(and what to look for in a rebuilt transmission)


Most of the time the cable just needs adjusting. On a level surface, get under the coach and reach the shift lever on the tranny. There should be some kind of pin or wire holding the shift relay rod to the tranny shift lever. Remove the pin or whatever is there. remove the relay rod.Try to move the tranny shift lever down. If it moves or if it moved when you pulled the relay rod out, it needs an adjustment. The adjustment is on the rod swivel bolt. Loosening bolt, the rod can slide. Tighten the bolt after the adjustment. Usually its moving the tranny lever down and the swivel toward the tranny.
It this doesn't make much since, call me. Manny Trovao

What is a Switch Pitch ?

Since you asked,
A 68 up THM400 or 425 trans uses a 3 element torque converter, which at stall (vehicle not moving yet) puts out about 2 times the engine input torque.  As rpm and output speed increase, that gradulally reduces from 2:1 to 1:1, stretching the useful range of each gear.  So the 400/425 will easily outperform the old 4 speed hydramatic or a 3 speed manual, by providing a wider overall range of gear ratios.

The switch pitch (SWP) THM400 and 425 of 65-67 had a torque converter 3rd element (stator) with VARIABLE VANE angle, changeable between performance and economy (instead of a compromise) as you drive.  This gave 2.5 times torque multiplication, a 25% increase.  It also gave a bit better mileage in the other mode.  In a heavy vehicle this can provide a reduction of 0-60 time of up to 10%, and also give smoother shifts. 

Some of us have been busy converting later vehicles to get SWP benefits, the parts bolt in if you can find them.  An electronic control has been added for max benefit.  Read about work on 70s GMC motorhomes and other links at

Bruce Roe


Here are some good articles on Switch Pitch  and 425 Transmission modifications


We are starting to think it might be good to monitor the Tranny pressure.  Now the question is, where.  Here is an interesting article about crank damage ( that old thing)  and they say T off of the cooling exit line.....

the other reference on my web page

says the same (could be the same source)


The shift point information on the tranny in the manual, is from the test port..... so it would seem if you wanted to know about the condition of the tranny this would be the best place....Easy to attach because you can remove a plug and insert a sender. gene

Article on using Main Line pressure tests

Found this place with these pressures. Not sure if its true.Main line pressure tap located on driver side of case right  behind manual linkage.

@Idle Hot
Park and Neutral 55-70psi
Reverse 95-150psi
D and 2nd 60-90psi
Low 135-160psi

@Stall Hot
Drive 145psi


There is a small plate on the driver side of your transmission that will tell you if your transmission is for a GMC motor home.  You can look down through the engine hatch cover with a light, and clean off and read the numbers on the plate.  It will not tell you if tranny has been rebuilt or what was done inside.

It is easy to tell if your GMC has a transmission that was built for the GMC Motorhome.  These Turbo Hydra-matic  425 (THM-425) were different than the ones built for the autos.  They had six clutch plates and other special stuff.  So are you curious what they put back into your MH when you or the PO had the exchange tranny ??  This will NOT tell you what they did inside the tranny during a rebuild, but would be nice to know what they are selling us !!!!!  Here is how to do it.  You have to promise to report back when you find out....

An identification tag is attached to the transmission bearing two code letters and the year as well as the serial number of the unit.  These letters identify the transmission, the car, series in which it was originally installed and the engine size and type to which it was originally installed. On the 425 units, it can be found on the left side of the Converter housing.

                   GMC MOTORHOME                  GM(DATE)  ie GM77
1974-78     ELDORADO, exc E.F.I.                 AJ
1975-78     ELDORADO, W/EFI                     AK
1974-78     TORONADO                                OJ
1974-77     TORONADO                                OM


You should rebuild the 425 transmission that came with your coach if at all possible.  The original transmission had some differences from a 425  transmission in a car.  Put back the parts that came out of the transmission.  There is a risk when switching parts.

The (motorhome) 1-2 modulator valve is different than most of the toro/eldo cars. Using the car 1-2 valve in the motorhome it shifts too early. Adjusting the vacuum modulator helps but now 2-3 shifts later. Using a shift kit on the coach tranny has a certain affect. Using the same shift kit on a car tranny has a different affect.


The way I check the chain play is by squeezing the middle of the chain and meassuring. If its less than 7.25", I discard it. Once it gets to around 7", it skips teeth. The 66-70 trannis are what we call first generation. 71-78 are second generation. The 67 eldorado had two different holes for the bolt partern.
The first two years of the first generation were the switch-pitch trannies. The first generation trannies had thinner pump gears, so less volume. The center support didn't come with a fretting ring. Most of the time when I tear one down, the tabs are worned so much that I get more the 80 thousands play. They didn't come with the return check valve and they have been obsolete for quite a while. The sun tube/sun gear are a matched set but because of front/rear
endplay, holes overlap and you get no lubing. There are a few more things but what I'm getting at is that a regular tranny shop probably doesn't know about these things and wil rebuild using what is in front of him

The direct (Third and Reverse) drum clutch packs in the GMC 425 had six clutch plates, and most of the auto transmissions had five.  This gives you a 20 % more clutch area in these areas.  It is possible to put the 6 clutches in a car transmission, but the rebuilder must use thinner separating plates and machine down the apply piston to allow room for the extra disk.  These are available but require extra effort to find and install.

The green clutch plates are not as good for a heavy duty application like the Motorhome.

The auto transmissions used anywhere up to 16 springs.  The Motorhome transmission used 14 of these springs in the Direct Clutch Piston assembly  and 16 in the Forward Clutch Piston Assembly.

A lot of the automobile transmissions used a steel piston made up of several parts and did not have the extra support ring for pushing on the clutch plates.  As a result, the clutch separator plates are often bent and broken for lack of support.  See the picture below

Often teflon rings were used in manufacture, because it was easier to get a seal on the assembly line, and some times rebuilders use them if the transmission shows wear.  The metal rings will last much longer with out giving trouble.

The original transmission had a LIP SEAL, the new improved CHAMFER seal, no lip..

This assembly requires a special tool that many rebuilders do not have.  As a result, they just do not always rebuild this unit.

Most of the rebuild kits the rebuilders use, do not supply the thrust washers.  These should be replaced on a rebuild

Again, most rebuild kits donot supply new bearings.  The tranny will be loose with old bearings

New solenoids are still available

The problem with the later trannies is that the plastic guides for the return springs on the apply pistons distort with heat  and interfere with the apply piston returning causing slippage. We should also know what kind of frictions are going in the rebuild.


The  switch pitch has the advantage of switching your torque converter stall speed 300 rpm.  This with a high ratio final drive will give the best of all worlds.  High torque at startup and climbing situations, and high speed efficiency at highway speeds.  The Switch pitch will require the above modifications since it was only available in automobiles.

The Chain cover and the pan gaskets should be neoprene not Cork.  the Cork ones leak




here are the details of my switch-pitch rebuild.
Initial cost of the 66 toro tranny $300.00  with final drive.

> "Replace all Teflon sealing rings with the cast-iron upgrades,  Their > temperature and pressure capacities are bullet proof"


The teflon rings are not a terrible product but are meant for 'production' not 'overhaul'. I think GM used them because they sort of melt into shape and can tolerate being jammed together by assembly line workers, where a cast ring is a little brittle and you need to watch what you're doing. The cast iron ones are good enough they generally *could be* reused, a very similar part is used on many turbocharger turbine shafts in fact as an oil seal on a shaft spinning circa 70,000 rpm. Covey

Thrust washer (inner) 8626372 AC Delco  $12.87
Thrust washer (outer) 8625404 AC Delco  $13.87
plus shiping and tax from local GMC dealer.

Saxco 408-453-6101 ask for John Tell them you want the TC that Manny gets.  They are making sure to add the special features
also has switch pitch.

13" Switch-pitch torque converter from P. A. E. Performance, 1-915-855-6009, ask for Jim. Let him know what you have. $150.00 plus shipping.
        Stall speeds--Jim says they are
                -----/2700 with a 455 and a 3.70 final  

Two aluminum apply pistons from late model TH400 tranny from local tranny core bank. I got some for five bucks each and some for ten bucks each. Here in the Bay Area they charge way more than elsewhere.If you're going to use six clutches, you need them. Machining must be done. No big deal though, I did the machine work myself. 

There is one special tool that I made for inserting the apply pistons in the forward/direct clutch housings.
For those interested I will loan the tool.

You also need a shop press. Manny


> Can you comment on the actual temps you saw when in both Low Stall and in High Stall>

In low stall, cruising on flat ground, temp in the pan was ~ 150. In the line going to the external cooler ~ 190. This is prety much the same temps I had with the other non switch-pitch tranny. In high stall during an uphill grade, it climbed to 250 at the crest. 200 in the pan.

> Are you using a controller from Bruce Roe ??>

Yes I am and it is working great.

> I just picked up  a 1967 Toro SWP-425 on Monday, last week, and have been following your rebuild. I know you installed a trans cooler a while back also. Please advise>

I installed the external cooler because I didn't like the way the temp raised during the long grades. Without the tranny cooler, the pan temp was 170, now its 150. Remember I'm not connected to the radiator. These tranny temps coincide with the tranny temps I had before this trip with the external cooler.
Steve Clevenger's Switch-Pitch

Is yours up and running ??
Yep.  It doesn't do as well on acceleration as I expected.
0-60 :
Stock 18.6 seconds
S/P 17.4 seconds

I need to drive it a bit to get the hang of when to use the S/P. Don't need a fuzzy logic controller---I control the toggle switch & my logic has been fuzzy for years.

When engaging S/P mode, the stall speed increases from ~700 RPM to ~2500 RPM.  It effectively puts the engine in the power band.  There is no free lunch...the trade off for more power is more heat.  As I keep my foot in it, I can see the temp rise on the T/C outlet line & also the pan temp goes up.  The T/C temp runs about 30* hotter than the pan.  Pan temp will climb to ~200*, & when I get out of it, it quickly drops back to 160*.  This is with external cooler & Mobil 1 ATF-- no "finned aluminum
pan" for me, thanks.  That $$ was better spent on the Mobil 1.  There is no "lock-up", as that is a different animal entirely. I think it would be useful if I were pulling a toad or big hills, to keep the RPM
up, though you can't stay in it too long, because of heat buildup.  I bet that's why GM discontinued them--eat up with warranty work. I don't use a switch on the throttle linkage--just a toggle switch on the panel left of driver's seat, so vacumn & WOT are not relevant.

You can engage the S/P at any time, in any gear.  I think it would work best split shifting with it.  I once had a setup with a Chevy three speed manual with overdrive that I put a slide switch on the Hurst lever to control the OD, & could split shift each gear--ala 6 speed.  The Caspro claim is just misleading marketing, IMHO.  Steve

 Bruce Roe is the guy makes the Switch Pitch electronic controllers.
Note that the 1966 and 1967 transaxles also originally came with a 3.21 axle. Brent Covey Vancouver BC
Bruce's electronic controller provides much more sophisticated control of the switch pitch function compared to what the original micro switch on the throttle linkage does.  It's based on manifold vacuum control vs. throttle position, which activated near maximum throttle position.  Unlike the original linkage
operation, which is either on or off, now the torque converter can be utilized for all driving conditions and not just WOT. 

For GMC applications, Bruce also has a time out feature to control transmission fluid overheating, however from various user reports I've heard, overheating has not been found to materialize.  Paul Bartz

Standard converters are about 2300 stall yes. This varies slightly with engine max output and converter condition.
A manual switch CAN do everything, but you will find the controller works harder for you than you might be willing to do manually.  In a car this is quite obvious.

I think the best thing for a GMC is to ask the people who started manual and converted to a controller.  I don't get much feedback; I guess that's good. Some things are hard to explain on a pure logic level.  I say if you don't think its worth the money, I'll buy it back.  I've never had to buy one back yet.

One feature of the controller is always starting you out in high stall after the brake is released.  The second is the drop "half a gear" to high stall when manifold vacuum is lost.  And of course the timers reduce the chance of overheating.  A SWP will actually run cooler (and better mileage) in low stall.

The SWP controller does not operate back and forth on each gear like a true 6 speed.  But it can give you half a gear more for any gear; I call it a poor mans' 6 speed.  Normal is to do 3 gears in high stall, then go to low stall.

I don't find SWP high stall is useful for downhill braking in a car; the extra slip cancels the lower range.

I think the Caspro SWP with power drive is the best GMC rebuild you can buy; I have one in my car.  Who else gives you improved gearing AND new chain?  I wouldn't describe the SWP controller exactly the same way Caspro does.

A controller that goes to low stall on high trans temp would be nice; haven't figured how to get the temp info to the controller yet.

Torque converters are not 100% efficient.  The power lost shows up as heat. Typically the efficiency is lower as you get more torque multiplication.  At the moment you hit the throttle but the vehicle has not moved, the efficiency is 0% (but rapidly increasing), a good trade off to get started and into the engines' power band.  In low stall, a switch pitch can be MORE efficient than a fixed

The size of the trans oil pump (bigger in 67) relates to oiling and somewhat cooling the trans; has nothing to do with converter efficiency.   Paul Bartz


> Now the SP converters we are getting are 1000/2400

Those numbers are a bit low- the numbers I am using are what results would be obtained with GM converters either in new 1966 cars, or from 1966 car transmissions if the converters are in good shape, in a seven litre engine vehicle making around 325-375 net ft/lbs torque. Its hard to get really specific, but in a healthy but not hopped up 455 vehicle the 13" SP converter should stall closer to 2800-3200 in 'performance' mode at wide open throttle. A 350 car with the smaller intermediate (think its an 11")
should also reach peak stall around 3000 in its performance mode with the lower output engine. In 'cruise' mode the converters should be pretty much stalled by 1100-1400.

There are a lot of possible reasons for variations in converter stall speeds in any specific vehicle, most are related to poor engine tuning, and some converter deficiencies like a slipping sprag clutch for the stator will make an enormous impact on stall speed.

Compared to the stock GMC converter the SP unit in the 1965-1967 big cars stalls several hundred to 1000 rpm later in performance mode. It is very noticably 'looser' than a GMC converter. You would definitely find a conspicuous difference Gene. Also, it should have a deeper reduction availible at stall, offering about another 150 ft/lbs torque at that point. As this is prior to the transmission and differential, this creates about 150 x 2.5 x 3.0 at the tires, or ~1100 ft lbs more at the wheels at stall. It also occurs at a speed the engine makes quite a bit more power, even a few hundred rpm makes a large difference.

> This does not seem to give us an improvement in the high Stall.  Which  would again only leave us with an over drive when we get into the low  stall.   Should be looking for a different SP torque converter ??

I havent got a great deal of experience with the aftermarket converters being marketed for the SP transmission. The GM factory big car unit was excellent and a nice balance for general use. As stall speeds are somewhat dependent on engine output, if you are getting a special converter from
somewhere it might behave quite differently in a GMC than a GTO. I feel most of the aftermarket ones would probably stall at a higher speed in a GMC, if they are given stall designations with drag racing in mind. Certainly you dont want a maximum stall over 3400.

Any used GM version can be rebuilt without tinkering with the factory stall speed. If you wanted a little higher stall speed than the 13" big car converter, an internmediate car smaller SP converter might be a useful thing. Theres still a lot of 1965-1967 Buick Special sedans with Super Turbine 300 transmissions in the wreckers, and they also have the micro switches- most used a plunger switch that was a quick and easy way to get the SP electricals working fast upon original installation. My pr eference is for the bigger converter however.

Using a junkyard converter is a risky proposition. I do that on my own transmissions, but I get free labour (me) if it blows up. I would be reluctant to encourage that as a general practice unless you are fairly certain of the history of the converter and from whence it came to be in your possesion. Converters are a sort of centrifugal junk and debris collector and anything that gets inside seldom gets back out on its own.

That place in Texas selling converters has a model that stalls at 3000 rpm that probably would work well in a GMC.

Hope this helps, if you can get a ride in a GMC that has the SP trans you'll be sold on the spot I suspect.
Brent Covey


> Question 1: Do you recommend adding the "Switch Pitch" to a rebuild of the  original transmission.

If the parts are availible (like a 1966-1967 THM 425 core) its a very worthwhile improvement, and I would recommend it. Its great to have, and a desirable refinement.

> Question 2: What is your estimate of the price for rebuilding a TM 425 with  and without Switch Pitch?

That depends quite a bit more on who is performing the rebuild than whats worn out nd how crooked the shop is. I would not be suprised to hear people paid as much as perhaps $2000 for a transmission overhaul at a chain type shop like AAMCO et al.

Removing and replacing the transmission from the vehicle is about a $175-$250 operation in many shops as well.

If you do the work yourself I feel good results can be obtained for under $500, and if you had enough cores you likely could rebuild a THM 425 for the cost of the gaskets ($~70) with enough good used parts to poach from a couple disassembled transmissions. In my view anything under $1000 for a
thoroughly inspected transmission with new seals, clutches, bands, modulator and converter is a pretty decent deal. Special converters and chain drives or other usually unneccesary parts may significantly increase the costs. Anything over $1500 is pretty suspicious. I would remind people that transmissions should usually not need an overhaul ever, and if they do quit or act up, they often can be repaired by good diagnosis and replacement of a smaller part rather than disassembly and inspection.

The Switch Pitch transmission is no more costly to overhaul than the ordinary type- even the special converter can be rebuilt for the same cost as the 'ordinary' ones locally in most Metro areas. The electric solenoid for the SP feature is the only extra part pretty much and its usually OK, and comparable to the kickdown solenoid if replacement is needed for cost (~$15). The extra cost would be obtaining a SP core in the first place to rebuild. At this time I am not aware of any vendors selling complete kits of
parts to convert a non-SP transmission to a SP type anymore- although years ago this was done. A SP transmission should be quite inexpensive (free for the picking to a couple hundred bucks) but there are few left, not a lot of Toronados and Eldorados survived. I have had good luck just calling at transmission shops where they have a pile of old transmissions in the back and seeing if they have any kicking around.

I recently bought a THM 400 with the SP converter from a 1967 Oldsmobile NinetyEight for $30 in excellent condition. Deals are out there if you arent in a hurry. Nobody wanted it because it has no speedo provision. I didnt care about that, I'm stealing the SP parts for a car that didnt have one

Any Toronado or Eldorado with the SP THM 425 also came with the 3.21 differential, so grab that as well with it if you're pulling one in the wreckers. Don't forget the jackshaft under the engine oil pan to the
passenger's side wheel, its special for the 3.21 axle assembly and needed. Only 1966 and 1967 cars had the SP THM 425. Later versions do not have this feature.Brent Covey