'69 427/390 TH400 will not downshift to second above about 35-40 mph

[Don W.]

I have a 69 with an automatic transmission. The car will not downshift from 3rd to 2nd at about 35 mph or higher, but instead climb slowly in 3rd gear. We were looking at the detent switch at the accelerator pedal area and found 1) that the metal piece on accelerator mechanism above the pedal shows weld marks at the top and 2) a piece of fuel hose was placed around a rod to increase its diameter to make contact with the detent switch. The transmission does downshift in other ranges and I assume based on that, the detent switch works.

So, I'm not an auto transmission guy, but I'm looking for on advice on what to have checked. Also, with the weld areas on the accelerator rod and rubber hose to compensate; it looks as if bubba got in there and messed up the whole mechanism geometry and I wonder if I should look at replacing the entire assembly.

Thank you for your assistance,
Don

[Edward J.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

Don the 400 trans. was not cable driven like the later 350 THM trans and could be somewhat adjust to delay shift points. the 400 had factors that did not let it downshift after a certain pressure was reached. The kickdown is operated by a switch at the gas pedal.
There are ways to get the 400 to shift later but requires the install of a shift kit that will let you see later and firmer shifts. when I restored my 400 this what I did to my corvette. most transmission shop can install these kits.

[Duke W.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

The CSM says the TH400 should downshift from third to second at up to 70 MPH, so something is amiss. I pulled the vacuum modulator hose off the manifold nipple and pumped it down to about 15" Hg vacuum and it held, so I assume the modulator is okay. Automatic transmissions are one automotive area that I have little experieice with, so we are looking for guidance. The connector at the switch resisted being removed, and I didn't want to force it. I got it off far enough to get test leads against the pins, but that test was invalid, so we don't know if the switch is engaging, but it did downshift to second below about 35 MPH with full throttle application.

[Jim T.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

How about holding it in second gear say 30mph and go full throttle will it shift to first? My original owner 1970 with turbo 400 will shift with very light throttle to 25 mph from second to third, I am only going to 30 mph, the street speed limit.

[Duke W.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

I don't think the kickdown switch is engaging due to how it's jury rigged, but given my lack of detailed knowledge on how the TH400 control system works, I'm trying to figure it out. My current thinking is that the modulator controls part throttle downshifts, but when the driver applies WOT the electric kickdown switch should immediately force a downshift below certain speeds including 3-2 below about 70 as stated in the CSM. Any TH400 experts out there who can explain the control system in detail?

[Douglas L.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

You pretty much got a handle on how it works, pretty easy to diagnose. First and easiest step is to get the car in a quiet/closed garage. Turn key to run but engine not running and slowly open the throttle to WOT. You should at some point (usually somewhat after 50% throttle opening) be able to hear the slightest "click" sound from the transmission indicating that the detent solenoid is opening. If not you'll need to go into further diagnosis using a test lamp.

[Don W.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

Hi Douglas...thank you for the information. I just went into my garage, put the key into run and slowly depressed the pedal. I then put the car into gear and repeated the process a few times. I did not hear anything sounding like a click. I heard the carb squirting fuel and some noise in pedal linkage but nothing like a click. It may also be my 61 year old hearing...Don

[Douglas L.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

Hello Don, next step will require a test lamp or voltmeter. Might as well start at the switch as it'll be easier to access. You'll need to check for battery power into the switch at one terminal(with the key on) and power out of the other terminal with the throttle depressed.

[Don W.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

I checked the feed wires to the switch using my ancient radio shack multi meter. I pulled the wire clip off and checked both points on the clip with key on and used the pedal shaft and a few different metal parts on the firewall as ground. With the key on to run, I got no readings. I checked across the battery posts a read a little over 13 volts, so my meter works. There is an orange wire and a pink wire leading to the detent switch. Also, the transmission does down shift at slower speeds, but not above 35 or so.

[Douglas L.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

Hmm, I'm not sure any of those ground points will work so well, are the leads on you meter long enough to ground directly to the battery?

[Don W.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

okay, I connected to the battery ground on my black lead, disconnected the wire clip and checked both both spots and got -0.20 volts on the 30 volt DC scale with the key in run/on position...Don

[Duke W.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

It sounds like there is no voltage at the connector. The other test you should run is the switch, with the connector removed. It's probably normally open - infinite resistance, but as you open the throttle at some point it should close at or before WOT.

[Douglas L.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

Don, We have now established that there is no power to the switch. Id like to say it could be something a simple as a blown fuse but I doubt you'll be that lucky. I can't seem to find a wiring diagram that shows the wires for the detent switch but I'm fairly sure that the detent circuit shares a fuse with a few other circuits like back-up lights and turn signals so you'd be aware if that fuse was blown. I'll keep looking for a diagram and hopefully someone will come up with one. That being said, I think you may have open in the feed circuit(pink wire). Hopefully a visual inspection of the pink wire will reveal the problem but C3s are tough to trace wires through, especially under the dash so determining the root of the problem may be a bit of a PITA.

Checking the rest of the components is fairly simple. To test the solenoid, using a fused jumper wire, jump battery power to the orange wire and listen for the click as soon as you jump power to it(if you don't trust your hearing, supposedly you can turn the radio on, tune to AM 1400 and you should hear the click through the speakers but I've never tried it). If you do indeed hear the click then test the switch by jumping power to the pink wire with the switch connected and listen for the click when approaching WOT. A third but optional test to see whether or not the solenoid is actually flowing fluid (I've never seen one go bad)is to jump power to the orange wire while driving in 3rd gear at about 1/2 throttle and the trans should downshift. If the tests check out then all you'll need to do is figure out where the open in the pink wire is and you should be off to the races. Doug

[Edward J.]

Re: '69 427/390 TH400 will not downshift to second above about 35-40 mph

I checked the feed wires to the switch using my ancient radio shack multi meter. I pulled the wire clip off and checked both points on the clip with key on and used the pedal shaft and a few different metal parts on the firewall as ground. With the key on to run, I got no readings. I checked across the battery posts a read a little over 13 volts, so my meter works. There is an orange wire and a pink wire leading to the detent switch. Also, the transmission does down shift at slower speeds, but not above 35 or so.

Don, with the transmission down shifting, it surly sounds like there would be power at the kick switch under dash. the transmission uses a pressure switch mounted to the valve body, this switch is closed, and is the ground side of the solenoid, and when the transmission pressure increases in 3rd gear the circuit is by-passed on the pressure switch opens, there are other factors involved such as the modulator. the orange wire should be the power side at transmission, and the kick down switch should be powered by one of the fuses in block. pretty sure that this how the system works. I would use a simple test light at switch and retest that circuit making sure to use a good grounding point, maybe the steering column.