TH400 downshift question...
Thread Starter
Le Mans Master
Joined: Aug 2001
Posts: 6,213
Likes: 1
From: South Alabama
TH400 downshift question...
This tranny has stopped downshifting when I put my foot down. It does not have a mechanical linkage, only a vac hose going to it.
I've dug through all my manuals and AIM but can't find any useful info on what makes it downshift :rolleyes: I was about to have it serviced at a local raceshops transmition bay but would like to have an idea of what could be wrong with it before I get there. I don't think they'll :bs me as they've rebuilt a few other Vette trannys for me but all the same.
Thanks! :yesnod:
I've dug through all my manuals and AIM but can't find any useful info on what makes it downshift :rolleyes: I was about to have it serviced at a local raceshops transmition bay but would like to have an idea of what could be wrong with it before I get there. I don't think they'll :bs me as they've rebuilt a few other Vette trannys for me but all the same.
Thanks! :yesnod:
Instructor
Joined: Jul 2001
Posts: 168
Likes: 4
From: central Washington state
Re: TH400 downshift question... (Cajun75)
Cajun;
Your TH400 has (or should have) two downshift modes. At speeds of about 35 mph or slower, you should have part-throttle 3-2 downshift that's controlled by the vacuum modulator. Under WOT, the downshift is controlled by an electrically operated detent solenoid in the transmission. The switch that controls the solenoid is located in the cockpit just in front of the accelerator linkage. Your problem could be a bad or simply misadjusted switch, a bad detent solenoid, or a bad wire between the switch and solenoid. If you're interested in learning more about TH400s, Ron Sessions wrote a very good one, "How to Work With and Modify Your Turbo-Hydramatic 400 Transmission." In it, he talks specifically about your problem. I'm sure Barnes and Noble has the book in stock or can get it for you. About $25 as I recall.
Your TH400 has (or should have) two downshift modes. At speeds of about 35 mph or slower, you should have part-throttle 3-2 downshift that's controlled by the vacuum modulator. Under WOT, the downshift is controlled by an electrically operated detent solenoid in the transmission. The switch that controls the solenoid is located in the cockpit just in front of the accelerator linkage. Your problem could be a bad or simply misadjusted switch, a bad detent solenoid, or a bad wire between the switch and solenoid. If you're interested in learning more about TH400s, Ron Sessions wrote a very good one, "How to Work With and Modify Your Turbo-Hydramatic 400 Transmission." In it, he talks specifically about your problem. I'm sure Barnes and Noble has the book in stock or can get it for you. About $25 as I recall.
Thread Starter
Le Mans Master
Joined: Aug 2001
Posts: 6,213
Likes: 1
From: South Alabama
Re: TH400 downshift question... (noxqsz)
Thanks a million!! The only switch on the accellerator that I found the first time was the one for the cowl induction door. Going to climb back under there and look again.
Thanks Again!! :lol: :lol:
Thanks Again!! :lol: :lol:
Instructor
Joined: Jul 2001
Posts: 168
Likes: 4
From: central Washington state
Re: TH400 downshift question... (Cajun75)
Cajun;
The same switch does double duty. That is to say, the switch that controls the cowl induction door also controls the trans solenoid. That makes it easy to check the switch. If your cowl induction door is opening, then your switch is working. Take a close look at the wire that goes to the transmission. It's about a 14-16 gauge wire that comes out of the harness in the vicinity of the steering column and goes down the firewall and to the solenoid on the driver's side of the transmission. Often, the wire is cooked and damaged by years of abuse. It may also be simply disconnected from the trans solenoid. Check the continuity of the wire, if you can, even if it looks good. If the wire checks out, it is probably the detent solenoid.
The same switch does double duty. That is to say, the switch that controls the cowl induction door also controls the trans solenoid. That makes it easy to check the switch. If your cowl induction door is opening, then your switch is working. Take a close look at the wire that goes to the transmission. It's about a 14-16 gauge wire that comes out of the harness in the vicinity of the steering column and goes down the firewall and to the solenoid on the driver's side of the transmission. Often, the wire is cooked and damaged by years of abuse. It may also be simply disconnected from the trans solenoid. Check the continuity of the wire, if you can, even if it looks good. If the wire checks out, it is probably the detent solenoid.
Thread Starter
Le Mans Master
Joined: Aug 2001
Posts: 6,213
Likes: 1
From: South Alabama
Re: TH400 downshift question... (noxqsz)
Yikes!!:eek:
I better get back under there. That switch is out of adjustment. Several months ago I noticed that the cowl induction wasn't opening when the throttle was fully depressed. Through the AIM and backtracking the wiring in the hood I came back to this switch. I couldn't get it to adjust properly as per the AIM instructions to the factory assemblers but it would open the cowl when I depressed it by hand so I know it works. (The Bubba in me took over and I simply removed the electric cowl door and forgot about it) :nono:
Had no idea it also controlled the tranny downshift and didn't make the connection mentally that my tranny stopped downshifting when my cowl induction stopped working.
And after looking again I did find a 16-gauge orange wire coming from the tranny into the firewall, probably the one you're referring to. I bet that I will find that wire going to the switch too.
Thanks!! It'll probably turn out to be a switch adjustment!
[Modified by Cajun75, 8:21 PM 8/30/2001]
I better get back under there. That switch is out of adjustment. Several months ago I noticed that the cowl induction wasn't opening when the throttle was fully depressed. Through the AIM and backtracking the wiring in the hood I came back to this switch. I couldn't get it to adjust properly as per the AIM instructions to the factory assemblers but it would open the cowl when I depressed it by hand so I know it works. (The Bubba in me took over and I simply removed the electric cowl door and forgot about it) :nono:
Had no idea it also controlled the tranny downshift and didn't make the connection mentally that my tranny stopped downshifting when my cowl induction stopped working.
And after looking again I did find a 16-gauge orange wire coming from the tranny into the firewall, probably the one you're referring to. I bet that I will find that wire going to the switch too.
Thanks!! It'll probably turn out to be a switch adjustment!
[Modified by Cajun75, 8:21 PM 8/30/2001]
Instructor
Joined: Jul 2001
Posts: 168
Likes: 4
From: central Washington state
Re: TH400 downshift question... (Cajun75)
Cajun;
You're right. That little devil can be hard to adjust as the instructions dictate. Part of the reason for that is that the inside of the switch is contaminated with congealed grease from the time it was new and, in my opinion, it was never the greatest design in the first place. What I did with mine was take it out and take it apart. It's not supposed to be a serviceable part, but with a little care and patience it will come apart without damage. It's very simple and you'll see immediatley what the problem is when you look inside. I simply cleaned mine up and relubricated it and carefully reassembled it and it's worked great ever since. By the way, if you should happen to break it, used ones are available from any Corvette recycler that handles C-3s. I have a source in the Seattle area that will sell them for $10. You might even be able to get one for nothing from one of the guys on the forum who is parting out a C-3. Nice to have a spare.
You're right. That little devil can be hard to adjust as the instructions dictate. Part of the reason for that is that the inside of the switch is contaminated with congealed grease from the time it was new and, in my opinion, it was never the greatest design in the first place. What I did with mine was take it out and take it apart. It's not supposed to be a serviceable part, but with a little care and patience it will come apart without damage. It's very simple and you'll see immediatley what the problem is when you look inside. I simply cleaned mine up and relubricated it and carefully reassembled it and it's worked great ever since. By the way, if you should happen to break it, used ones are available from any Corvette recycler that handles C-3s. I have a source in the Seattle area that will sell them for $10. You might even be able to get one for nothing from one of the guys on the forum who is parting out a C-3. Nice to have a spare.
