1973 C6 inaccurate shifting - fluid related?

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Mark IV VIenna
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1973 C6 inaccurate shifting - fluid related?

Post by Mark IV VIenna »

Hi everyone,

Another issue on my 1973 Mark IV. Tranny double-shifts from 1st right through 2nd to 3rd in gentle to moderate acceleration. A slight double bump is to be felt in the process. This occurs mainly when the fluid is at operating temperature. Shifting down by column lever or by kickdown works. Also, forward engagement from neutral to drive is slow (reverse is fine). The fluid level is correct, the fluid itself does not appear to be dark or smelly. The two vaccuum lines are hooked up (one to manifold vaccuum, one to ported) and check out air tight. The medium band was adjusted by a shop recently. However, the fluid is ten years old. And I have no idea what type was used. So here are the questions:
- Does ATF age in a way that could cause these problems? Maybe it is worth mentioning that the car has been sitting in a barn most of the last ten years.
- Could it be that these symptoms are caused by some wrong ATF?
- Or is it normal C6 operation to just jump through second gear in slow acceleration and go to 3rd at 15-20 mph? I never had a C6 before, just GM trannies. And they all go to second gear around 15 mph and to third around 30mph (again in gentle acceleration)
- In case a fluid change makes sense: The dipstick mentions Type F to be used - as does the 1973 manual. However, the 77 manual says that DEXRON/MERCON can be used. Was there any change in the C6 between these model years that actually calls for different spec ATF? I still have lots of DEXRON III / MERCON V which I would use it if I can.

The car is driveable, this is not a major issue. But it still puzzles me :roll:
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Dan Szwarc
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Re: 1973 C6 inaccurate shifting - fluid related?

Post by Dan Szwarc »

Rubber hardens. Seals leak. Transmissions start to shift later or not at all.

The cheapest and easiest thing to do is replace the modulator on the back of the trans. That usually solves most upshifting issues, especially 2-3. Check it out.
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Re: 1973 C6 inaccurate shifting - fluid related?

Post by action »

The age of the fluid as an only factor is not an issue. Old fluid is still good.
Wrong type of fluid will cause shift issues
Debris in the fluid/screen can cause issues. Usually delayed shifts

Delayed engagement to Drive is from low fluid or as stated above hardened rubber seals. Hard rubber seals is very common on older transmissions. Transmission removal and disassembly is necessary to fix this.

Upshifts are controlled by the governor. The vacuum modulator influences the governor. There is a rod in the vacuum modulator. That rod comes in different lengths. Either the governor, modulator or rod is an issue.

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Mark IV VIenna
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Re: 1973 C6 inaccurate shifting - fluid related?

Post by Mark IV VIenna »

Thanks so far - I guess I`ll live with the late engagement of "D" then. It takes around two seconds. So. it is not worth dismantling the transmission.

The upshift issue seems to be unrelated, and maybe easier to tackle. I found the vaccuum diaphragm (named "governor / modulator") on rockauto, but no rod or governor. Does it make sense to get a new vaccuum diaphragm even if it doesn`t "leak"? I checkd that with a vaccuum pump, it is tight. Also, would that explain why the issue gets worse with rising fluid temperature?

About the fluid: Should I use Mercon V in case of a fluid change or is it better to stick to Type F?
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Re: 1973 C6 inaccurate shifting - fluid related?

Post by tomo »

Here is a link to the specs for Type F ATF
https://www.mobil.com/en/lubricants/for ... ype-f-atf/

The vacuum modulator and the governor work together to tell the clutches to engage and shift the transmission. A stiff diaphragm in the modulator can affect shift points.

Old fluid may still be good, but it may have some contaminants such as moisture or debris that can cause shifting problems. If it were my car, I would change the filter and have the trans flushed and filled with Type F ATF. I would also check the vacuum hoses for flexibility and replace any that are hard as they can have small crack that leak air into the system.
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Re: 1973 C6 inaccurate shifting - fluid related?

Post by action »

The vacuum modulator has the rod internally. When replacing the modulator the rod could fall out.

+1 to a fluid & filter replacement. Especially if you just bought or if you do not know how long ago the fluid has been replaced. Drain the torque converter too.

The delayed engagement will not get better. It will get worse. If you do not repair it do not drive any distances.

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2006 Lincoln Navigator Limited 5.4l 3V
1996 Lincoln Mark VIII 2DR Coupe Diamond Anniversary 4.6l DOHC, 4R70W, 3.07
1970 Continental Mark III Triple Black 460 4v, C6, 2.80 (Used for Woodward Dream Cruise or just generally stored in Michigan)
1966 Lincoln Continental 4DR Convertible 462 4v, C6, 3.00
1966 Mercury Park Lane 4DR Breezeway 410 4v, C6, 2.80
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Mark IV VIenna
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Re: 1973 C6 inaccurate shifting - fluid related?

Post by Mark IV VIenna »

The fluid is roughly ten years old. But looking at the other work on the car that has been done by the same shop I highly doubt that they spent too much thought on using the correct ATF.

Anyway looking at the amount of work involved I`ll try the easiest solution first and mount a new vacuum diaphragm. If the early-upshift issue persists I will change oil and filter as a next step. Or think about having the whole transmission overhauled - which naturally comes with an ATF change.

The retarded engagement of "D" has been there since two years, and it did not get worse so far. However I concur that such conditions tend to get worse at some point.
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Re: 1973 C6 inaccurate shifting - fluid related?

Post by papawayne »

Only a few things heal up all by themselves. Wayne
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