TH400 Won't Shift

[CARC686]

Engine runs fine. First and reverse normal. Intermittently shifted high. Attempted to adjust shift points at IP. No effect. Took it for some hard wheeling for the first time yesterday morning. It stopped shifting into third last night. Wouldn't shift at all this morning. I did not dismantle the governor because it would require replacement pins which I do not have, but the plastic gear looks new. Vacuum pump is operational. Sufficient to suck hose to fingertip. Pressure unknown. What should I be looking at?

 

[Tow4]

Plug the vacuum hose from the transmission directly into the vacuum pump and see if it shifts. Start in first and manually shift it up. With full vacuum, it should/will go straight to 3rd gear if you put it in drive.

 

[ehuppert]

Can't speak as to no shifting, but the pins for the governor weights can be fabricated from finish (trim) nails. Cut to length and crimp end once installed with vise grips to flatten (opposite side of nail head) Believe they were #4's, but that process was from my bracket racing days many moons ago!

 

[chevymike]

Might be your vacuum modulator at the trans. If that is not working, it will only allow it to shift gears at almost redline, which sounds like the engine is going to explode. I would look at replacing that first. Common issue.

 

[CARC686]

Can't speak as to no shifting, but the pins for the governor weights can be fabricated from finish (trim) nails. Cut to length and crimp end once installed with vise grips to flatten (opposite side of nail head) Believe they were #4's, but that process was from my bracket racing days many moons ago!

I do recall once upon a blue moon flattening nails to put drum brakes back together. I'm pretty limited as to what I can accomplish at present, but things will improve with time.

 

[CARC686]

Might be your vacuum modulator at the trans. If that is not working, it will only allow it to shift gears at almost redline, which sounds like the engine is going to explode. I would look at replacing that first. Common issue.

R&R'd modulator. No effect. I thought @chevymike wanted me to install a rubber hose from the modulator all the way to the vacuum pump, maybe because hard line cracking is a known issue or something. Y'all are the experts here, not me. It only just dawned on me that I can do a simple bypass at the IP. I figure it won't change anything either because I have inadequate vacuum or debris in the valve body.

Did I mention something started grinding when I let off the throttle? Sounds like a ringing bell or a dragging chain.

 

[cucvrus]

First off, do you have vacuum at the vacuum pump? Pull the hose off and see if it chirps or sucks your fingertip fast while the engine is running. Use a long piece of hose if you are afraid of the running engine. Then if you do have vacuum run a smaller hose with an adapter to the bottom of VRV on the left side of the injection pump. Run another hose to the tube that runs to the transmission modulator. I always paid $25. to the transmission shop to set up my vacuum so I wasn't guessing and doing the slip and slide shift with an indecisive transmission that shifts right most times but slips and slide the other times. I bought a vacuum gauge and still spent the money at the transmission shop, so I knew it was right. I could drive it like a new truck when it was shifting correctly. Good Luck. Them old time transmission guys are getting harder to find. I lost my friend to COVID. Take Care.

 

[CARC686]

First off, do you have vacuum at the vacuum pump? Pull the hose off and see if it chirps or sucks your fingertip fast while the engine is running. Use a long piece of hose if you are afraid of the running engine. Then if you do have vacuum run a smaller hose with an adapter to the bottom of VRV on the left side of the injection pump. Run another hose to the tube that runs to the transmission modulator. I always paid $25. to the transmission shop to set up my vacuum so I wasn't guessing and doing the slip and slide shift with an indecisive transmission that shifts right most times but slips and slide the other times. I bought a vacuum gauge and still spent the money at the transmission shop, so I knew it was right. I could drive it like a new truck when it was shifting correctly. Good Luck. Them old time transmission guys are getting harder to find. I lost my friend to COVID. Take Care.

All new vac hose since I R&R'd the IP recently. The pump sticks the hose to my fingertip. Per the TM, I unplugged the sensor and it started hunting 2nd, but won't shift into 3rd. Since reverse was lazy and IIRC, it shares a band or clutch or something with 3rd, I'm guessing doing 4-wheel burnouts over boulders finished it off. Rather than flushing a bunch of money down this 3-speed, I'm thinking about a 4L80 or NV4500. Whichever'll be cheaper, I guess. Either way, I can't afford it this year, so it'll just have to be a lawn ornament until then. Sorry to hear about your friend.

 

[adf5565]

I just sold a working TH400 for $300. Typically there are a lot of them on marketplace for cheap, and it is not a hard swap. Probably your best bet unless you want the truck to sit a while, 4l80 and nv4500 are doable but definitely a lot more time and money.

 

[CARC686]

I just sold a working TH400 for $300. Typically there are a lot of them on marketplace for cheap, and it is not a hard swap. Probably your best bet unless you want the truck to sit a while, 4l80 and nv4500 are doable but definitely a lot more time and money.

I don't doubt it, but I'd like to be comfortable driving faster than 50 MPH.

 

[CARC686]

@Tow4 - I finally got around to messing with it again today and when I pulled the vac hose, it had a bunch of crap in the end of it. Rat's nest stuffing from the look of it. I guess not particularly surprising given I cleaned a big nest out from under the manifold when I R&R'd the IP, but I do wonder how it got inside there. With the IP throttle control looped out of the equation and the pedal sensor unplugged, I can manually shift, but it feels like stirring a bowl of mush. Leaving it in D, it's doing something that feels like shifting, but I don't know if it's just floating in and out of lockup or what. Transmissions are inscrutable Rube Goldberg machines to me.

 

[Tow4]

@Tow4 - I finally got around to messing with it again today and when I pulled the vac hose, it had a bunch of crap in the end of it. Rat's nest stuffing from the look of it. I guess not particularly surprising given I cleaned a big nest out from under the manifold when I R&R'd the IP, but I do wonder how it got inside there. With the IP throttle control looped out of the equation and the pedal sensor unplugged, I can manually shift, but it feels like stirring a bowl of mush. Leaving it in D, it's doing something that feels like shifting, but I don't know if it's just floating in and out of lockup or what. Transmissions are inscrutable Rube Goldberg machines to me.

With full vacuum on the transmission, it shifts almost immediately to 3rd gear. That causes the torque converter to slip until the truck is moving fast enough for it lock up. My M1028A2 has high ratio differentials so it shifts to 3rd by 15 mph unless I'm really on the throttle hard. By-passing the vacuum modulator made almost no difference. Now I just manually shift if I need the extra rpms for acceleration or power to pull a load.

 

[chevymike]

FYI: TH400 transmissions do NOT have a lockup convertor. They always slip some. I think there was a very rare model of TH400 that had a lockup convertor but they were never put into CUCV's or I think trucks in general. Unlike a TH350 which uses a kickdown cable, the TH400 relies on vacuum modulate shifting, until high RPM when the governor takes over.

 

[rdc0000]

Check all the vacuum lines with a mytivak . Replace line that has become hard. Also, get a new modulator for a diesel turbo 400. I bought a Rostra 51-0020-00-00 Black/Red Stripe Vacuum Modulator. Fixed my problem on late hard shifts. Seems like my truck all the rubber died. Trying to bring it back. That cheap truck is costing me mucho dinero.

 

[CARC686]

Replaced all the topside rubber when I did the IP. Rockauto didn't offer B&M, so I searched up the part number on eBay, but they sent me a Rostra single red stripe. By this point in time, we've got layers upon layers of superseded part numbers. Ordering parts feels like gambling. One thing I've noticed is that every time I spend extra hoping to get something decent online, I receive the cheapest cut rate junk on the market, and LMC is just as guilty as the hucksters on eBay. I guess it wouldn't kill me to actually go hunting for vac leaks. I just haven't bothered with the truck for a minute 'cause my dog went and got himself run over and I've been looking after him.

 

[cucvmule]

Check your kickdown switch voltage at the transmission switch connector. If you have 12v there it is locked in passing gear.

You can check this with the engine off and the key on.

 

[cucvmule]

Also check to see if your shift lever grommets are in place, one ay steering column and one at transmission connection. It may not be going into drive on the transmission selector at transmission. Rubber grommets will eventually break and disappear.

 

[CARC686]

When I unplugged the switch and bypassed the throttle control on the IP, it started shifting properly. Anybody who knows what that means is probably cringing right now, but it matched my budget, so that's how I drove it for a month. It started shifting at redline again today, so my intuition is that the vacuum pump diaphragm is cracked. Once that's replaced, if it keeps this up, I'll try a different IP throttle control valve. If the transmission switch hot shorted, leaving it unplugged removes that from the equation for now, assuming it can't short internally? I've no idea. Haven't got a multimeter available to me here. I'll make a test light out of a light bulb when I get around to it. It's clearly multiple issues, which muddies the troubleshooting, but one thing at a time, I'll get it sorted.

Oh, and never in my life have I known a Chevy three speed to have bushings on the linkages, but that's probably because I'm accustomed to owning beaters and junk. I suppose it makes sense that it wasn't designed to be metal-on-metal. Does anybody sell them in poly?

 

[CARC686]

Very Available Vacuum Pump Fix:
Cardone 641023 (or 64-1023).

I used a 5% discount code at Rockauto and forfeited my core charge for a total cost just under $100 shipped (October 2023). The pod is a direct swap. When you remove the pod, it will dump about a fluid ounce of oil, so be ready for that. Vacuum hard line out angle is different, but it makes no difference.

My modulator was leaking ATF up the line, the hard line had a pin hole in it, and the vacuum pump had failed. These issues are remedied. Need to look into the switch, because the truck doesn't know what to do with the pedal to the metal, but I am getting consistent shifts at 25 and 40 pre-adjustment. To lower the shift points, do I rotate the top of the vacuum valve on the injection pump toward the front or the rear of the vehicle? I installed it at dead center.