TH400 Shifting Problems

[86M10086.2L]

The transmission in my truck (1985 M1009 with stock gearing/axles/tires) started acting up the last week or so. First it started with my partial throttle shifts being very high. Even just lightly tapping the throttle the truck would not upshift from 1-2 and 2-3 until about 2500rpms. But if you manually shifted it it would upshift around 1800 rpms. Foot to the floor was around 3500 rpms. Today while driving to work it seemed to be laboring in first gear coming from a dead stop. Moving ok, but it felt almost like it was starting in second gear. But it would still make it's two gear changes. So I shifted from D to 1 at the next light and it accelerated normally. At the following light I tried it from 2. Same thing accelerated normally, although it held out the upshift into 2nd to almost 2600 rpms. However, much to my surprise before I could shift it into drive myself the trans shifted into third while still in the 2 position. It jumped into third, came out and then shifted back into 3 again. Did the same thing at the next light, same result. To further test this I put it in drive, started from a dead stop, let it get into 2 at about 30mph dropped ithe shifter into the 2 position and it shifted right into third like it was made to do it. Shouldn't it not be able to do that? I was always under the impression that putting it in 1 or 2 held it or stopped it in that respective gear. To make matters worse. After my run on the highway (in the D position). At the first light it didn't want to shift out of 1st. After some acceleration and deceleration as well as manually shifting the column linkage. It finally upshifted. After that it seemed ok. Only hanging in 1st once more. I'm at work now. Also I just recently moved so my TMs are buried in a box somewhere. I can get them just have to find them. The transmission was professionally rebuilt at a respected local shop a little over a year ago. The owner has since retired and closed his business luckily at least after my 12/12 warranty expired. Any ideas what might be causing these issues?

 

[Chaski]

Drop the pan and go treasure hunting? In all honesty the only transmission I've ever worked on was a 700r4, but if you do drop the pan at least you will be able to take a look.

 

[86M10086.2L]

So I had some time this morning to start testing on the truck. I started at the vacuum modulator. Pulled the line off, hooked it up to my Mighty Vac with the truck at a warm idle and got a reading of about 1-2 in lbs of vac, No beuno. So I then went directly to the source. Hooked up to the vacuum pump to the might vac. ran the truck at idle again got the same 1-2 inch lbs of vac. If memory serves me correctly I should be seeing at least 17-20 in lbs of vac. Goosed the engine, and even up to 2000 rpms it barely read over 3-5 lbs directly at the pump. This may not be the solution to all my problems but it's certainly a significant start. Ordered a remanufactured pump. It'll be here tomorrow morning. After I install it, I'll update my progress.

 

[Skinny]

Ruling out any internal issues, you are going to boil it down to a problem with the vacuum signal (pump, throttle valve, lines, modulator) or your governor valve sticking/malfunctioning. You should get a nice steady source from that vacuum pump so I think you found the problem. Just to be sure, I would check or replace your rubber lines and maybe even pull a vacuum on it to make sure you don't have any pinholes in the metal line.

 

[86M10086.2L]

Well, that's the next step. Once I have a reliable vacuum source then I'll move down the line. I did some quick tests and I didn't notice any alarming or noticeable leaks. But I won't rule that out.

 

[86M10086.2L]

Finally found my Binders of TMs! According to TM 9-2320-289-20 I should see about 20 Hg coming out of the vacuum pump and then with proper adjustment a reading of 8 Hg out of the vacuum valve on the injector pump. Considering I couldn't even get that out of the vacuum pump I'm feeling a little better about my transmission. I'll have to fab up that little testing block to make the adjustment on the valve if need be.

 

[Skinny]

You really don't have to. Dial it into to about 18" of vacuum at closed throttle then road test it. You can tweak it from there. It isn't rocket science and the gas engines don't even have that adjustment because it is manifold vacuum.

 

[86M10086.2L]

So I installed the new vacuum pump this morning. Tested the other vacuum lines with no leaks to report. Warmed the truck up. Adjusted the vaccuum valve as best I could to the TM specs and took it for a test drive. I'm pleased to report that it is a night and day difference. The truck shifts earlier and better than it ever has. No more speeding up just to get into third to cruise on a slow street or in traffic. Which is a very good thing. That was annoying me to no end. I still have a little more testing and tweaking to get it dialed in just right for me. But even if I left it "as is" I would be totally satisfied. I must admit, I'm a bit ashamed at myself for not addressing this issue earlier. But now that I have I feel a lot better about the truck.

 

[Tinman84]

Thanks for this. Gives me a couple ideas to try with mine. I'm a CUCV greenhorn (had mine about 6mos) and it's seemed to dislike upshifting ever since I got it.

 

[86M10086.2L]

The TM's and google are your friend here. I searched TH400 shifting problems and the like. I found numerous topics discussing it. Reading in various forums, including this one. Various tips/methods in tracking down the problem. Just weed out the ones that don't seem credible. When you see the same or similar info more than once. No guarantee but chances are its at least plausible info.

 

[MarcusOReallyus]

Thanks for this. Gives me a couple ideas to try with mine. I'm a CUCV greenhorn (had mine about 6mos) and it's seemed to dislike upshifting ever since I got it.

Vacuum problems are the most common reason. Bad lines are a few bucks to fix, and the pump & modulator aren't too pricey , either. You can rent a vacuum gauge from Advance Auto for free.

 

[Tinman84]

That's a good thought. We don't have an Advance here in Pendleton, but we do have an O'Reiley's Auto Parts and I know they do loan (with a deposit) some specialty tools.

 

[cucvrus]

And then when you get tired of try this try that method of repair. Take it to a reputable / reliable transmission shop and use the best tool you have. Good old US dollars. Get it fixed right and know that it is fixed right. I hear the US dollars are a very popular trade item world wide these days. I send people there all the time. None have left unhappy. Good old US dollars can fix many issues. Sometimes when you think you fixed it with others advise it comes back and costs you three fold. I know as a fact. Been there done that. I had a a $20. (Crossed vacuum lines) at the transmission shop cost me $220. on the self help plan. But I do have all good spare parts that I didn't need. That a positive thing I guess. And aren't you on another thread getting the same advise?

 

[Tinman84]

I had a a $20. (Crossed vacuum lines) at the transmission shop cost me $220. on the self help plan. But I do have all good spare parts that I didn't need.

Sorry you had a bad experience with advice you were given. I tend to like trying to figure out how to fix it myself first, and if it's too difficult or technical for my skillset, THEN I pay someone else to work on my project. I must say, you have a reasonably priced transmission shop in your town. I've never managed to get anything more than a free battery test from any shop in my area for under $35. I don't have lots of cash to spend on my truck, and since I get a kick out of tinkering anyway, I will try that first. Forums like this are a great place to get leads to guide my research in the TMs, and info to discuss with mechanics I know. It's also fun just to shoot the breeze about what projects people have going.

And yes, you did see me on another thread, but not discussing shifting issues. I was contemplating trying to swap my transmission for one with similar ratios for gears 1-3 and an overdrive on top to save on fuel, and was kindly advised as to the cost vs. benefit of such a swap by others who had been there before me. Someday if I have a couple grand to drop on an experiment in power and economy, I may try it. For now, thanks to some friends here on SS, I think I shall save a few bucks and learn more about low vacuum, its causes, and the possible remedies that are within my reach. So far I've got a pretty good track record of figuring out if I can do it myself, ranging from doing timing belts on a couple of cars to helping rebuild an ATV engine. No catastrophic failures thus far. I am pretty good at knowing my limits. I'm still early in my learning curve for CUCVs and 6.2 diesels, so I'm asking questions and looking for info. If I hit a brick wall, then, as you suggest, I'll be figuring out how to put together some more good old U.S. dollars to work on the transmission in the third person.
Thanks for the suggestion.

 

[MarcusOReallyus]

That's a good thought. We don't have an Advance here in Pendleton, but we do have an O'Reiley's Auto Parts and I know they do loan (with a deposit) some specialty tools.

You might want to start with just replacing the lines. They probably need it anyway, and it might cure your problem. If not, well, it's good preventive maintenance, and will leave you with change back from a $10, so why not?

If it doesn't fix it, time to get the vacuum gauge and consider a pump or mod valve replacement.