[MDGOAT69]

Hello,

My father's GTO is running excellent and the transmission shifts into all gears with no problems. I have noticed after putting about 100 miles on it that it's not downshifting the way it should. If you are cruising at 30mph and mash the gas it will kick down to 2nd gear from 3rd, but shouldn't it go to 1st? It makes the car feel more sluggish than it really is because it's almost never in the powerband.

I checked under the dash and the kickdown switch is there. I have power at the switch but I wasn't getting power to the wire going to the transmission with the plunger depressed. I swapped out the switch with another I had and NOW I get power to the wire going in to the transmission. Laying under the car I can hear the solenoid now faintly click when the pedal is floored with the ign on.

Took the car for a test drive to see if it works and it's still not right. The car will only downshift to 1st at under 20mph. (Still an improvement I suppose) I should get 3-1 downshifts up to 30 or 35mph right?

What else could cause this? Only other thing is the WOT shifts are pretty early, I don't have a tach to check but they are at most probably 3800-4000rpm. Could that be part of the issue?

The car is a blast when you shift manually with the his and hers but I feel like it should still be better in drive?

Any advice is appreciated!

[rexs73gto]

Check your vacuum supply. It should be on hard vac. & not ported. Also check where the vac. come in to the trans. You should have it on the back of the carb where the power brakes are connected.

[Formulajones]

Part of that is controlled by the governor. If your full throttle upshifts are short, which they are (you said 3800-4,000) then that governor setting isn't going to let you find first gear at WOT when going 30-35 mph, because depending on the rear gear ratio, 30-35 mph 1st gear downshifts will probably throw the engine at or just above 4,000 rpm. Time to play with governor weights and springs.

[MDGOAT69]

Originally Posted by Formulajones

Part of that is controlled by the governor. If your full throttle upshifts are short, which they are (you said 3800-4,000) then that governor setting isn't going to let you find first gear at WOT when going 30-35 mph, because depending on the rear gear ratio, 30-35 mph 1st gear downshifts will probably throw the engine at or just above 4,000 rpm. Time to play with governor weights and springs.

I was leaning towards that being the issue, time to start researching that.

That does make sense, a 35 mph downshift would make it rev higher than it wants to.

Originally Posted by rexs73gto

Check your vacuum supply. It should be on hard vac. & not ported. Also check where the vac. come in to the trans. You should have it on the back of the carb where the power brakes are connected.

I actually don't have it hooked up there, I used an accessory port on the front of the carb. You think that's an issue? Should I tee the line for the booster and run the transmission line there as well?

[OCMDGTO]

Your downshifts sound pretty normal to me, and I have had several T400s in different cars over the years. You should get Cliff's T400 book, great reference material.

[Formulajones]

To give you an example. My 70 Formula shifts on it's own at about 5200-5300. Matt the throttle at 35 mph and it will grab first gear and go. Won't be in first for long but these cars, when working properly, will do this. Most of them had governors setup from the factory to shift at 5,000 rpm or more.

My fathers 69 GTO he bought new, 400/350hp, shifted at 5,000 like clockwork. It would also grab 1st gear around 30-35 mph if you matted the throttle.

[Greg Reid]

Originally Posted by MDGOAT69

I actually don't have it hooked up there, I used an accessory port on the front of the carb. You think that's an issue? Should I tee the line for the booster and run the transmission line there as well?

I'm pretty sure on the '68 the tranny governor should be attached to a full vacuum source on the carb. In this photo, it is the metal line attached to the right front of the carb by a short length of rubber vacuum line and is routed down between the choke stove on the manifold and the valve cover. It runs down the back of the block where it is attached to the Gov by another short length of rubber line.

[Formulajones]

Are you talking about the vacuum modulator?
The vacuum modulator will control part throttle shifting, some are adjustable inside the hose nipple.

The governor I'm speaking of is further back on the side of a 400 turbo, with a stamped steel plate held on with 4 bolts and a gasket. Remove this cover and there will be a centrifugal device that pulls straight out with a gear on the end of it. This device has weights and springs on it and works similar to centrifugal weights in a distributor.

These weights and springs can be manipulated to lengthen shifts, higher shifts, or stack the shifts closer together. You can even have stacked shifts at part throttle, and then long wide high rpm shifts at WOT throttle by manipulating the springs. There is a kit available with weights and springs for modifying these governors. Very tricky and extremely time consuming. A lot of "beating" on the car to get the shifts exactly where you want them. This is where electronic transmissions have a huge advantage, just a few key strokes and you're done.

Most 400 turbos built up into the 1970's started having very short shifting governors installed where upshifts under WOT may only occur at 4,000 or maybe 4500 rpm. Low performance applications. My 79 454 pickup is this way. When a governor like this is installed, it's almost impossible to get the trans to grab first gear at any mph when matting the throttle. I don't even think my pickup will find first gear at 20 mph. It won't even downshift to second if I'm going over 50 mph. It has very short stacked WOT upshifts. Which is okay in this application, pulling trailers you don't have a transmission downshifting all the time. It just grunts along in 3rd gear.

It sounds as though someone has changed the governor in your 68 GTO, or maybe the trans is out of another Pontiac not meant for a performance application. Any 67-70 GTO with the base 400/350hp engine should have a 400 that upshifts around 5,000 rpm on it's own.

[OCMDGTO]

Vacuum line goes to the modulator not governor

[MDGOAT69]

Originally Posted by Formulajones

Most 400 turbos built up into the 1970's started having very short shifting governors installed where upshifts under WOT may only occur at 4,000 or maybe 4500 rpm. Low performance applications. My 79 454 pickup is this way. When a governor like this is installed, it's almost impossible to get the trans to grab first gear at any mph when matting the throttle. I don't even think my pickup will find first gear at 20 mph. It won't even downshift to second if I'm going over 50 mph. It has very short stacked WOT upshifts. Which is okay in this application, pulling trailers you don't have a transmission downshifting all the time. It just grunts along in 3rd gear.

It sounds as though someone has changed the governor in your 68 GTO, or maybe the trans is out of another Pontiac not meant for a performance application. Any 67-70 GTO with the base 400/350hp engine should have a 400 that upshifts around 5,000 rpm on it's own.

I wouldn't be surprised if the transmission was swapped at some point, so that's a good possibility.

Thanks all for the feedback!

[MDGOAT69]

Got a tach hooked up, 1-2 shift is 3500rpm, didn't check 2-3. That seems crazy low, so time to start playing with the governor.