:16 |
tgeiger
Messages: 518
Registered: February 2006
Location: kansas city
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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?
Tom Geiger
76 Eleganza II
KCMO
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| Re: Modulator for Transmission [message #321712 is a reply to message #321504] |
Sun, 06 August 2017 18:28 |
tgeiger
Messages: 518
Registered: February 2006
Location: kansas city
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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 Geiger
76 Eleganza II
KCMO
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| Re: [GMCnet] Modulator for Transmission [message #321730 is a reply to message #321712] |
Sun, 06 August 2017 21:18 |
Richard Denney
Messages: 920
Registered: April 2010
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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
On Sun, Aug 6, 2017 at 7:29 PM tom geiger wrote:
> 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?
>
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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
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| Re: [GMCnet] Modulator for Transmission [message #321737 is a reply to message #321730] |
Mon, 07 August 2017 01:06 |
Bullitthead
Messages: 1411
Registered: November 2013
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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 
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: Modulator for Transmission [message #321753 is a reply to message #321715] |
Mon, 07 August 2017 09:03 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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JShot wrote on Sun, 06 August 2017 19:21Tom,
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 intake. 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
[Updated on: Mon, 07 August 2017 09:43] Report message to a moderator |
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| Re: Modulator for Transmission [message #321759 is a reply to message #321753] |
Mon, 07 August 2017 09:44 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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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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