The 5-speed on my new (to me) '91 F-150 is whiny when in gear. Not obnoxiously so, but certainly audible. Is this at all typical? I must confess I haven't even checked the fluid level yet - that's on my to-do list before I take it any significant distance.

I know the spec'd fluid is ATF with a capacity of 2.5 quarts I believe. I'm sorta figuring to go with synthetic ATF unless someone tells me otherwise.

 

Get 4 quarts of the Walmart dextron 3 and fill it till ATF comes out of the fill plug the actual spec for the fill amount is 3.9 quarts but a extra tenth of a quart wont hurt it. What gears does it do in?

 

I would crawl under the truck and make sure it has fluid in it before I moved it. These transmissions are bad about slowy leaking fluid from the rear of the top cover; so slow there isn't an oil spot under the truck, or an oily/ greasy transmission.

The M5 OD isn't that hard to rebuild, but keeping fluid in it will prevent having to do that.

When I rebuilt one of mine the tech in Nevada where I bought the parts recommended 10W30 synthetic motor oil.

Robert

 

It's whiny in all gears but 4th. Makes sense, as there's a lot less going on in the gearbox when it's running 1:1.

 

Interesting... Same advise I've heard from some for the Borg-Warner T-5, which is also spec'd for ATF.

I'll get under it this weekend and at least check the fluid level - if time permits, I'll just go ahead and change it.

 

I did get a chance to check this a couple weeks ago. Partially un-screwed the fill plug and fluid started coming out - I took that as a positive sign and screwed it back in.

At this point I'm just tackling the items most in need of attention. Since the gearbox has a full fluid fill, I'm going to move on to more pressing items and come back to a fluid drain and re-fill at a later date.

 

You may want to check the rear wheel wells and make sure that there is no oil there as a indication of a slow sleeve leak. The play on the pinion on these can create a whine that sometimes sounds like it from up front when actually its in the rear or even the midship carrier bearing (if it has one). I have also seen lift kit placed on the vehicle that placed stress on and off of the wrong areas. It happens when reinstall. Some un bolt them and reconstruct on un level surfaces and then dont measure and mark right when putting it back together and force it in place. Then they get axle roll too.

Just some other things to consider. Besides the tranny bearings

 

Note that the whine only occurs in 1-2-3-5. It clearly goes away in 4th. That tells me it's most assuredly coming from the 2WD gearbox.

 

Just to update this thread with something of a resolution, replacing my worn 1-2 shifter fork cut down on the noise level when in 1-2-3-5. You'll note above that it was always quiet in 4th.

I'm not sure why this is the case - it doesn't really make sense to me - but I won't argue with positive results.

For further discussion of the worn shifter fork and replacement of same, see this thread... https://www.ford-trucks.com/forums/1173238-m5od-shifter-fork-wear-diagnostics-pics-inside.html

 

Just to update this thread, although the M5R2 is certainly quieter since I put new fluid in it (10W-30 synthetic), and replaced the worn 1-2 fork, it certainly still isn't quiet in 1-2-3-5 like it is in 4th. Just stumbled across this comment in another thread....
...have the M5OD-R1 in my 89 ranger, they are tough but the input shaft bearing fails 80% of the time and whines loudly in all gears except 4th.

I'm guessing the M5R2 (AKA M5OD-R2) is essentially the same in this regard, yes?

 

I'd agree with that statement , except the tough part. The M5OD R1 and R2 are pretty much the same except for size.

After rebuilding both versions, I definitely recommend putting a new oil slinger in any time you take the input bearing cover off; they are a delicate ~$6-8.00 part that will let you get 423 miles out of a transmission before the input and main shaft weld together if the old one cracks while reassembling.

Robert