72455

Since I bought my '72 Supreme, I've always wondered if the kickdown on my TH400 was working properly. On the highway when I'm cruising around 65-70 mph and hit the pedal, there's no kickdown.

Yesterday I noticed that around 40 I hit it and she kicked down to second, then back up to 3rd at around 50-55.
​​​​​​
So, unless I'm wrong and it's pressure related, my kickdown is coming in at a lower speed than it should. A while I tried the rubber hose trick around the switch by the pedal, but it didn't seem to make any difference, so I just left it.

What options do I have to try and have the kickdown come in at highway speeds?
​​​​​​
​​​​​​

72455

Thanks Norm. I'll check all my vacuum lines...didn't think of that. Would that possibly be the cause of the kickdown coming in at a lower speed, and should I remove the piece of hose on the switch?

rocketraider

TH400 uses electric downshift. Check adjustment of the downshift switch per CSM. Also TRANS fuse and all wiring. Downshift solenoid possibly weak.

Why do you want downshift at those speeds? Carb secondaries are more effective in higher speed passing maneuvers.

72455

So I'm good where I am? If so, I'll leave it alone.

joe_padavano

Exactly what RPMs are you turning at 65-70 MPH? I'd be amazed if the trans kicked down at those speeds. As an example, consider a car with 3.08 rear end and 225/70-14 tires (about 26.5" OD). That car would be running at nearly 2800 RPM at 70 MPH in third. A detent kickdown to second gear at that speed would jump the engine RPM to over 4000 before you started accelerating. It sounds to me like your kickdown is working exactly as it should.

72455

Joe, you know your stuff, cuz your numbers are pretty much dead-on. I'm running 3.08 gears and 275-60-15s. So yep...at 70 mph I'm turning around 2800.

Fun71

They both have a vacuum modulator that affects the part throttle shifts based on engine vacuum, but the downshift on the TH350 is cable controlled vs the TH400 which is electric switch controlled.

matt69olds

Joe beat me to it. The exact downshift/upshift threshold depends a lot on gear ratio and transmission valve body calibration.

The fact that it downshifts and then upshifts is a clear indication the electric kickdown works.

The vacuum modulator has zero influence on WOT shift points. The governor, modulator and kickdown (either electric or cable) combined with the valve body calibration all determine shift points. You can vary the part throttle shift points a couple mph up or down by moving the small screw inside the modulator neck

72455

So based on all the replies, it seems like a good idea to leave well enough alone😀

oldsmobilejim

In regards to vacuum leek, do you have a mechanic shop that you patronize. If so, ask if they would hook up ( EVAP Smoke Machine ) Automotive Vacuum Leak Detection Tester. Make a appointment they will probably do it while you wait, this will test the whole system, including intake, carb. brake booster, transmission, all ports and rubber lines. Good luck
,

Dream67Olds442

Guys,
To follow-up on this discussion, I have a question.. With the OP's set-up, if he were to accelerate from a stop at WOT, and he noted the speed/rpm at which the car shifted from 2nd to 3rd, should he not expect the kickdown to operate at speeds above which he observed the car shift from 2nd to 3rd?

Fun71

Correct, the downshift will not happen at vehicle speed (or engine RPM) higher than the upshift point.

72455

Hmmm...🤔 I'll have to try that tomorrow.
Just to understand, it should downshift at speeds/rpms higher than when it upshifts at WOT?

Fun71

72455

Got it...this is what confused me...

"should he not expect the kickdown to operate at speeds above which he observed the car shift from 2nd to 3rd?"

Dream67Olds442

I wrote it, and I see why you were confused, it was correct, but poorly worded. I should have wrote - He shouldn't expect the kickdown to operate above the speed where the car shifts from 2nd to 3rd at wide open throttle.

72455

Took it out today and made 3 WOT passes, all from a standing start. Each time, 2nd to 3rd came in around 35-40 mph and around 2800-3000 rpm. Then I took it down to about 30 mph and hit it, and kickdown came in around 40 mph...does that sound right?

Fun71

WOT shifting at 2800-3000 RPM sounds as if your trans is shifting too early. The factory shift point on my car with a TH350 is 4800 RPM in drive.

For comparison, my car has 3.23 rear gears and 245-60-15 tires (26.5" tall) and shifts 1-2 at around 35-40 MPH (at 4800 RPM).

matt69olds

You may need to play around with the governor to get the shift points where you want. It sounds like either the trans or the governor came from a car calibrated for much lower shift points.


TCI, B&M, and other manufacturers offer a governor calibration kit. I would suggest getting a governor from another transmission. Go to your nearest transmission shop, if the don’t have a governor they can locate one. Get a couple governor cover gaskets. Install the replacement governor, go for a test drive, and carefully record the 1-2 and 2-3 shift points. Once you have this info, read the instructions and replace the weights. Reinstall the governor, and go for another test drive. Hopefully this time you got the shift points where you want them. If not, make another weight change.


if you raise the rear of the car, you will loose very little fluid. The governor cover is held in place by 4 1/2 bolts. It’s pretty easy to remove and reinstall compared to a TH350 or 700 trans.


here is a article I copied from the buick forum.

[img]blob:https://classicoldsmobile.com/234867c8-0e8f-4e36-a286-b4870251692b[/img]

72455

I'm running a 3.08 gear and 275-60-15, so with you running a steeper gear, would it not seem logical that your shift points would come in a a higher RPM?

matt69olds

Or if you have a small scale that will accurately read in grams you can disassemble your current governor and modify your existing weights. Keep in mind, the actual weight isn’t the only determining factor, it’s WHERE the weight is. Removing weight closer to the center of rotation won’t make nearly as much of a difference as taking weight from the end due to centrifugal force.

As crazy as it sounds, the axles the weights pivot on aren’t a fancy machined part. They are actually finishing nails. HydraMatic spared no expense building these transmissions!!! Cut the flared end off the nails, disassemble the governor, carefully weigh the weights, carefully grind the weights, then reassemble. Once it’s all back together, use a punch and hammer to flare the end of the nail so it can’t fall out of the governor.


It will take some trial a d error to get the shift points where you want, but that’s a perfect excuse to go run your Olds hard and have some fun. That’s a good enough reason in my opinion!!!

72455

According to Joe, it's good, or am I missing something? Because his numbers are dead on with my setup, except the rear tires... again, I'm turning 275-60-15 out back.

matt69olds

your transmission or governor might have come from a car with much different rear gears. Keep in mind, the valve body doesn’t know what the engine rpm is, the governor is spun by the output shaft. The driveshaft speed is what the governor is influenced by.


Let’s say the original calibration was set up for a car with 3.91 gears. The 1-2 shift is made at 35 mph (5200 rpm, I’m just throwing numbers out there).

Put that same trans in a car with a 2.56 gear. That same 5200 rpm shift point now results in the 1-2 shift being made at 45-50mph. Swap the trans in a car with 5.13 gears, 5200 rpm might result in a 1-2 shift at 20mph.

I suggested starting with a different governor so if something happens to the replacement you have the original to reinstall.

Here is the governor calibration kit. https://www.summitracing.com/parts/b...hoCxLYQAvD_BwE

72455

It's the original tranny Matt, but it also has a 2800 stall converter.

matt69olds

Has it ever been rebuilt? If so it’s entirely possible the governor was replaced. Occasionally the drive gear wears out, it’s far easier to replace the governor instead of the gear.

If the trans upshifts/downshifts normally, just not at the desired speeds you should be able to correct it with some governor modifications.

72455

It has been rebuilt, and it upshifts/downshifts fine.

joe_padavano

That can't be right. At WOT, the TH400 should upshift in the 4500 RPM range. With 3.08 gears, that's about 45 MPH for the 1-2 shift. The 2-3 upshift at WOT would happen at about 65-70. This is entirely controlled by the vacuum modulator and the governor.

72455

Ya know what...I was mistaken...it's the 1-2 shift that's coming in around 40 mph..as far as the RPM, I looked at the tach after the shift, and was looking at around 3,000. So is it possible that it's shifting correctly, but I'm not seeing the actual shift point?

joe_padavano

As noted above, the upshift RPM will vary depending on the calibration of the trans. A trans from a non-performance car will have a governor that allows upshifts in the 4000-4500 RPM range. Transmissions from performance applications might go 5000 RPM or even a little more. Sounds like yours is working about as it should.

Fun71

The shift points occur at the same engine RPM no mater what rear gear is in the car. When I put a 2.56 rearend under the car to rebuild the 3.23 rearend, the shift points were still at 4800 RPM. As Matt said, with the lower gear (3.23) the trans shifts at a lower vehicle speed (35 MPH), and with a higher gear (2.56) the trans shifts at a higher vehicle speed (55 MPH).

And yeah, you gotta look at the RPM as the trans is shifting, not after it is in the next higher gear.

joe_padavano

The shift point RPM is determined by the weights and springs in the governor, period. The governor is geared to the output shaft of the trans, so it's the RPM of the driveshaft that matters. Slippage in the converter will typically cause the engine RPM to be slightly higher than driveshaft RPM. The vehicle speed that corresponds to that RPM will vary depending on the axle ratio and tire diameter, but that is unrelated to trans calibration.

EDIT: To clarify, I'm talking about upshift RPM at W.O.T. here. Obviously at less than W.O.T., manifold vacuum is not zero so the modulator has an effect on upshift RPM also.

72455

Ok, just so I'm clear...with 3.08 gearing at WOT, the 1-2 upshift should be around 35-40 mph at around 4500 RPM. Kickdown to second should come in no higher than the 1-2 upshift, correct?

matt69olds

That should be pretty close. The kickdown threshold might be a little higher than the 2-3 shift ( ideally you want the trans to stay in 2nd long enough to actually do something besides make a lot of noise) but should be pretty similar.

72455

Ok ..so I should not expect a 3-2 downshift at highway speeds, right? At those speeds, it's the secondaries that do the work? Again, not to be a pain, but just trying to fully understand....not looking to fix what isn't broke😀

matt69olds

From Ron Sessions TH400 book.
The exact upshift speeds depends on gear ratio.

72455

Ok.. just got back from another run. Under normal pedal, both the 1-2 and 2-3 upshifts come in around 1800-2000 rpm. WOT upshifts come in around 3800-4000 rpm. At highway speed (65-70 mph/2800 rpm), I get no kickdown at WOT.

joe_padavano

Personally, I think you are loosing too much sleep over this. I don't know what the "ron sessions book" is, but the factory Chassis Service Manual says that kickdown should occur at speeds below about 70 MPH. More to the point, that factory information assumes that it is the original trans in the original car with the original rear end gears and the original tire diameter. Where did your trans come from and what was it calibrated for? Just for grins, what does the ID plate on the trans say for model year and two letter code? If this trans was intended for a full size with 2.73 gears and you have it in a Cutlass with 3.08s and smaller diameter tires, guess what? The driveshaft RPMs at 70 MPH will be higher than the factory intended, which means that the governor will assume the speed is higher than it is.

What's the highest speed at which it does kick down? What's the RPM before and after that? If it currently upshifts at 4000, it isn't going to downshift if the RPMs would be higher than that in second. If you think it should kick down at a higher speed, manually shift into second while you floor the throttle and see what happens. If you want to increase the upshift RPM, you need to play with the governor weights and springs.

72455

OD trans Joe...numbers matching, and original 3.08 gears. Highest kickdown speed is around 40-45 mph. As far as the before and after RPM, I'll haveta check that next time.

matt69olds

How to Work with and Modify the Turbo Hydra-Matic 400 Transmission (Motorbooks Workshop)
Probably the most informative transmission book available. In my opinion, better than the factory manual.

This book covers production changes, parts interchange, what parts and assemblies can be combined, specifications, build processes, and high performance upgrades.

How comfortable are you with removing the valve body? There are different detent valve calibrations, if the trans valve body was changed during overhaul that may be the reason why your trans won’t downshift at the speeds you want. If it’s really that big a concern to you that may be the solution.

read this

http://www.buickperformanceclub.com/SPTrans.htm

joe_padavano

I would play with the governor and the modulator long before I touched the valve body in this case.