1970s THM 425 Question

[Big Fins]

Another item not addressed in the FSM. 1976, THM 425 is shifting early at light throttle. It shifts, 1-2, 15mph and 2-3 at 22mph. These numbers aren't close to the trouble shooting listed numbers. Even at moderate throttle they grab way too early. They give you all sorts of tests for late shifting, but nothing for early shifting. The closest I can come is the modulator valve may be at fault. Second would be the governor.

The THM 425 has a huge modulator can on the side with a screw running through the middle of it like it was for adjustments. I don't like sticking my fingers where they don't belong on an expensive item like one of these. You don't just pull it out and bench it.

[TJ Hopland]

I'm for sure no transmission expert.... So my opinion may not even be wroth 2 cents.

How does it act when you are harder on the gas?  Does it delay things or does it still seem too early?  How about if you floor it?  Floor is an electrical circuit so different than the gov or vacuum.

Thinking out loud about the governor for it to be making the trans think its going too fast it would have to be spreading out sooner than it should.  I assume the weights are held 'in' with a spring or springs?  So if a spring was broken or damaged perhaps that could cause this issue?

It was years ago that I had one out of a 425, I think to deal with the speedo and oil leaks.  I don't remember seeing the weight part so I'm guessing it wasn't obvious or easy how to expose that part of things?  I think it was just an O ring for sealing it back up. 

The vacuum modulator with a strong vacuum signal would tell the transmission the load isn't that great so would shift earlier vs no signal that would delay and firm shifts.  Failure would give you the delay so that doesn't seem like whats happening unless its just stuck in the early mode which would seem unlikely since it takes vacuum to get there so most of the time especially when its sitting its in delay mode.  Unless maybe the transmission internal pressure (or lack of) effects what the at rest mode is? 

I don't think you have been able to find the giant ones for a long long time now so if you still have that that means its an older one.  The replacements are much smaller.  Similar to the gov I think its just held in with a clip and an O ring for a seal.  Nothing is going to explode or fall out if you remove it.  It just acts upon a plunger that I think goes into the valve body. 

[Big Fins]

The governor is the easiest to access. Like the rest, it sits right under the steering input shaft. A spring clip holds the cover on. The modulator has a plunger in it that enters the valve body. I didn't study it too closely.

Other models such as the THM 400, just use a simple vacuum modulator similar looking to a choke pull off. It works, or it doesn't.

I have to read the modulator section a little closer to see if this is like a modern version of the TV rod on the HM transmissions. A very slight adjustment can make the difference in a smooth shift or one that slams into gear.

[TJ Hopland]

I didn't think most of the modulators were adjustable?  Thought that was an aftermarket thing.  I sort of remember isn't there a colored stripe that identifies them?  At least what is currently available?

And I was thinking the adjustment is just a restriction to slow down the response time?  Or does it change the spring tension?  Which I would assume would have a similar effect?

[Big Fins]

I'm reading and trying to digest it. The 'governor' is what has the weights and springs in it. The 'modulator' is vacuum controlled and has an adjustable stop for the sliding diverter valve, (if I'm using the correct terminology) that goes into the valve body of the transmission itself.

One of these two, controls the up shift points. I'm not a turtle by any means. If I'm first at a light, I'm gone. I get away from the crowd. I'm not burning rubber either. In the FWD cars, it's not hard to do.

[TJ Hopland]

There is a rod that goes into the valve body and the rod in the modulator presses onto that so there has to be a spring or hydraulic pressure that pushes the rod out of the valve body so its in contact with the modulators rod.  The stop screw thing makes some sense vs altering the vacuum signal.  I wonder if its stopping on the high vacuum end or low?  High would be idle or light load steady cruise.  If its shifting early maybe its pulling too far out? 

I don't know specifically on this model but the gov ultimately also ends up moving a rod in the valve body.  I think to get a better idea whats going on you have to start looking at the fluid flow diagrams.

What happens if you disconnect the vacuum line and go for a test drive?  That should delay the shifts.

[TJ Hopland]

Does the manual have pics and talk about servicing the governor?  Or is it just a replacement assembly?

["Cadillac Kid" Greg Surfas 15364]

John,
What happens when you shift manually from L-D1_D2? Does it hold gear until you shift the lever?
Greg Surfas

[Big Fins]

Does the manual have pics and talk about servicing the governor?  Or is it just a replacement assembly?

I believe it is serviceable. The book is out in the garage at the moment.

John,
What happens when you shift manually from L-D1_D2? Does it hold gear until you shift the lever?
Greg Surfas

It will hold whatever gear it is manually placed in. It will also kick down under a heavier throttle just like it should and also when you go wide open, the kick down switch operates as it supposed too.

I'll get some numbers out of the book in a bit and post them along with the car numbers.

["Cadillac Kid" Greg Surfas 15364]

John
Disconnect the vacuum line to the modulator and plug the line.Check the modulator with a hand vacuum pump to see if it holds vacuum for at least 10 seconds and you can perceive movement of the modulator shaft. If the same condition (early shifts) occurs you can probably discount the modulator as the source of the problems. The next most likely issue would be low line presure caused by a dirty filter (or if the filter has just been replaced a poor connection of the pick up tube/o-ring) or internal issues.
Greg Surfas

[Big Fins]

Greg, it's been like this since I bought the car. The filter and fluid has been changed and done correctly. Because it's something that didn't hinder the way the car operated, I just didn't pay it much mind. Now that I don't have to work 12 hours a day, I have time to do the things that I want to do.

I will check both the modulator and the governor for vacuum leaks, weak or broken springs and go from there as far as parts replacement.

Now, if I just had an air conditioned garage to work in without the door being open.

["Cadillac Kid" Greg Surfas 15364]

John I feel for you, but I just can't reach you. Yesterday here we had a heat index of 104, and it's just May
Greg Surfas

[bcroe]

Do make sure the vacuum connection on the Vac mod is good.  It
is the same as used on the TH400, and performance should be
nearly the same as a TH400.  The vac mod is not intended to be
field adjustable.  After market vac mods (typically 6 and 8 cyl types)
are adjustable with a very small screw driver.  Takes a lot of turns in
to tighten up shifts, best keep a record of where you are doing this. 
I believe they are much smaller because they do not have the
vacuum bellows to compensate for altitude.  I try to use originals. 

The vac mod might be compared to the TV rod, but is far superior. 
The engine torque is nearly directly proportional to absolute manifold
pressure (lack of vacuum), which is part of what trans shifting needs
to know.  At idle only a slight cracking of the throttle will produce a lot
of torque.  At high rpm, the throttle must be pushed far open to get
the same amount of torque.  SInce the TV rod only follows the throttle
opening it is a terrible indicator of torque over the rpm range.  The
vac mod solves this problem, all my cars have them. 

The governor is not so easy to pull with its flat rubber gasket, might
need lube to replace.  It is an assembly crimped together, not
intended to have weights and springs changed to affect shifting.  I
have on paper a design to replace the flat band with an O ring, and
allow removing a cover to change weights and springs.  But not
being a machinist, it is not yet built.  Have not needed it. 

Another issue with the governor is the plastic gear which may strip,
it has to drive the speedo too.  If yours is failing, it may affect shifts,
and of course speedo readings too. 

The 400 and 425 have a pressure port to check if pressure is tracking
shifts, torque, and rpm.  Special gauge with long hose used.  A broken
pressure reg spring could throw off things, but I doubt it.  More likely
an original trans at this age has failing rotating and clutch piston seals,
perhaps losing enough pressure to throw everything off, and perhaps
warning of a catastropic failure very soon if not renewed.  As I write I
am replacing all these on a low milage TH425 (minimal cost) while
retaining all the original hard parts (that may be better quality than any
replacements). 

A dirty filter can be a problem for a TH425, they do not have that nice
open mesh used on the TH400 starting for 68.  I actually have been
looking at modifying a TH425 pan to use the later filter, might just be
possible, at some loss of ground clearance. 

Shifting manually will allow the trans to shift up only to the gear
selected.  Normally there is no engine braking possible in 1 & 2,
not needed while accelerating, the trans will go to 3 if you back off. 
Pulling down the selector will engage engine braking in lower gears,
the reverse band is engaged.  Bruce Roe

["Cadillac Kid" Greg Surfas 15364]

Actually the FSM shows an adjustment screw on the modulator and according to the cut way, it controls spring pressure. There is absolutely no mention of how or why to adjust it so IF this has ALWAYS been the shift pattern perhaps the modulator is not adjusted correctly.
Greg Surfas

[bcroe]

Actually the FSM shows an adjustment screw on the modulator and according to the cut way, it controls spring pressure. There is absolutely no mention of how or why to adjust it so IF this has ALWAYS been the shift pattern perhaps the modulator is not adjusted correctly.
Greg Surfas

Yes there is a screw, but my feeling is, it was not
intended for field adjustment. Bruce Roe

[Cadman-iac]

  The factory original modulator was preset and the adjustment screw spot welded in place to prevent movement from vibration or tampering.
It's possible to break the spot weld, but you can also damage the screw in the process.

The aftermarket sells an adjustable modulator to "fine tune" your shift points.
It's easy to tell if you have an original modulator or an aftermarket one. The original is large. The "can" is about 3" in diameter and sticks out of the transmission about 4 inches.
The aftermarket one is only about 1.25" in diameter and only about 3" long. (The diameter of the can doesn't include the pinched lip that holds the pieces together).
To adjust one on the car, (which is the best way to test as well), to increase the shift points, turn the screw in. To decrease the shift points, turn it outward.
I would start with a half a turn either way so you are not making extreme adjustments. It may take a while and a few adjustments before you are happy with it. A lot of up and down with the car and you to reach it. Be patient.
Before you try to adjust the original one if it's still on the car, I would consider buying an aftermarket one instead. If you don't like how it performs, you can easily reinstall your original one.

Hope this helps.

Rick

[TJ Hopland]

What are you actually adjusting in the aftermarket modulators?  I was thinking it was basically response time, not travel.

[Cadman-iac]

 It's the spring pressure in the modulator on the valve inside the transmission. More pressure will cause a later shift, less will mean an earlier shift.

 

[Big Fins]

I went for a ride this morning to get some shift numbers.

Normal acceleration from a stop and we have a 1-2 shift at 15mph and a 2-3 shift at 23mph. The book says 15 and 30.

Half throttle: 1-2 at 25, 2-3 at 40. No reference for this in the FSM, but it felt good.

Full throttle, which you have to slowly get into, otherwise the front end breaks loose: 1-2 at 40, and 2-3 at 70. The book says 44-48 and 77-83.

So there is a lower shift point than what it's noted to be.

Base timing is set at 10^oBTDC, 36^o cruise advance, no Cat, no EGR, direct to manifold distributor vacuum, ALC hooked up correctly and works, Climate Control all correctly piped and the rest of the emissions vacuum lines have been removed and/or plugged.

I don't drive the car hard, I drive for enjoyment. At light throttle, I'd like to see later shift points, but it doesn't affect how the car runs and drives. Further hands on investigation is forthcoming. A handheld vacuum pump and a pressure gauge with a long lead to it would make it easier. As of now, I own neither.

["Cadillac Kid" Greg Surfas 15364]

Sounds like you are just about where you should be worth the shift points John. FWIW i keep my big motor (650 HP) to a maximum of 34 degrees total mechanic advance.But that's just e
Greg Surfas