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| 904 shift problems https://slantsix.org/forum/viewtopic.php?t=24642 |
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| Author: | Chuck [ Sun Aug 26, 2007 5:52 pm ] |
| Post subject: | 904 shift problems |
Fellow Slixers: I rebuilt a '66 904 for a friend and it doesn't shift right. The original problem was that it seemed to slip in 3rd, but he pulled it before I had a chance to drive it. We did a basic rebuild (kit with seals, clutches, etc.), and it worked OK the first time we drove it except for a weak 2-3 upshift. We tried playing with the kickdown band without much effect and ended up setting it out 2 turns, per my FSM. Now we have 2 problems. First of all, it slips in reverse. We didn't back up under a load before so that problem may have existed after the rebuild and we didn't notice it. The second problem is that it does not want to shift into 3rd. It will shift sloppily if you back off the throttle, but as soon as you step on the gas it shifts right back into 2nd. I am thinking it is a low pressure problem. Whatever the cause, it must be something that wouldn't get covered in the general rebuild, because it seems that the original problem is still here. Sticking governor? I need some ideas. |
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| Author: | SlantSixDan [ Sun Aug 26, 2007 6:15 pm ] |
| Post subject: | |
Sounds like low pressure to me, too. I'm thinking low line pressure. How 'bout the line pressure adjustment on the valve body? Don't think it's the governor, 'cause you're mentioning slipping in reverse. |
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| Author: | Chuck [ Sun Aug 26, 2007 7:52 pm ] |
| Post subject: | |
I don't think it would be an adjustment because the trans worked fine for years, then just started acting up. It has to be a failure of some kind. |
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| Author: | SlantSixDan [ Sun Aug 26, 2007 8:08 pm ] |
| Post subject: | |
I meant, was the line pressure adjustment checked during the rebuild? Has the line pressure itself been checked with a pressure gauge attached to the various test ports on the outside of the trans? |
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| Author: | Chuck [ Sun Aug 26, 2007 10:47 pm ] |
| Post subject: | |
I haven't checked it yet. I need to go buy a gauge. Still, the same question applies. If the pressure is low, what did I miss when I rebuilt it? The pressure will not be low unless there is something broken. The adjusting screw is hard enough to turn that it can't move by itself. So what could make it drop and not be fixed by a rebuild unless I specifically looked for it? (This is assuming that there was a low pressure problem before I started.) |
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| Author: | Charrlie_S [ Mon Aug 27, 2007 6:59 am ] |
| Post subject: | |
Did you change any bushings, (pump, drums)? they may be worn, allowing pressure leakage. You might want to drop the valve body, and do an air pressure test, to see if you have clutch pack leakage. |
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| Author: | SlantSixDan [ Mon Aug 27, 2007 7:04 am ] |
| Post subject: | |
That might or might not be a valid assumption. The "before" problem might not be caused by the same problem as the "after" problem. Low pressure will be either a clog or a leak...somewhere.  Sorry I can't be more specifically helpful. But somebody on here ought to be able to be!
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| Author: | Chuck [ Mon Aug 27, 2007 11:02 am ] |
| Post subject: | |
We changed all the seals and a few of the bushings. There were more in the kit than we needed, and a few, like the rear bushing, we did not change. I remember we did the large bushing in the pump and 2 smaller ones, I think in one of the clutch housings. My assumptions may be wrong, but I was hoping this would be obvious to someone who has had the same problem. If not, I need to throw out those assumptions. |
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| Author: | Doc [ Mon Aug 27, 2007 11:15 am ] |
| Post subject: | |
How did the front pump look? Was the impeller assembly and case in good condition? Look at the FSM for the pressure routing chart, a slow reverse and 3rd sounds like a big pressure leak in the forward drum / clutch pack. (I am going by memory... look at the apply chart an "logic it out" then run the pressure and / or air checks to confirm. DD |
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| Author: | Chuck [ Mon Aug 27, 2007 12:57 pm ] |
| Post subject: | |
Thanks Doug. The pump looked good and the air checks seemed OK. I didn't think the forward clutch pack affected reverse. On the pressure checks, should I jack the rear wheels off the ground and run in gear, or jusr run in neutral? I saw a chart that showed speed vs. pressure, but that sounds like a pain to check if I have to drive the car with a gauge attached somewhere. |
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| Author: | Dusty Desks [ Tue Aug 28, 2007 9:12 pm ] |
| Post subject: | |
This is definitely a front clutch slippage problem, because the front clutch engages in 3rd and reverse. If there is a pressure test port to apply air or oil pressure to the front clutch while watching for leaks with the oil pan off, this would be the next thing to check. Leaks could be caused by a worn clutch piston seal, the new seal damaged or installed wrong, scored bore, and on some A-904 units of that era, the clutch pressure plate disc could actually be broken, allowing the piston to move out to far, though this would be caught during any rebuild (it isn't visible unless the transmission is torn down and front clutch assembly pulled out). If the front clutch works ok, then likely it is not getting enough pressure & oil flow due to an internal hydraulic leak or other hydraulic problem, most likely inside the valve body. |
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| Author: | Charrlie_S [ Wed Aug 29, 2007 6:02 am ] |
| Post subject: | |
Dusty is totally correct. That is why I asked about the bushings and suggested an air pressure check. If the front clutch "thunks in" and does not have much (it will have some) air leakage, it sounds like a valve body problem. If the clutch has leakage the trans will have to come out and be dissassembled. A lot of trans kits are like carb kits. They will fit many years, and you get different size lip seals, and seal rings. Got to match them with the old ones. It is also very easy to twist the lip seals on the clutch pistons, when installing the piston in the drum. The last trans I built, had the wrong front pump seal rings in the kit. They were just a little too small dia, when they were "latched" and would leak badly. Luckily, I air test all my transes, before installing the valve body. Knew I had a problem, but had fun finding exactly what it was. |
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| Author: | Chuck [ Wed Aug 29, 2007 2:50 pm ] |
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Thanks Dusty & Charlie. I checked the fluid flow diagrams I have and came to the conclusion that I probably damaged the front clutch seal on assembly. It was very difficult to get the piston in on this trans. I damaged the rear when I did my Cuda, but I thought I got them right on this one. The air check seemed OK but I have not done enough of these to remember the correct "thock" sound that indicates it is working properly. I have only done one pressure check so far with the tranny still in the car because I forgot to bring my book over to where the car is. I checked the Governor Pressure port at the rear and got about 60lbs in 3rd with the rear wheels off the ground and the speedo reading 60. This looks good. (My chart shows 60-69.) I am doing the checks to better understand how to troubleshoot a trans while still in the car. I plan on testing the Line Pressure next, but I haven't located a chart for that yet. If you know what the pressure should be, please let me know. |
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| Author: | Chuck [ Tue Sep 04, 2007 3:11 pm ] |
| Post subject: | |
Well, they work better when you install ALL the parts! I did some pressure checks and came up with about 15 psi less at the front servo release than at the line pressure test port, which, if I am reading the book correctly, indicates a leak in the servo or front clutch. When I took it apart again and inspected the front clutch seal, it looked OK, but I also noticed 2 grooves in the clutch center hub. I dug out an old clutch assembly and discovered that I left out the inner seal on the hub! Usually I will remove a seal and immediately replace with a new one during a rebuild, but my friend was helping with this rebuild because it is his daughter's car. He removed the seal and didn't install the new one, and I didn't notice it when I assembled it. I had done the air checks and it didn't sound right, so I checked everything out and discovered that I had left off the front pump gasket. I assumed that was where my leak was coming from so I didn't do the air checks again. If I had, I would have discovered the problem right away. It doesn't help that it has been 2 years since I did the last trans. Bottom line, install ALL the parts and air check, air check, air check. Thanks for all the replies. 
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