Transmission slipping when hot?

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Trevor B

Trevor B

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As I was practicing launching (preparing to drag race the Duster for the first time), I noticed that my transmission seems to "slip" a little or sometimes a lot between 2nd and 3rd gear, but only after several runs.

Driving to the track on Wednesday night, I was stuck in awful traffic, just poking along (in drive) for 45 minutes.

My first run (right off the highway)... I get up there and stage, and take off, and it feels like someone has his foot on the clutch a little. Except it's an automatic. More exaggerated slipping from 2nd to 3rd (at wide open throttle).

Second run, I feathered the gas a little (starting in 2 and then shifting to D manually) and it was better but still a good slip from 2 to 3. The car had been sitting a while and cooling down.

Third run, almost no slipping - really cooled down (45 degrees outside) but still a little between 2 and 3.

Here are some details:
1973 Duster
904 transmission, all stock
stock rear end
Transmission leaks fluid toward the front (haven not opened it up)
For some reason, never have been able to shift all the way to low - only 2 and drive. In fact, I never needed low before.

I was sort of hoping not to destroy anything on the drag strip but here we are.

Thoughts? Is this something I should even take on?
 
The fluid would burn on that first slipping run if it is the trans and not the converter.

Is it dark and smell a little like burned wood?
If so then it already burned up.
If not then it's not the trans but the converter or something you are misinterpreting as a trans slip all together.
 
If it's slipping, the damage is done. Period.
 
I would hope it was the tires slipping, heehee, but I fear the guys are on the money. First lesson; racing ain't cheap..... :(

As far as trannys go the 904/727 is pretty simple. With a camera, an FSM, a few tools , and a bit of mechanical smarts, a stock rebuild is easy-peasy.
 
just a quick thought, is the kickdown linkage adjusted properly? I had this problem on my 727 for a few years, I finally adjusted the kickdown linkage and everything was fine. No damage to the tranny at all.
 
AJ/FormS said:
I would hope it was the tires slipping, heehee, but I fear the guys are on the money. First lesson; racing ain't cheap..... :(

As far as trannys go the 904/727 is pretty simple. With a camera, an FSM, a few tools , and a bit of mechanical smarts, a stock rebuild is easy-peasy.
Click to expand...

2nd lesson, racing ain't cheap. My ex-neighbor ran a 10:90 340 powered Dart. He figured a good weekend, with nothing breaking would typically run $400.00+.:burnout:
 
transman said:
2nd lesson, racing ain't cheap. My ex-neighbor ran a 10:90 340 powered Dart. He figured a good weekend, with nothing breaking would typically run $400.00+.:burnout:
Click to expand...

So about the same as owning a boat. :D
 
44067gtdart said:
just a quick thought, is the kickdown linkage adjusted properly? I had this problem on my 727 for a few years, I finally adjusted the kickdown linkage and everything was fine. No damage to the tranny at all.
Click to expand...

Incorrect. If there is ever any slippage, there is damage done. Period. End of story. Over and out. Whether it's enough damage to make a difference will remain to be seen, but anytime clutches slip, material IS removed = damage.
 
If your transmission doesn't shift to low you should adjust your linkage until it does. Then try again.
 
Hi All - thanks for the responses.

So... obviously need to do something but the question is "what"?

The fluid does not smell burned and looks nice and red, both on the dipstick and dripping off the bottom of the bell housing (it has always leaked).

There is no slippage when the car is cold or cool, only after I'd been actually racing. I also got it to slip the previous weekend when I was practicing on the highway: dead stop through all three gears, and it only slipped a little between 2nd and 3rd on my 6th "run" (so after 5 runs). This is what made me think it was heat-related.

If it is a torque converter issue, is that something that I can rebuild, or should I buy another one?

If it is inside the transmission, can I rebuild it myself or am I looking for a world of hurt?

Transmission guys around where I live (north of San Francisco) are very, very expensive and one guy won't even work on the tranny unless he takes it out and puts it back himself.

I could buy a rebuilt 904 with torque converter for around $700 from a sort-of local guy who claims to be a retired transmission guy who now rebuilds mopar in his retirement. He sells transmissions on Craigslist.

I could buy one off of Summit but they seem to only have TCI in my price range (as little as possible) and there is no end of questionable reviews in the TCI department!

Obviously, this is an expensive thing to do. But I got into it to learn how stuff works and ended up really wanting this car to tool around in and race once in a while.

Thank you much!
- Trevor


p.s. the problem with "low" was a linkage issue. The shaft was bent funny and we got it straightened out - now can shift the car into all positions.
 
can you elaborate on this 2-3 shift slipping. Describe it more. You have a tach, what does it do under normal driving when everything is warmed up or hot as you stated?
 
I have had a Turbo 350 slip in the opposite situation.... slipped when cold but was fine when warm. No burned smell in the fluid or discoloration. Tore it down and the problem was obvious: the friction material was mostly worn off on one pack of clutches and the metal backing was showing on all of them to varying degrees in that pack. You could just have a worn out trans.....since it is original; that would not be too surprising.
 
Is it slipping, or just slower to shift when it's warmed up and your foot's on the floor? Since no one has stated the obvious.....how's the fluid level look?
 
AJ/FormS said:
I would hope it was the tires slipping, heehee, but I fear the guys are on the money. First lesson; racing ain't cheap..... :(

As far as trannys go the 904/727 is pretty simple. With a camera, an FSM, a few tools , and a bit of mechanical smarts, a stock rebuild is easy-peasy.
Click to expand...

I burned up a 904 in my 71 Demon about 30 years ago. I did not have the kickdown working properly. :eek:ops:

I would be very tempted to rebuild it myself like the poster above mentions. I am actually looking at doing just this with a 727 before I even put in the car.

Does your car have a transmission cooler? Heat kills automatic transmissions. :violent2: A big cooler will not reverse the damage but in the future it may help save a transmission. Also an electric fan as an accessory to use when traffic stops is an idea. I have been stuck in that Sears Point traffic more times than I can count. If you have room place an electric fan in front of the radiator with transcooler between it and the radiator.

They are not as cheap as they used to be but a transmission temp gauge may be helpful to see how hot the trans is getting. If you buy one of these make sure it is a complete kit with the sending unit.
 
FredS said:
can you elaborate on this 2-3 shift slipping. Describe it more. You have a tach, what does it do under normal driving when everything is warmed up or hot as you stated?
Click to expand...

I do not have a tach. Under normal driving there is nothing out of the ordinary. The only slipping I'd experienced was under wide open throttle, shifting from 2-3, after 5 runs with no problem. The first time it happened I heard the engine suddenly rev a little higher, as if I had not timed a shift perfectly in a manual. The really bad slip I experienced at the racetrack felt like someone had left his foot partially on the clutch as the transmission shifted from 2-3. I could feel the engine revving much higher than before and could also feel that the engine was not pulling the car anywhere near what it should. Then when I feathered the gas, it settled into 3rd - no clunk or anything, just sort of settled.

Fluid level is normal. On the advice of the transmission guy I cannot afford, I've been adding TransX in place of ATF to keep the level up, hoping it might slow the leak. It hasn't.

The car has only its stock transmission cooling lines running to the radiator, which has a new core. I did not clean out the transmission cooling lines, nor have I ever replaced the fluid that was in there. It had been sitting a long time and the level was low but it didn't look or smell weird.

Regarding the kickdown - I installed the Lokar system about 8 months ago. I followed the instructions pretty carefully and the transmission kicks down from 3-2 on the highway as it should - seems fine for the most part and works according to the factory specs (about 65 mph or lower). I have occasionally wondered if maybe it shouldn't be adjusted slightly as I am used to automatics downshifting at any speed, even over 70 mph.
 
If the fluid isn't burned then trans itself didn't slip, because what you are describing would have fried that thing like an over done wonton.
Sounds like a converter problem to me.
 
When it does it, pull over, put in reverse and power brake it and see if still feels like it is slipping. What I'm trying to determine is if you have a 2-3 overlap issue or direct clutch issue, and the kickdown could not be set right. Did you confirm if throttle is to full position, the kickdown lever is 100% stroked?

Also, verify the fluid level is not low at all. If it's only 2-3, then I'm not leaning towards a converter, but most likely the direct clutch pack is on its way out, possibly. Need the info first, above.
 
FredS said:
When it does it, pull over, put in reverse and power brake it and see if still feels like it is slipping. What I'm trying to determine is if you have a 2-3 overlap issue or direct clutch issue, and the kickdown could not be set right. Did you confirm if throttle is to full position, the kickdown lever is 100% stroked?

Also, verify the fluid level is not low at all. If it's only 2-3, then I'm not leaning towards a converter, but most likely the direct clutch pack is on its way out, possibly. Need the info first, above.
Click to expand...

If it slipped like he says it did the fluid would show it and so would the pan contents.
It almost has to be an overlap issue misunderstood as a slip, and being a stock 903 and racing it wouldn't surprise me one bit.

That is if it isn't the converter.
 
who...or what is shifting this transmission ?
are you allowing it to shift itself, or are you shifting it?
and if you are shifting it, are you using the stock shifter linkage or a ratchet type shifter?
and if your using a ratchet type shifter, when was the last time you cleaned and lubed it...and maybe inspected and adjusted the cable?
 
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