Over last few months I've solved a variety of leak issues on my '74 that all turned out to be simple fixes but yesterday when I took the car out it acted like there was no 2nd gear under very light driving in neighborhood i.e. under 30 mph.

I checked fluid level and it was right on the money.

Took it out again and same thing but eventually it shifted into 3rd OK. 2nd seemed to be slipping. Then I took it out on road and gave it some gas and it shifted fine through all the gears but when I got back into neighborhood at low speeds it didn't want to go into 2nd again ... just slips with engine rev's but doesn't want to hook up. I did hear some squealing noise which I think came from tranny. Didn't sound good.

In the process of solving some of the leaks I had, I've dropped the pan several times. Changed the filter & fluid the 1st time and then just added new fluid after that. Never had a problem before but I'm wondering if the tranny fluid which was supposed to be compatible with Dextron II could be the issue.

Any thoughts on what to start checking?

 

How many miles on the transmission? It maybe overhaul time.

 

The odometer works and shows just under 94,000. I'm the 3rd owner and have had the car for 31 years.

 

I have had my 1970 since new with the turbo 400. No shifting problems, great transmission having only fluid and filter changes. Over 160,000 on it now.

 

Good to hear 70ZZ3 96LT4! I've done the fluid and filter changes too and other than the recent leaks that developed, it's been a great tranny.

 

Sounds like a bad sprag. Last time I rebuilt a th400 its was for a similar problem, shifted fine under med/heavy load but would freewheel/jerk into 2nd under light load.

 

On a th350 it would be a modulator or vacuum line. I don't think the th400 has a modulator but I may be wrong?

 

hugie82 ... mine has a modulator ... I'll check out both the vacuum line and the modulator. Appreciate the tip on something easy to check.

dugsgms74 ... thanks for input ... I hope it's not the sprag or anything internal but the symptoms you described does sound like what I experienced today. If checking vacuum line & modulator doesn't turn up anything and I don't get anything else that's easy to check I may need to start looking for a good repair shop.

 

There is a détente or kickdown switch on the bottom of the tranny, These wear out. Park the car, turn the ignition on (do not start the car) and depress the accelerator to the floor and listen for a relay operating in the tranny. You should be able to hear it operate. A new relay costs around $20 and is simple to change (must drop the pan). There is a wire connecting the tranny switch to a switch under the dash at the accelerator.
Good luck

 

I checked out vacuum line to modulator & modulator for any cracked lines or fluid in Mod. and that all seemed OK. I did slide the rubber vacuum hoses on a little further and re-angled the vacuum port on the Modulator so the steel line actually slipped inside the Modulator vacuum port. Just didn't find anything that looked significantly wrong/bad.

I don't hear the kickdown switch clicking when depressing accelerator to floor with ignition switch on. If it's not working would this have anything to do with normal driving?

This coming week I'll try to actually check the amount of vacuum at Modulator and check for a switch under dash to see what's up with that.

It rained a lot last week so I didn't take the Vette for a spin until today but I got some different symptoms today .... ARGH

Last time it was 2nd gear that was slipping under light throttle. Today reverse was fine but it started slipping right away when I put it into Drive (3rd), to start driving. When I manually shift it ... it seemed to shift fine but wanted to delay and get to a higher speed/rpm before it shifted. When I decelerate or down shift it manually, I can feel the engine braking in all gears.

Since I only fooled with the vacuum lines and orientation of the Modulator since the last test drive, I'm getting very suspicious of Modulator or vacuum.

Any more advice from what I found?

 

Once it starts slipping its all done , pull it get it over hauled .

 

Check the kick down switch on top of the gas pedal linkage and make sure it's not stuck.. Or better yet unplug the wire from the kick down solenoid on the side of the Trans, then try to drive it normal. Sounds like it's stuck in "passing gear". If you disconnect the wire to the kick down and it shifts normally, then you know you have either a bad gas pedal switch or the kick down solenoid is bad.. I'd lay odds that the gas pedal switch is worn out and stuck.

How does the Trans fluid look/smell?

 

That would stop up shifts , he would hold first gear a long time same with second gear. Also he would not slip in reverse or forward gears.

He can try every guess fix and maybe get lucky but my money is with a rebuild.

 

I've had his exact symptoms, it was the kickdown circuit activated. Sometimes it will shift to 2nd but will slip severely...sometimes it won't shift at all.. They will do weird things if he kickdown circuit is flowing..sometimes they'll shift into 2nd with very light throttle and even into 3rd, but anything more then 10% throttle and it'll start slipping almost like it's in neutral..--kick down circuit is engaged.

Easiest thing to check, just reach up on the driver side of the Trans and unplug the kickdown solenoid, go for a drive.. You'll know by the end of your street if that is the problem or not.. If this doesn't fix it, it's likely an internal issue... Just check all the easy stuff first.

Not saying his trans isn't smoked but....I'd check this first.

 

pull that wire. You'll have normal up shifts but no kick down unless you do it manually.
The modulator usually will give you a warning before it goes completely. A tell tail sign is getting it up to 30mph in first and let off the gas and it will grab 3rd gear. Sometimes if you play with the gas pedal or up shift manually it will pop into 2nd. Checking the diaphragm is good but a modulator is very cheap and it's 1 bolt to change it. Most guys don't even mess with it and just replace it.

 

Thanks everyone ... I really appreciate all your input. Experience can be worth a lot.

Today I checked the amount of vacuum at manifold and at both ends of the steel line that runs from a tee at manifold to the modulator ... it was 12" at all places. Article I had said it should be 20" but I've got an LT-1 engine from a 70 Vette so I'm thinking the 12" is about right. I also pulled 20" of vacuum on the modulator and it held rock solid and didn't loose any vacuum.

Then I pulled that wire on drivers side of transmission and took it for a drive ... just got back and no change from previous symptoms when I let it shift automatically. When I manually shift it ... it drives fine except for the delay in shifting ... maybe 3 or 4 seconds to shift from 1st to 2nd and from 2nd to 3rd. It really bugs me that it doesn't have any slipping issues when out on the road ... just at low speeds i.e. 30 & under and also when I start out with shifter in 3rd (auto).

The symptoms dugsgms74 posted in his reply is the closest to what I'm experiencing ... he said it was a bad sprag in his case. If I can figure out how to get a new modulator (the right one ... I'm not sure this one is stock as the tranny has a shift kit in it), I'll try that as a last resort before planning to get it rebuilt.

Thanks again for all the ideas!

 

Because manually from low 1 more is going on then in D .. Not gonna explain that because I am sure someone will tell ya to check a u-joint or some other crap .. Pull that unit and get it gone over .. GL

 

I hear you DIEHRD! I'm getting close to pulling it!

I ordered a new modulator and that will be the last thing I’ll try before pulling it, if that doesn’t fix it. Since my last post I found a ’74 Service Manual and it actually lists some of what I’ve been experiencing as being associated with a bad modulator. I figure I might as well try everything external that I can first.

1974 Chevrolet Service Manual info.:

A defective vacuum modulator can cause one or more of the following complaints.

· Harsh upshifts and downshifts (Don’t have this symptom.)
· Delayed upshifts (I have this symptom.)
· Soft upshifts and downshifts (Don’t have this symptom.)
· Slips in low, drive and reverse (Have had slip in low & 2nd)
· Transmission overheating (No signs of this)
· Engine burning transmission fluid (No signs of this)

While waiting for the new modulator, I hooked a volt meter up to the wire that connects to the right side of transmission for the internal kick-down solenoid. I turned the ignition switch to run position and meter read 0v. I figured that at least means tranny issn’t stuck in the gear kick-down mode all the time. I depressed accelerator pedal and still nothing on meter. I think that means either a bad or mis-adjusted kick-down switch under the dash on gas pedal, a blown fuse or a broken wire somewhere along the line.

I can’t find a kick-down relay listed in any auto parts stores and so far I can’t find anything in the wiring diagrams from a Chevy ‘74 Service Manual about the Kick-down circuit at all.

Does anyone know if the wire at transmission supplies 12 volts directly from kick-down switch or if the switch engages a relay somewhere that might also be the problem with no signal at tranny?

 

The switch supplies 12v to the solenoid in the trans somewhere between 1/2 throttle and WOT. Should have 12v with key on at one side of the switch and 12v on the other wire with the throttle opened around 1/2 or more. You can supply 12v to the wire heading down to the trans and listen for the click. If the solenoid is stuck on it will do all kinds on weird stuff like ajrothhm said, it it stuck closed or has no power going to it for whatever reason it just wont kick down a gear properly when you give it the beans while cruising.

 

Thanks dugsgms74 … I checked it out and kick-down solenoid in tranny clicks when I hit the connection terminal on driver side of transmission with 12 volts so looks like my problem with it not working is between the 12v source and the wire that connects to tranny … I’ll work on that.

ajrothm … BTW … fluid looks perfect with no smell or indication of being burnt.

Also got the new modulator and installed it today but no change in drivability.

Looks like diehrd & others that thought it needed an overhaul were right so I’m working on finding a reliable place to get it overhauled … wish me luck on that.

If possible I’ll pull it myself and let someone else rebuild it. Not in any rush at this time of year but I’ll update this thread after I get to check out the results of rebuild.

Thanks again for everyone’s input … I now know more about the TH400 than I knew when I started this thread.