TH-400 Engagement Delay?

[KCKKen]

Hi everyone!

In my ongoing saga of the 78 2WD 3/4-ton "Blackie" I recently replaced the fluid and filter, plus replaced the pan with an aftermarket pan with a drain plug. After replacing the fluid it seemed to take a lot more than normal (per CSM) to get the trans dipstick to read "full" -- I chalked this up to sitting a rather long time due to the rear spring/shock replacement project. Once I got it to what appeared to be "full" the truck moved quite well, and I was able to get it on the road.

The truck sits for extended times since it's not needed in a daily-driver situation. After it sits for these extended periods it feels like it's going back to being low on fluid, and nothing is showing up as leaking. Any advice on what else to look for? Reverse seems to take longer to engage than Drive. Am I sitting on a transmission that's about to give up on me? If so I want to get it to a shop ASAP, since I will be unable to drive for a few weeks after knee surgery.

Picture is from the barn-to-garage renovation project -- Blackie has been quite helpful with hauling supplies!

Really appreciate all the advice on this forum, it really helped me through the rear spring process!

Take Care,
Ken Snyder
In KCK

 

[NickTransmissions]

Hi everyone!

In my ongoing saga of the 78 2WD 3/4-ton "Blackie" I recently replaced the fluid and filter, plus replaced the pan with an aftermarket pan with a drain plug. After replacing the fluid it seemed to take a lot more than normal (per CSM) to get the trans dipstick to read "full" -- I chalked this up to sitting a rather long time due to the rear spring/shock replacement project. Once I got it to what appeared to be "full" the truck moved quite well, and I was able to get it on the road.

So your symptoms didn't materalize until after you installed the pan? What specific pan did you install?

 

[Matt69olds]

If you installed a deep pan, did you also install the spacer and filter extension? Did you install the filter correctly?

A delayed engagement after sitting for awhile isn’t really an issue. The torque converter will slowly drain fluid into the pan. Once you start the engine, it takes a few seconds to refill the converter.

 

[KCKKen]

Hi Nick and Matt!

Thanks for the responses, guess I need to clarify my initial post.

First of all, the transmission was doing this before I changed the pan, it's a standard-depth version with a drain plug that came in a bunch of odd parts I bought at a sale. It was an annoyance, but after a considerable amount of time (ten to fifteen minutes) it responds normally. In each of these situations the truck had sat for several weeks, the times before the pan/filter change were really cold so I also thought temperature might have contributed to it. However, after the pan change the weather was nice and warm, so that's what had me concerned. When I changed the pan/filter the fluid looked fine (nothing burned) and there wasn't any residue in the pan after I drained everything.

Would there be something that would cause the torque converter to be slow in refilling after the truck has sat for a considerable amount of time? I had a devil of a time getting the fluid up to the "full" mark on the dipstick after the change, took several quarts over what the CSM called out.

Again, thanks for the responses -- I want this truck to get me by until I finish the 85 GMC, so I'm being extra careful with it.

Take Care,
KS

 

[NickTransmissions]

Hi Nick and Matt!

Thanks for the responses, guess I need to clarify my initial post.

First of all, the transmission was doing this before I changed the pan, it's a standard-depth version with a drain plug that came in a bunch of odd parts I bought at a sale. It was an annoyance, but after a considerable amount of time (ten to fifteen minutes) it responds normally. In each of these situations the truck had sat for several weeks, the times before the pan/filter change were really cold so I also thought temperature might have contributed to it. However, after the pan change the weather was nice and warm, so that's what had me concerned. When I changed the pan/filter the fluid looked fine (nothing burned) and there wasn't any residue in the pan after I drained everything.

Would there be something that would cause the torque converter to be slow in refilling after the truck has sat for a considerable amount of time? I had a devil of a time getting the fluid up to the "full" mark on the dipstick after the change, took several quarts over what the CSM called out.

Again, thanks for the responses -- I want this truck to get me by until I finish the 85 GMC, so I'm being extra careful with it.

Take Care,
KS

You're welcome, Ken...The first thing I'd do is get it up to normal operating temperature and to the point where it's responding normally then check fluid level. As Matt said, converter drain back can occur if the vehicle sits for ages but if it's happening again shortly after you've driven it, or it's taking more than a few minutes to respond that's not normal.

I'd also recommend sticking a pressure gauge on it (grab something that goes from 0-300PSI off Amazon, eBay or your local auto parts store) and see what your pressures are at idle both cold and at operating temp...If they're real low and build slowly, you'd then have to do more diag and troubleshooting but low pressure can be caused by a few diff things (PR valve, modulator system, x-leaks between circuits, loose valve body bolts, etc).

Questions:

When you changed the filter, did you put two new O-rings on the case side of the filter pick up tube?

Have you checked vacuum at the modulator?

How old is the modulator - has it been replaced at any point in time (it's not original, is it lol?)

How many miles on the trans/converter?

Anything coming out of the tail pipe such as white smoke?

Going forward, assuming you resolve the current crop of trans symptoms, I'd drive it around more frequently - it's generally not good to let vehicles sit for months without firing up and driving for a bit...

 

[Rusty Nail]

Yea I vote for vacuum leak in the modulator hose.

 

[KCKKen]

Thanks, Nick!

Here's the answers I know about:
When you changed the filter, did you put two new O-rings on the case side of the filter pick up tube?
-- Yes, I was concerned about how "loose" the filter was even after tightening down the bolt, but I read that this is common on the TH-400s.

Have you checked vacuum at the modulator?
-- Not yet, as soon as I can crawl around under it I will put a gauge on and see what I get.
How old is the modulator - has it been replaced at any point in time (it's not original, is it lol?)
How many miles on the trans/converter?
-- Unknown age/miles on the transmission, previous owner "changed" it so I'm not even sure of the actual year of the transmission or whether it was "rebuilt" when it was changed.
Anything coming out of the tail pipe such as white smoke?
-- Not white smoke, valve cover seals are worn so I get a little blue smoke at start-up that goes away quickly. Nothing after warm-up.
Going forward, assuming you resolve the current crop of trans symptoms, I'd drive it around more frequently - it's generally not good to let vehicles sit for months without firing up and driving for a bit...
-- I hear ya, my knee issues have kept me restricted as far as activity goes and the truck has had to sit way too much. I keep it at the house I'm renovating (starting with the barn/garage in the picture, have to have the priorities straight) but my plan is today the truck comes back to the house I'm living at so I can get it driven over to the muffler shop for a totally new exhaust while I'm off my feet. I hope to have more info on it once I get it back here.

Rusty: I'm putting the modulator at the top of my list to check. Once I see what I have there I'll post an update.

Thanks again for the advice!

Take Care,
KS

 

[NickTransmissions]

Thanks, Nick!

Here's the answers I know about:
When you changed the filter, did you put two new O-rings on the case side of the filter pick up tube?
-- Yes, I was concerned about how "loose" the filter was even after tightening down the bolt, but I read that this is common on the TH-400s.

Have you checked vacuum at the modulator?
-- Not yet, as soon as I can crawl around under it I will put a gauge on and see what I get.
How old is the modulator - has it been replaced at any point in time (it's not original, is it lol?)
How many miles on the trans/converter?
-- Unknown age/miles on the transmission, previous owner "changed" it so I'm not even sure of the actual year of the transmission or whether it was "rebuilt" when it was changed.
Anything coming out of the tail pipe such as white smoke?
-- Not white smoke, valve cover seals are worn so I get a little blue smoke at start-up that goes away quickly. Nothing after warm-up.
Going forward, assuming you resolve the current crop of trans symptoms, I'd drive it around more frequently - it's generally not good to let vehicles sit for months without firing up and driving for a bit...
-- I hear ya, my knee issues have kept me restricted as far as activity goes and the truck has had to sit way too much. I keep it at the house I'm renovating (starting with the barn/garage in the picture, have to have the priorities straight) but my plan is today the truck comes back to the house I'm living at so I can get it driven over to the muffler shop for a totally new exhaust while I'm off my feet. I hope to have more info on it once I get it back here.

Rusty: I'm putting the modulator at the top of my list to check. Once I see what I have there I'll post an update.

Thanks again for the advice!

Take Care,
KS

If the modulator's age/mileage/condition is not known, I'd just replace it as they are super cheap...Pull the existing modulator and use a pencil magnet to pull out the modulator valve from the case then insert it back in to see if you feel any drag...If the valve slides in and out smoothly, no further action is required. If there's drag felt on the valve use a bench buddy and WD40 to polish the bore a little bit then re-check. ...Reinstall the new modulator and check...Note that you may need to adjust the modulator after installation if part throttle shift points are too early or too late...Screwing in the little adjustment screw (use very small flat head screwdriver) will result in slightly later, firmer p/t shifts while screwing it counterclockwise will result in slightly earlier, softer p/t shifts...After you exceed a certain throttle angle up to WOT, the governor takes over as the primary locus of control over shift timing...

Note that these things may or may not fix/resolve your symptom but are simple, inexpensive diagnostic and/or preemptive steps to take to help keep the trans running right and/or rule other problems in or out.

Don't forget to do a pressure test as well.

 

[Rusty Nail]

They  are really cheap. Prolly ougta take a 3/8 and 7/16 open end under there with ya. Only takes a couple of minutes to swap out. Maybe a pocket sized flat head too.

 

[gsuburban]

Hi everyone!

In my ongoing saga of the 78 2WD 3/4-ton "Blackie" I recently replaced the fluid and filter, plus replaced the pan with an aftermarket pan with a drain plug. After replacing the fluid it seemed to take a lot more than normal (per CSM) to get the trans dipstick to read "full" -- I chalked this up to sitting a rather long time due to the rear spring/shock replacement project. Once I got it to what appeared to be "full" the truck moved quite well, and I was able to get it on the road.

The truck sits for extended times since it's not needed in a daily-driver situation. After it sits for these extended periods it feels like it's going back to being low on fluid, and nothing is showing up as leaking. Any advice on what else to look for? Reverse seems to take longer to engage than Drive. Am I sitting on a transmission that's about to give up on me? If so I want to get it to a shop ASAP, since I will be unable to drive for a few weeks after knee surgery.

Picture is from the barn-to-garage renovation project -- Blackie has been quite helpful with hauling supplies!

Really appreciate all the advice on this forum, it really helped me through the rear spring process!

Take Care,
Ken Snyder
In KCK

If fluid is not keeping its level, its got to be going someplace right? Maybe when the pan was replaced the converter was already drained and low on fluid itself when the trans was refilled however, after driving it a ways, the dip stick level should indicate that and more fluid would be needed. I'd have it inspected by a good independent trans rebuilder shop as this isn't normal and the leak down on the converter shouldn't be happening even though many suggest that is "normal". I'd guess the O rings and piston seals are shrinking so if you plan to keep this truck and use it, its best to have it looked into by a decent shop and if there are questionable issues found, simply rebuild it with all of the 700R4 upgrades that will remove all of the weak operations they have, such as third gear upshift and weak "shells" that tend to crack and weak servos. YouTube has a top notch trans shop called Precision Transmission in Amarillo, TX. Richard & Theresa run the shop and after a few videos of the 700R4 rebuilds, you will see what I'm referring to. He now works from his home shop. First class shop plus he has tons of core parts in stock. Well worth your time to see his abilities. There's another shop in TX run by Hiram called Automatic Transmission. He is knowledgeable as well.

https://www.youtube.com/@PrecisionTransmission/videos

https://www.youtube.com/@richardteresacricheducatio3542

 

[Matt69olds]

This is a 400 transmission, not a 700.

Converters will drain into the pan, that’s why you usually find big puddles of transmission fluid under cars/trucks that have sat for awhile. The fluid drains into the pan, the fluid level will eventually rise enough to leaks places it normally never would.

It shouldn’t take more than 30-45 seconds, a minute at the most, to refill the converter. If it’s taking longer than that before the truck moves, it’s time for some additional diagnostic work. As nick suggested, a line pressure gauge would be ideal. If line pressure is low until it begins to move, then that would suggest the seals have gotten hard and leak internally.

If the seals have gotten hard, you can try some transmission fix in a bottle (sometimes advertised as stop leak) but in my opinion it’s like putting a band aid on a gunshot wound. It might be ok to try it if your just trying to keep it going until you can get it rebuilt, but it’s not a permanent fix.

 

[Bextreme04]

Also make sure you aren't using a paper element filled filter. My TH400 will start to whine when it's cold if you use a box store paper filled filter in it. Use an AC Delco TF231, it's the high flow version that has a brass screen inside instead.

 

[KCKKen]

Thanks everyone for the advice, it's all welcome!

I'm at a spot where it may have to sit for a little longer, unfortunately -- I am facing knee replacement surgery on 8 November and I'm having to prioritize what I can get done in the time I have left. Never mind that the weather turned to dog doo and I wasn't really big on crawling around under the truck outside.

The other thing that just hit was either the alternator, battery, or both decided to take a dump on me. Not too upset, it happens -- just at the worst possible time. The last time I went to start it up (Sunday) it took well over 15 minutes to start to move with ample throttle applied (<2500 RPM). But as it usually does it appeared to be fine once it started moving.

Depending on if I can get the time to get the battery/alternator checked/replaced and move it to the house I'm living in I may just punt and ask a local transmission shop to look it over while I'm out of commission -- I was going to have my local muffler shop do their magic on the pipes while I can't drive anyway. Once I know what the cause is I will make sure I update everyone -- that way, if someone else has the same issue they can learn from what I did (or didn't do).

Thanks again to everyone!

Take Care,
KS

 

[idahovette]

Good luck on the knee surgery, Ken. Be sure and follow the physical therapy regimen, really helps!

 

[KCKKen]

Hi Everyone!

Been way too long since I updated this, so I thought I'd catch everyone up.

The problems were all my own fault: I already had a deep pan on the truck, and I was taking the advice of the factory service manual for a 1981 (this is a 1978 if that matters) with a shallow pan. When I took my time in checking the fluid level I found out that the "fluid" I saw on the dipstick was being scraped off of the tube as it traveled down to the engine, and not a true indicator of the level. When I finally saw an accurate fluid level on the dipstick it turned out to engage just fine.

The battery/alternator was fixed by replacing both -- I had suspected the alternator for a while due to flickering lights in the truck, not on/off but bright and slightly dimmer all the time. New parts fixed that, and everything seems to be doing just fine.

Got the truck back out on the highway, and it runs just fine. The one thing I really like is the effortless power steering, I hope I can duplicate this on my 85 GMC as I rebuild it.

FOR NOW I'll consider it "fixed" -- thanks to all who offered advice, it's greatly appreciated.

Take Care,
KS