69VetteAngel

O.K. guys,

I am exhausting possibilities on a problem that developed with the 400 tranny recently. Our vette has a new 350 motor that runs very well. It is nothing crazy, but a nice strong street motor. 9.8:1, edelbrock alum heads, shorty headers, 750 vac sec holley on an edelbrock performer and into a homemade 2 1/2" full flow xpipe exhaust through Dynomax ultra flows. That is the basic rundown. I put a brand new hughes 2500 converter in based on your suggestions to go with the comp Magnum 280 cam.

The car ran great for quite a few weeks, and always shifted fine. Practically overnight, the transmission started shifting very delayed from 1-2 (about 3,000rpm), and pretty hard. I tried shifting manually from 1 -2, but same thing. I have to bring the rpms up to about 3,000 for it to go into 2nd. same thing with 2nd to 3rd.

I discovered the tranny was down a quart (However, the fluid is still red and not burnt at all). I added fluid and it still does the same thing. I do have the modulator valve hooked up to manifold vac, as well as the throttle detent switch. Nothing changed from before the condition to after. Just , possibly the low fluid level. My wife took the car, and when I got home from work, she told me it was not driving right.

If I am in for a rebuild (don't know what else to check at this point - I will pull the pan and check for clutch or band material), What rebuild kit do you recommend for the street? Do you think I need the HP one with the alto clutches, etc? I want it to hold up to the additional power it has, but it is not a race car.

Thanks for your recommendations.
Greg.

'75

The first thing I would do is pull the vacuum line off the modulator and see is there is trans fluid in the line to see if the diaphram in there is bad. A new modulator may cure the problem.

7T1vette

The modulator diaphragm has failed and the 'missing' tranny fluid has been sucked up the vacuum line into the intake manifold (and burnt with the fuel charge). Remove the vacuum line from the modulator. If the fitting has tranny fluid on/in it, the modulator is bad. You will have trouble finding an 'original' design modulator, but all the auto supply stores sell a replacement that works reasonably well.

69VetteAngel

Thanks, I did that , and it is dry. But, I have not actually tested the modulator to see if the diaphram leaks. I will check it out tomorrow.

A bad modulator will actually cause the symptoms I have?

Thanks again,
Greg.

69VetteAngel

I just responded to the above post as your post came through.

Thanks very much...I will do that.

69VetteAngel

I took the vette out today and after checking the original modulator(previously replaced by old owner), purchased a new replacement. I took the car out and with light throttle, the tranny shifts from 1 - 2 at 22-25 MPH and 2-3 at no earlier than 40 MPH.

Obviously, something is wrong. The fluid level is correct and still red, but twice when I checked it after driving, it was aerated (lots of small bubbles). Anyone know what could cause this and the shifting?

I am thinking more that a rebuild might be in order. Just trying to hope for the best here.

7T1vette

It could be many things. But the simplest of those is one you can check fairly easily and is a possibility because of the 'clue' you offered on "aerated oil". If the O-ring seal on the pickup/filter in the tranny sump has gone bad, air will be sucked in along with the oil. That will create an aerated oil that will not build as much pressure as 100% oil would do. That 'weak' oil may be what is causing your late shifts.

Drop the tranny pan and check the pickup seal. If you think that could be the problem, put on a new seal and try it again. You should also look for any metal filings in the oil and/or pan while you are in there. You will see some black 'muck' at the bottom of the pan. If you drag a finger through it, it will feel like putty. This is a mixture of very fine clutch plate facing material...which naturally wears off the face of the plate under use...and the tranny oil. It should not have any particles in it of any size you can feel. You should not find any metallic chunks, particles or flakes in the oil. Significant debris should cause you not to reassemble the tranny but remove it or take it to your favorite mechanic.

69VetteAngel

Thank you very much for your help...I will definitely check that next. I am a pretty competent mechanic , but I have no experience at all with automatic trannys. I have had manuals all my life.

Thanks again and I will report back with the results when I have the time on the weekend to inspect it.

Regards to you all,
Greg.

JimT

Replace the detente relay in the tranny (less than $20). A weak relay can cause erratic/late shifting into second gear.

JimT

Any luck with fixing your TH400?

69VetteAngel

Yes, I just got some time today to check into it. I am grateful for all the suggestions. A few days ago, while testing the car, the left headlight started popping up and going down by itself while driving. This led me to believe I have a vacuum problem and maybe it is related to the tranny problem ( modulator).

I started checking the car with a vacuum gauge and I only had 5" vacuum, when previously I had 13" at idle. After narrowing it down and backtracking, I found the main vacuum line coming out of the carb was kinked. This reduced the vacuum and was giving the modulator false low readings.

Once I shortened the line and had normal vacuum, guess what?
Normal shifting and headlights function as normal.

The whole ordeal really just cost me 18 bucks for a new modulator and a couple of hours and probably a few hundred hairs that I pulled out.

Thanks again for all the help...I knew the th400 was a strong tranny and couldn't have wimped out on me.

By the way, JimT, Your vette in your heading looks just like ours...red, t tops, and same alum wheels. Nice looking vette you have there.

Greg.

JimT

Thanks,
I do love it!!! Glad you fixed the tranny problem. My th400 never would go into kickdown mode so I replaced the detente/kickdown relay. Then the tranny would not shift into second gear until a very high rpm.
I tested the "new" relay but found it to be weak (slow to operate)- you should be able to hear the relay operate while sitting in the car. I purchased a new one and solved the problem. The first relay I obtained came from a tranny shop and I think they gave me a "returned unit" and they had never tested it.

Jim