My TH 400 was rebuilt several years ago and sat in my shop wile I finished my 68 442. When I installed the tranny and drove the car for the first time it shifted from 1 to 2 as it should, I only drove it around the block and did not get much speed. Drove it again a week or so later and it tried to shift from 1 to 2 but felt like it was slipping, again did not go far or fast just around the block. Checked the fluid and it was low so I filled it up. Drove the car around the block again and it wont shift at all but does not have the feeling of slipping it just will not shift. Tried starting in 1 and then manually shifting to 2 and that did not work either. I have checked the vacuum line from the intake manifold to the modulator and it is connected. What do I try next?

 

I know you checked the vacuum line to see if it's connected but is there vacuum there? Or it could be the modulator itself and it could also be the kickdown switch.

 

OK, how do I check those to determine if they are working properly or not?

 

On the kick down make sure there is not 12v there all the time first. With a jumper wire you would hear a solenoid activate when 12v is applied. On the modulator, if you disconnect the hose you will feel vacuum there with the engine running. If in doubt of the modulator working correct change it.

 

OK let me make sure I have this straight.
For the kick down I will run a 12v jumper from the battery to the kick down switch and listen for solenoid to actuate. If it does not I will have to replace the kick down switch.
For the modulator I will disconnect the vacuum line from the modulator and see if there is vacuum to modulator, if there is I will replace the modulate.
If this is correct I will give it a try this weekend.

 

First check for 12v at the kick down solenoid at the trans without activating the switch. Then check for 12v there moving the pedal to wot. It should only be there @ wot.

With the engine running remove the rubber line at the modulator and see if there is vacuum present, it should be connected to manifold vacuum. If there is vacuum then I would try changing the modulator.

 

was able to do some work today but no success. The kick-down does not have constant power and works properly. There is vacuum to the vacuum modulator but when I put a vacuum gauge on it, it is unreadable the needle was moving rapidly from about 10 to 20 or so, not sure why it is doing that there are no obstructions in the line. I also removed the governor and checked the driven gear and it is fine. I will pickup a vac modulator tomorrow and try that next.

 

You should have relatively steady manifold vacuum at the modulator. Is your vacuum fluctuating at the carb, which may be another issue?

 

I have two vacuum line attached to the brake brake booster it has steady 17 or so and the other hooked to trans and fluctuates as I described. I took the line to the trans and hooked it to the outlet that was running to the brakes and it still did not shift. What do you think about the fluctuating vacuum?

 

I don't understand 2 lines at the brake booster. If you have steady vacuum at the booster it should be steady at the trans. Check the steel line and both rubber end lines for cracks, if they are hard and brittle just replace. If the trans does not shift after that I would get a modulator. Perhaps a trans shop has a used one they can throw your way to test with.

 

sorry my reply was not clear there are two vacuum lines connected to the intake manifold, one goes to the brake booster and one goes to the trans. The one to the brake has a good reading on the vacuum gauge the one to the trans fluctuates. I connected the one from the brakes to the trans to see if better vacuum would solve the shifting problem, it did not. All the lines are new. So I will try the modulator next but now I have to solve the fluctuating vacuum as well.

 

Ok looks like the problem is vacuum. I have run a compression test and all are 170 to 180. Where next

 

Have you changed the modulator?

 

No not yet the reason being I need good vacuum to the modulator. I was getting a good reading at the brake booster because I was checking at the one way valve not at the manifold. When I checked at the manifold it did the same thing. So I figured before I change the modulator I should fix the vacuum situation. But if you think I should change the modulator first I will do that. Any thoughts on the vacuum?

 

Won't shift at all or won't shift unless at WOT or close to it? The latter indicates you might have a vacuum leak. Follow the modulator line up to the engine and check every connection to it. Especially those bogus plastic "T's" GM liked to use.

 

If you put a gauge on the line at the trans it's still fluctuating there?

 

If you have a leak in the vacuum modulator, then when you test the vacuum at the modulator, it will be low or will vary because it drops when the vacuum leaks out through the hole in the diaphragm, which may not be a constant leak, but one that depends on the position of the diaphragm.

If you have an irregular vacuum reading at the modulator end of the pipe, with the modulator disconnected, then plug the modulator line and pull a vacuum on the other end of the line - it should hold vacuum without leaking down - if it leaks, then there's your problem.

- Eric

 

 

Yes I think I have a manifold vacuum leak what do you think? And if so what are your suggestions to fix it? I rezc in another forum to re torque the manifold bolts?

 

I have determined so far.
1. the line to the modulator is fine, holds vacuum no leaks
2. the modulator molds vacuum no leaks.
3. The trans will not shift out of 1st gear when in drive or manually from 1 to 2, reverse works.
4. When I connect the vacuum gauge to either of the two ports on the intake manifold the gauge fluctuates very rapidly from 0 to 20, hard to tell the exact number as the needle is moving so fast it is difficult to tell where it stops on either end.
This a Edelbrock performer manifold and edelbrock aluminum heads. It seems I have a vacuum leak but I cant seem to locate it.

 

You can spray some brakeclean or carb cleaner around the manifold and carb to see if the rpm increases. If it does you found a leak. It can also be a rubber vacuum line.

 

 

I think I will have to try the brake cleaner l have disconnected and tested all the hoses and tested pressure directly at the manifold. Will the brake cleaner harm the finish of the valve covers or intake

 

The chemicals might discolor some paint, what some use is a portable propane torch, unlit of course.

 

OK this kind of turned into two problems, the trans not shifting and bad vacuum and I have solved both. The vacuum was an intake valve not adjusted correctly, I adjusted the valves and the vacuum got much better at idle and and rock solid off idle. I had hopped that would solve the trans not shifting as well but no such luck. So I changed the modulator as suggested and that didn't work either. So I figured the next logical place was the governor, removed and inspected the shaft movement and there was non, fixed that and what do ya know it shifts! Thank you all for your help once again, this is the best forum going!

 

Glad you got it fixed. What did you repair in the governor?

 

Excellent - we love it when we can help folks get back up and running again!

- Eric

 

The gov shaft does get gummed up after years of burned fluid and poor fluid change maintenance. I would recommend to everybody even if you had the trans rebuilt to pull the gov out and spray it with brake cleaner and make suer it's not sticking. It's a simple operation and will cure a bunch of ill's in the trans.

 

That is very good advice my trans was rebuilt, installed a new governor and worked great, I will be doing good maintenance, when would you recommend a fluid change on this newly rebuilt trans?

 

I would guess it depends on how you drive and miles per year. I do mine every 10 years. I don't put that many miles on my car.