Downshifting your overdrive

[johnrb3b]

I decided to move these posts to a new thread as they have nothing to do with the thread they were in - my bad.

[johnrb3b]

I was told not to downshift the overdrive unless I was stopped . . .

I'm curious where you heard that. I've never heard it. Herm never said anything like that to me and we discussed all things overdrive for a couple of summers. If you can't split-shift both ways, you lose half the advantage of an overdrive.

I downshift mine all the time. Sometimes I double clutch, sometimes I don't bother. It shifts easily and positively.

Has anyone else heard this? And more importantly was a reason given that makes any sense?

[Ryan_M]

I was told not to downshift the overdrive unless I was stopped . . .

I'm curious where you heard that. I've never heard it. Herm never said anything like that to me and we discussed all things overdrive for a couple of summers. If you can't split-shift both ways, you lose half the advantage of an overdrive.

I downshift mine all the time. Sometimes I double clutch, sometimes I don't bother. It shifts easily and positively.

Has anyone else heard this? And more importantly was a reason given that makes any sense?

I had a couple phone conversations with Herm when I was troubleshooting my OD shortly after install. He walked me through a procedure that involved downshifting as a way to try and unstick a synchro ring but he quickly followed up with a warning about downshifting. Here is a line directly from one of his e-mails:

I would recommend keeping the downshifting to a minimum as that is the hardest operation on the overdrive and the synchromesh.

I realize that line does not specifically prohibit downshifting but in follow-up conversations he strongly advised against it too. I was a little bummed as I had expected the ability to downshift out of OD would be big help when direct 3rd is too much but 2nd isn't enough. I still do downshift from OD to DD on very rare occasions but really try to avoid it at anything above a crawl. Even with that limitation, the OD unit is the best thing I have ever done for the Jeep.

[johnrb3b]

Hmmm.
I'll have to give Herm another call.
In the meantime, I guess I'll double clutch all my OD downshifts just to be safe.

[Ryan_M]

I'm curious what Herm says if you speak with him. He's been very helpful and I like dealing with him so I'm not trying to put him down at all. It would be nice to get some better clarification on the downshifting though.

[1955CJ-5]

I either read about the downshifting or Herm also may have mentioned it. I thought it was mentioned in the installation instructions...

I sort of took it to mean that it's not recommended as a means of slowing or using a downshift for engine braking.

I do downshift the overdrive, when I need more power or when more or less coasting and anticipating needing a lower gear. But I don't treat it as I would a transmission with another gear...no speed shifting!

[Quebec Jeep]

Herm put some advise on his site:

http://hermtheoverdriveguy.com/?page_id=972

Completely at the end he said:

 OVERDRIVE IMPORTANT POINTS:

Along with the instructions I would like to point out some important points:

1-One very important thing is the positioning of the  oil scoop #1133. The oil scoop is the lifeline of your overdrive. It must be properly positioned so that oil can lubricate the complete overdrive. It must be properly positioned so that oil can lubricate the complete overdrive. This oil scoop can not function properly unless your transfer case is full of oil. Fill oil to the bottem of the fill plug. The oil scoop install approximately 7/8[ch8243]. Making sure the oil scoop is tight and does not move. This can be accomplished by taking a screwdriver and twisting it into the trough after installation.

2-Please make note of any oil  leaks you have had previously as if the leaks continue it will lower the level in the transfer case and the overdrive will not get the proper lubrication. If the oil level gets to low it will not be able to lubricate the bearings and planetary pins, and failure will occur.

3-If your overdrive pops out of gear, please call me and we will determine a correction. This is caused by a misalignment of the syncromesh caused by a non-origional replacement transmission parts or transmission adapter kits that are not set up with the proper tolerances for the overdrives.

4-If you encounter difficulty shifter, this can be caused by a precision machining of all component parts. They have a tendency to stick when new. They need a little break-in period. If you have questions please call.

5-Above all, keep the oil level up and the overdrive will perform for many years.

6-I recommend minimum downshifting to prolong the operation of your overdrive. Even though the overdrive is syncronized, it is hard on the syncromesh to downshift at a high speed.

Herm Tilford, 360-256-3843, hermtheoverdriveguy@yahoo.com

[johnrb3b]

Wow.
Herm has added a ton of information to his web site since I bought my overdrive 4 years ago. And the downshift advice is also new since then.
So
The easy answer is just double clutch downshifting the OD just like you do the transmission. Problem eliminated.

[Jeff_M]

Double clutching a constant mesh or non-synchro transmission (like a Jeep first gear) is OK, but I'm not sure double clutching a synchro transmission does the synchros any good.  Having said that, not downshifting the OD until it's practically stopped is a bit of a nuisance to say the least.  I'm thinking--but I certainly don't know--that Herm is erring on the side of caution here. 

[johnrb3b]

Double clutching a synchro transmission virtually eliminates shifting wear on the synchros as the gear speeds are almost matched thus giving the synchros almost no work to do.

The job of a synchro is to force the two gears that are spinning at very different speeds to match speeds so the rings just snick together.

Same principle as a non-synchronized transmission, bringing the gears to almost the same speed so they'll mesh quietly with minimum clashing.

Of course you double the wear on your throwout bearing, but that's cheaper and easier to fix than synchro rings.

[Jeff_M]

So
The easy answer is just double clutch downshifting the OD just like you do the transmission. Problem eliminated.

That's probably what I would do, too.  I had a Farmall Cub and everyone warned me about shifting on the fly.  Well, after careful experimentation I got the technique down and used it for over eight years.Fellow I sold it to was most impressed with how tight the tranny was.  Key phrase here is "careful experimentation", which I'd use when downshifting the overdrive.

[johnrb3b]

I had a Farmall Cub and everyone warned me about shifting on the fly.� Well, after careful experimentation I got the technique down and used it for over eight years.Fellow I sold it to was most impressed with how tight the tranny was.� Key phrase here is "careful experimentation", which I'd use when downshifting the overdrive.

That's where I learned double clutch technique - with farm tractors. We had at least five IH Farmalls on the farm and I quickly learned to double clutch, especially when running the gravel roads from farm to farm. I could shift up into road gear at 10 mph with not a single clash of gears. Our grain truck was even more fun to drive. It was an Army surplus 2 1/2 ton with a totally NOT synchronized transmission and a 2-speed rear axle. Every shift, up or down, was a double clutch. Throw in split shifting with the rear axle and it was lots of fun. Kept you busy.