Trans techs -- Hydramatic 3T40 question

I’m trying to do a full rebuild on a GM Hydramatic 3T40 (formerly 125C). I bought it from a junkyard as a “working pull”, but decided to rebuild just to freshen everything. When I got it all apart, I found that most of the clutch plates were in good shape. However, the two plate friction surfaces that run against the wave plates of the low-reverse clutch pack are completely worn down to the metal. All other surfaces in the same clutch pack are fine. It looks like the clutch was “dragging” slightly, so pressure from the wave plates wore off the friction material when the clutch was not applied. I’m installing all new clutch plates anyway, but I wonder if there might be a defect somewhere else that caused this problem, and may cause it again with the new plates. Does anyone have any insight into this? Thanks,

-Andrew L

So trans is 15yrs old? Many miles? And is worn? I expect a rebuilt trans to also wear. Do u plan on driving it for an additional 15 yrs?

Well, the low-reverse clutch only applies in reverse and manual low. So, 99% of the time, the clutch is released and should not be wearing. I would expect to see more wear on the forward and direct clutches, which spend a lot more time applied, but those ones were fine. This is why I thought something may be wrong with the low-reverse clutch assembly.

After posting, I found this:

It explains that there was a problem with the low-reverse clutch sticking on in older 3T40s. However, mine was built in 1993 and appears to have the new design that was supposed to correct this problem. So, maybe even the new design didn’t fully correct it.

-Andrew L

(deleted double-post)

I was hoping Transman618 would post in on this matter as he is the resident GM transmission guru.

I notice that the backing plate for the low/reverse stack is selective but there is no part call out for available thicknesses. Also there is a snap ring above the stack up that may set the clearance. But then with the low/reverse piston and one way clutch inner race in place you cannot check the clearance.

If this were my rebuild, I would check for minimum clearance between the low part of the wave plate and the first friction plate surface and look for 0.010 to 0.030 inch clearance. Then I would measure down from the snap ring for the low/reverse housing; measure the thickness of that snap ring; and the distance between the back of the low/reverse housing and the front of the piston. Subtracting the last two from the first should give a clearance number of the piston from the wave plate.

Your find above is for the redesign and modification for the forward clutch engagement. The thermal restrictor should also affect the engagement of the reverse clutch. But then you should see wear on all the frictions of that stack if there was slippage on the reverse engagement…

I suspect, as you do, that the installed clearance of the low/reverse stack up was not right. But all my sources of information do not give a specification nor a method to determine that clearance. Also, as I noted before there are no part call outs for alternate thicknesses of backing plates.

Hope this helps if Transman618 doesn’t see this.

Thanks for the detailed answer. I found that the factory shop manual does describe a test for correct thickness of the low-reverse clutch housing spacer (item 660 in GM’s diagrams). It looks to me like a thicker spacer would move the clutch piston away from the top wave plate. Maybe the current spacer is too thin, allowing the clutch piston to ride against the wave plate at all times. I’ll do this check carefully when reassembling and see what I find.

-Andrew L

The low/reverse clutch housing is located by snap rings #647 at the bottom end and #650 at the top end. This housing does not move because movement would disrupt the seal on the oil passage passage from the case to the housing hence to the pistion chamber.

The selective spacer #660 sets the running clearance for the reaction planet carrier to the output ring gear by moving the one way roller clutch inner race with respect to the low/reverse clutch housing. If you thin that out the reaction carrier and/or the internal ring gear might move into something critical or push the reaction sun gear into the clearances above.

The element you should be looking at is the low/reverse clutch stackup backing plate (selective) #659. The way to determine the clearance is to do the measurements mentioned previously as it seems impossible to gain access to the clutch stack up with the l/r housing in place. I thought of another way to check that clearance. Install the backing plate, wave plate, the clutch stack, the other wave plate, and the first snap ring. Place modeling clay around the rim of the l/r piston contact surface. Put the housing into position and install the snap ring. Remove the housing and measure the maximum compression of the clay, i.e. the thinnest points. That would be the clearance you are shooting for. Of course clean all evidence of the clay off the wave plate and piston before finally assembly.

That still leaves the question open about what to do with the selective backing plate #659 for which I see no callout or table for selective sizes.

I see what you mean about #660. I was thinking the low-rev clutch housing position could be adjusted, but I guess it can’t. I like your idea for measuring the clearance – similar to the plasti-gauge method of checking engine bearing clearances – but I have no spec on what this clearance should be. I have the factory service manual for this car ('95 Lumina Minivan) but it doesn’t seem to mention this…I’ll look again to make sure.

My thinking about the #659 backing plate is that it may be “selective” in the sense that different models of this trans use different numbers of clutch plates in the stack. So, other versions may use a thicker backing plate to make up for fewer plates. This one has 5 friction plates in each of the 3 clutches, which I believe is the max, used for the minivan version (IMHO, GM should not have put this wimpy trans in a 7-passenger vehicle! This will be at least the third one going into this van…)

Today I assembled and installed the final drive unit…I need to locate a new reaction ring gear (mine has a broken tooth) before I can proceed. Then I can test-fit the whole clutch and planetary set and see if I can see any obvious problems. I’ll check that clearance just to see what it turns out to be. Obviously if the clutch piston turns out to be flush up against the wave plate with the clutch not applied, that would seem to indicate that something is wrong somewhere.


-Andrew L

I was thinking that you could delete one or both of the wave plates and substitute one or two standard plates. That would make Reverse and Low downshift more severe i.e. a noticable jolt. That might give you the necessary clearance. The shift feel is probably moved to the high clutch engagement in Reverse anyway. That seems to be the clutch pack that wears out on the Simpson gear arrangement from Reverse engagement problems.

I would think a clearance of 0.010 to 0.040 inch for the low/reverse clutch would be appropriate. You might use the forward clutch clearances as a guide.

Thanks for sharing your find of the broken tooth on the ring gear. Are you talking about the final drive ring gear; the ring gear on the output shaft, or the ring gear driven by the forward clutch? I would have thought that the ring gear would be the last place for a tooth to break. I opine that the sun gear teeth would be under more stress.

Thanks again for following up on your progress.