Speedo Sensor Leaking Trans Fluid in Cab?

[Gary Lewis]

Yes, that's the guy. And your cable should plug right into it just like into the transmission. But you do have to move the driven gear as you said.

Here's what I've found.

Second, here's a snippet from the calibration parts list for '87 and '88 trucks. Note that the part numbers match the Rock Auto listing.

Gary, thank you very much kind sir! You are awesome...it is so nice to so quickly remove all doubt on buying a part;). Thank you for the details.

Having found a thread where this cured the problem, I think it's a cheap solution to try.

Must have something to do with the design of the M5OD combined with the watery thin Trans Fluid;).

Welcome.

Yes, it may have something to do with the transmission design as well as the thin ATF.

I think the key is the o-ring I've circled on the adapter, below. It keeps lube from going outside of the adapter, and they apparently block the fluid internally, so it is contained. As you say, a cheap solution.

[85lebaront2]

Welcome.

Yes, it may have something to do with the transmission design as well as the thin ATF.

I think the key is the o-ring I've circled on the adapter, below. It keeps lube from going outside of the adapter, and they apparently block the fluid internally, so it is contained. As you say, a cheap solution.

Gary, there should be a seal in the adapter, however, I would check the transmission vent, if it is clogged then pressure from heat expansion will push fluid through the path of least resistance.

[Gary Lewis]

Gary, there should be a seal in the adapter, however, I would check the transmission vent, if it is clogged then pressure from heat expansion will push fluid through the path of least resistance.

Bill - I agree that something is amiss. There should be a vent which keeps it from pushing fluid out.

[Rembrant]

Bill - I agree that something is amiss. There should be a vent which keeps it from pushing fluid out.

The M5OD does not have a dedicated vent...not that I could find. According to my web searches, it vents through the shifter seal. The old seal was torn, and most definitely venting...lol, but the new one was pretty snug on there.

In the FTE thread, the guy said he actually added a vent to try and cure this issue, and it was no help. He had a new cable from LMC, and the only way he could get the leak to stop was to install the VSS from the 1988 truck (that the trans had been removed from). Mine is from a 1991, so I assume the same.

It must be that the fluid is wicking up the cable? I dunno. I'm sure it's no problem with the thicker gear oil in a traditional cast iron trans, and maybe it doesn't show up in transfer case mounted speedo cables? Maybe an issue specific to the 2wd M5OD?

I guess I'll find out after I install a VSS....if it's pressure, the fluid will find another way out.

I wonder...did all of the 5spd trucks have this VSS? I know it didn't show up until 1988, so the trucks were all EFI by then weren't they?

[Rembrant]

...if it is clogged then pressure from heat expansion will push fluid through the path of least resistance.

I just thought of something else...

I really haven't gone anywhere with this truck since installing the transmission. I've put about 200 miles on it, but just running around locally within the city really, and some short rural drives (I live 10 miles outside the city, on a road where I can barely do 45-50 mph). I've been on a half a dozen drives with it, maybe 30 miles each time.

I guess my point is, I wouldn't think that the transmission fluid would have had any opportunities to get very hot. I could see steaming along the hwy at 60-70 mph for a while, but I've only been on a few little drives with this thing.

Maybe it would get hot anyway, I dunno.

 

[Gary Lewis]

...if it is clogged then pressure from heat expansion will push fluid through the path of least resistance.

I just thought of something else...

I really haven't gone anywhere with this truck since installing the transmission. I've put about 200 miles on it, but just running around locally within the city really, and some short rural drives (I live 10 miles outside the city, on a road where I can barely do 45-50 mph). I've been on a half a dozen drives with it, maybe 30 miles each time.

I guess my point is, I wouldn't think that the transmission fluid would have had any opportunities to get very hot. I could see steaming along the hwy at 60-70 mph for a while, but I've only been on a few little drives with this thing.

Maybe it would get hot anyway, I dunno.

I can see them expecting a tranny to vent through the shifter seal. But when it is new and tight it probably takes more pressure to escape there than it does out the speedo cable hole, so it pushes some out there instead.

As for getting hot, did you replace any parts or just the seals? If you replaced parts, like bearings, they'll take some time to wear in and during that time they'll be creating some heat.

Assuming the VSS is on order then I'd try it first. But another approach would be to slip a straw in the shifter seal to try to create a vent. But if it fits really tightly then it'll probably mash it shut and defeat the purpose.

[Rembrant]

I can see them expecting a tranny to vent through the shifter seal. But when it is new and tight it probably takes more pressure to escape there than it does out the speedo cable hole, so it pushes some out there instead.

As for getting hot, did you replace any parts or just the seals? If you replaced parts, like bearings, they'll take some time to wear in and during that time they'll be creating some heat.

Assuming the VSS is on order then I'd try it first. But another approach would be to slip a straw in the shifter seal to try to create a vent. But if it fits really tightly then it'll probably mash it shut and defeat the purpose.

Good point on the tighter seal Gary. That may very well be the case.

I didn't replace any hard parts, no. I bought the transmission used, supposedly with approx 100k miles on it. It came out of a light duty F250 2wd w/302, but that is all I know of it. I replaced every seal in it, and even replaced the rubber plugs in the top cover with pressed steel plugs (a common mod for the M5OD).

Now that I've had a little time to drive it, it is clear that the trans is not 100%. It is fine for what I need for now, but it's a notchy shifter between 1-2, and 2-3 unless I shift it very slowly. 3-4-5 is as smooth as new. I won't be breaking any 1/4 mile records with it...lol.

[Gary Lewis]

I can see them expecting a tranny to vent through the shifter seal. But when it is new and tight it probably takes more pressure to escape there than it does out the speedo cable hole, so it pushes some out there instead.

As for getting hot, did you replace any parts or just the seals? If you replaced parts, like bearings, they'll take some time to wear in and during that time they'll be creating some heat.

Assuming the VSS is on order then I'd try it first. But another approach would be to slip a straw in the shifter seal to try to create a vent. But if it fits really tightly then it'll probably mash it shut and defeat the purpose.

Good point on the tighter seal Gary. That may very well be the case.

I didn't replace any hard parts, no. I bought the transmission used, supposedly with approx 100k miles on it. It came out of a light duty F250 2wd w/302, but that is all I know of it. I replaced every seal in it, and even replaced the rubber plugs in the top cover with pressed steel plugs (a common mod for the M5OD).

Now that I've had a little time to drive it, it is clear that the trans is not 100%. It is fine for what I need for now, but it's a notchy shifter between 1-2, and 2-3 unless I shift it very slowly. 3-4-5 is as smooth as new. I won't be breaking any 1/4 mile records with it...lol.

I know that feeling. The ZF5 I had in Dad's truck had worn synchronizers and it needed to be shifted slowly into most gears. And Big Blue's T19 is getting that way in 3rd. But I've had the ZF completely rebuilt and it'll go into Big Blue this winter, so that should resolve that problem.

Anyway, if you didn't have hard parts replaced then it shouldn't be needing to wear in, so I'm surprised that it is pushing lube out on short trips. You might want to slide under and see if you can find a hot spot after a trip.

[Rembrant]

You might want to slide under and see if you can find a hot spot after a trip.

Good idea. I'll give that a try.

I've spent a lot of hours under there, one more slide on the floor won't kill me;).