Here is what I understand to be true.

While cruising, the Vacuum Modulator on the transmission handles shifting. (This has been replaced and cruise shifting works great)

While at WoT the switch beneath my pedal activates the governor within the transmission, which should then handle shifting the transmission.

Starting off the line, when at WoT my engine winds out to 6500 RPMs, and the transmission still does not shift. As soon as I let off the gas, it immediately shifts to 2nd gear, and I can get back on it, and as soon as I hit 6500 RPMs again I let off, and then it shifts to 3rd.

Tranny fluid is at its proper level.

I have not touched the inside of the transmission since purchasing the 79 Vette. What I do know it is has an after market ribbed pan, so someone could have been inside and made changes, shift kits possibly?

The previous owner had the WoT switch / sensor disconnected under the gas pedal, and I reconnected it.

So I am looking for the cause of this.

Do you think this is a governor issue itself, as in it is bad?

Or is it a calibration issue and I need to purchase a B&M Governor Calibration kit for a TH400?

Or is it some sort of valve issue not actuating to allow the tranny to shift?

Any tips on how to troubleshoot this better etc are much appreciated!

Thanks guys you have always come through for me in the past!

 

Sorry didn't get a single view besides myself, bumping the thread.

Thanks

 

You got a bit of an oddball there. The THM400 didn't come in a '79 so you're looking at a transplant. They also don't shift at 6,500 rpm stock, so you've got something going on with the governor. It would be hard to say what was done to the trans. Have you tried readjusting the kickdown switch?

 

Doesnt the vacuum modulator on the passenger side of the trans control shift points. Turning the screw inside one way or the other changes shift points by vacuum. Not sure which way lowers, and this may not even be the problem. But I thought I would throw that in there.

Sean

 

OK. Here goes.... The governor in the tranny tailhousing turns with output shaft speed. It regulates "main" pressure so that the faster it turns the more pressure it sends out. This is Governor pressure. Governor pressure is what signals the shift valves along with some 'modification' being performed by the vacuum modulator. It's function is to regulate the pressure according to how much manifold vacuum is present. At idle there is a maximum vacuum...at full throttle there is none. So the vacuum modulator trys to make the shifts earlier when there is more vacuum present. At WOT [since there is no vacuum], the modulator has no effect. So, to summarize, governor pressure is the primary signal; the vacuum modulator just modifies that shift schedule.

Now, after all of that: your problem must be related to the governor. When it won't shift soon enough (especially at 6500 rpm!), the governor regulator valve {built into the shaft of the governor} is probably sticking OR the gear at the end of the governor shaft is about wearing out. Take the 4 bolts out of the governor cover, then pull the governor out and check it out. The flyweights should move freely and the regulator valve should move with the weights with no resistance. The plastic gear (if worn or damaged) can be replaced by removing the roll-pin in the shaft. Hope that helps. The "correct" governor should be selected according to the WOT shift point you want to have.

 

Just to be clear, turning that screw inside the modulator does change WOT shift points to some extent right? Or does it have no effect ever at WOT and that is handled by the governer pressure? maybe I misunderstood the explanation. Sorry if thats a dumb question, I will be dealing with this soon and I need to learn sometime.

Not trying to steal OP's thread, just figured I would ask while we were on the subject.

Thanks,
Sean

 

I can't say positively that the adjustment screw causes no change to the overall shift pattern....but I don't believe that it does. I'd have to see the hydraulic schematic to know how that adjustment impacts the whole circuit. But, that adjustment is a 'fine tuning' feature for the transmission test operators to twiddle on. It won't make much difference in the overall shift schedule either way. Again, the governor pressure and the shift valve spring rates really determine the shift schedule; the vacuum modulator just lowers the overall schedule when there is some level of vacuum present.

 

It does, but it's not a major contributor. The adjustment, min to max, will alter the shift scheme by around 10mph. Anyone who's had a vacuum motor go bad or had the vacuum line pop off -creating what the trans thinks is WOT as far as the modulator is concerned- can tell you shift firmness, both up and down, as well as timing changes. But the commanded upshift timing is still an exclusive function of the governor -even overriding the downshift solenoid. I believe you are correct; he's going to find the issue in the governor. It's not hard to modify this function (http://www.dragracingonline.com/tech...vi_9-bm-1.html) and it appears his has been tweeked.

 

Great man thanks for the info.

So my next question is, is the B&M Governor Calibrater kit about the only way to go or is there a way to buy a pre-setup governor just to make the job easier?

I kind of doubt I would get that lucky, but is there a way to calculate which weights / springs etc to start with to get in the ball park of what I am looking for?

I would like to have the car shift around 6,000 RPM, I only pushed it to 6,500 to see if I could make the @#$% thing shift hehe.

Thanks again guys

 

The primary 'adjustment' in the governor assembly are the springs which restrain the fly-weights; the stronger the springs, the later the shifts. Or, you can change weights; the lighter the weight, the later the shifts. So, if that "governor calibrator" package comes with different weights and/or different springs, it should allow you to do what you need. The governor is easy to remove and work on (I'm assuming you can get enought access in a C3 to remove and replace it) if you have a way to get under the car. Oh, buy several governor cover gaskets...you'll need them.

 

I am going to ty this... I cant get 3rd in my TH400 C3 LSX. Regular and part throttle shift no problem. running 1/4 mile going from 2nd to 3rd nothing. So I am guessing the Govenor needs to be adjusted. NO vaccum on my Motor at high RPM

 

If I assume that the trans is functioning properly with good clutches, clean valve body, no internal wear, etc, most likely is the governor has been modified by narrowing the inner flyweights to raise the shift points. By removing the governor and replacing the modified inner weights with larger/heavier weights, the shift rpm will be reduced.

The kick down switch activates an electrical solenoid which activates detent regulator pressure. If the detent regulator spring had been shimmed, it will raise the WOT shift points also. You might want to check for a washer inserted to shim the detent regulator spring - if so, remove it to lower the WOT shift points.

Inspecting the governor flyweights is easier to do and I'd recommend that you do this first.

 

trans is brand new... about 1000 miles on it and 3 passes on the 1/4. 1-2 shift is perfect and on time. 3rd works great unless it is WOT... then nothing. I think it is vacuum related, the line i have on it is really a brake line... a little large. But I will try the governor after I change the line... mine is not electric but vacuum.

 

The vacuum line to the vacuum modulator is for modulating part throttle shifts. Don't think you will find any difference in operation by changing the vacuum line.

The springs in the governor affect the 1-2 WOT shift mostly, the inner flyweights affect the 2-3 WOT shift the most.

 

The 2-3 shift valve determines when that shift is made...well, actually, the spring on that valve is what is changed to modify the shift point. (There may also be an 'adjuster ring' at the end of that valve which can be reset to raise or lower that shift point.) I suspect that you just haven't revved it high enough to make the 2-3 shift happen. If you have reached your 'redline', then the transmission wasn't built correctly to your shift point specifications.

 

Both good points but is shoft at part throttle just fine... I shift the car about 6500 on the 1-2 and try to shift 2-3 at the same time... but doesnt happen under WOT.

I guess I cplay with inner flyweights affecting the 2-3 at WOT, but I dont want to change my 1-2 shift...its perfect even when in full auto mode.

 

OK, if you are shifting this manually, then the issue is the 2-3 inhibitor valve. It is in the circuit to 'force' a 2-3 shift, even though you are keeping it in 2nd gear manually. Apparently, that valve is stuck so that it will not force the shift or the spring is too strong and it would require higher rpm for it to shift. If it shifts at the right time in auto mode, why do you shift it manually?