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Ultramatic Rebuild - Several hundred miles on this rebuild now. Everything seems solid so far. One particular item I need feedback on if anyone has experienced this. - Once fully warm after 30+ minutes driving, in D>, when stopped it feels like it is in N (will roll back on a hill, doesn't roll forward when brake pedal is let off). Needs RPM's to engage the TC. Prior to being warm, it engages/holds as normal.

Posted on: 10/29 12:52

I have a more modern car that does that (2007 Volkswagen EOS) which is very disconcerting, especially when someone pulls up close behind me at a stop on an upward slope but apparently they ll do it by design. The Ultramatic was not designed to do that I hope the experts here can help you!

Quote: Kinetic viscosity decreases as temperature increases, so some of that should be expected. Check the Data Sheet. Mobil Type F ATF has similar kinetic viscosity specs to the Amsoil. I have noticed a decrease in creep hot versus cold. I do get roll back on a hill, but I haven't done any temperature based comparisons. Have you tested in Low? The results should match D>.

Both the viscosity and density decrease with heat. Density is only about 5% as you go from 100F to 210F, but viscosity is much more. A reduction in density decreases the energy transferred essentially through the conservation of momentum, as there is less mass flowing per area.

Are you sure that your idle speed is the same in all cases? As the car starts at high idle but, for example, if your choke stove is broken, it will take forever to warm up.

Rather wondering if your High clutch pack isn't letting go for lack of pressure when the trans is hot and rpms are low. A pressure check on the high port will confirm. I am removing a 50 to look for this exact trouble later today.

The other unpleasant possibility is that your reactor clutch is not holding (making your convertor a very inefficient fluid coupling), but usually when they start to slip they never grip again til you put the trans in neutral and start all over again.

Convertors don't care about the fluid thinning out as it gets hot--in fact it makes them more efficient. The actual work is done by the mass of the fluid being accelerated and decelerated and deflected by the vanes.

I was also wondering if the pressures were inadequate. However I interpreted "D>" to mean Drive, so he would be in low range. But the low range servo also defaults to off via its springs, so inadequate pressure would cause the band to start to slip. As a side note, the pump valve is specifically designed to cut off flow to the converter at low gallery pressure conditions, to boost the pump pressure. So if it's still problematic it would probably have to be REALLY low. Does the trans do the same thing if you are in 'D instead of D' ?

The root cause could be a worn front pump. Did you check the condition of it per the instructions in the manual when the trans was apart?

Another possibility would be a loose/distorted low range servo jumper tube, or if you have the old style low range cap, a distorted cap. Do you have a picture of the transmission with the pan off or something showing the low range servo?

I will be interested to hear your findings Ross.

Everthing else is working excellent.

In response to HPH's question I missed.

'D does not have this issue.

Only D' and L

You already sent pictures showing that you have the new style servo cap. If you really want to nail down this issue you will need to attach a pressure gauge to your low range brake. Unfortunately this is a bit goofy. There is a plug in the back of the case, I would take a union fitting to replace that plug, a fitting such that you can drill and tap it for pressure gauge hoses on both sides. The interior one goes to the brake takeoff and the other one goes to a hose which you should take off the flex hose on the driver side air duct, and route it into the cabin attached to a gauge.

However this would not solve the problem, just get more data.

I would suggest instead that you loosen the 3 servo attaching screws, and push the servo toward the valve body to really make sure that the jumpers are seated, then cinch the screws back down.

Also did you ever have the servo apart? It could have a broken piston ring... they are hard to reassemble.