Staghound Hydramatic Transmission Question

[rob love]

I am working on an M5A1 Stuart tank, which has the two hydramatics coupled with the Cadillac flatheads. There is a two terminal switch on the top of each of the transmissions (two wires running to each) which I cannot find on the wiring schematic in the TM from June of 43. Any ideas on those? One is hooked up, and the other side has bare wires and is hanging on the bellyplate. You need to be a 3 foot high contortionist to do anything to this tank.

I'm gong to get a light and a mirror up into there, but am hoping someone can save me some time and a hairdo full of leaking dexron.

[Malcolm Towrie]

That'll be the low oil pressure switches for the warning lights on the dash.
The Hydramatics in the Chaffee and the Staghound have the same switches.

On the Staghound, the lights just warn that the engine has stalled as described above. On the Chaffee the switches are also tied into the starter circuit to prevent cranking an engine that is already running.

The switch set point is 60psi, since the trannies are supposed to generate at least 60 psi at hot idle. In reality, one of our Chaffee transmissions only puts out about 55 psi (although it rises above 60 psi as soon as rpm is raised, so it can come at idle with engine running.
Malcolm

[rob love]

Thanks for the reply Malcolm. I wonder if they are inline with each engine's low oil pressure switch? I'll pull the multimeter out tomorrow and do some testing before I finish installing the belly plate.

[marco]

This is what I could find in the M5A1 manual TM9-727C from December 22, 1942.
It seems that these are just warning lights, so not wired in the cranking motor circuit.

[rob love]

Thanks for that Marco. This tank is a little bit later, so there are some changes, but I think your wiring is valid. I still have some sorting out to do on the wiring, but I think it is under control. The switch in question turns out to be bent from the re-installation of the engine/transmission some years back. All of the lightbulbs for the three warning lamps I also found to be laying in a bin near to the control panel. I can barely reach the left hand switch, never mind testing or replacing it. I think I can pull a panel underneath the turret basket to access the tops of the trannies.....that will be Monday's job.

How come, for every job I want to do on this thing, three other jobs show up? Where does it all end?

[rob love]

As I said in my last post, every job ends up with 3 more. It turns out that one of the switches was only loosely in the transmission while the other switch (the bent one) was in so tight I had to modify tools to get it out. It turns out neither switch was appropriate... the right one was a 60psi normally open, so the light would have come on at 60. The other one was 4 psi normally open, so the light would have come on at 4 psi. Both switches were two terminal, so required a new wire running from ground to one post of the switch. Both sides merely had the ground wire end taped to the wiring conduit, which is by no means a suitable or dependable ground.

Going by the Honeywell chart for Hobbs switches, a suitable switch would be a Hobbs 78154 (60 psi) which is a single terminal normally closed. An alternate will be Hobbs 76064 (60 psi) normally closed 2 terminal, which would require a ground wire to be provided. In the case of your lower pressure transmission Malcolm, these switches can be adjusted, so you could lower the pressure to 54 and get rid of the annoying warning light as well as prevent accidental application of the starter. Here is a link to the Hobbs chart on the switches: https://sensing.honeywell.com/index.php?ci_id=49633 Watch out as there are variations to the pressures within each part number given, which will be reflected in two extra digits after the part number. They are easy enough to adjust however by removing the little rubber plug at the top and turning the screw in or out as required.

I noticed that the two transmissions in the tank are slightly different. One has only 1 port in between the two adjustment screws on top, while the other has that port as well as another forward of the adjustment screws. I'll have to investigate if the pressures are the same in both ports, or if I should be using the same port location on both transmissions. Currently the last people used the one center port and one forward port.

I tried doing some of this work from underneath, but it is near impossible. So I am now laying in the turret basket, having removed the appropriate ammunition bin and floor plates. There is no comfortable way for a 240 pound man to be laying in the bottom of the turret basket.

[Malcolm Towrie]

Rob, great info on the Honeywell switches. Just what i needed.

Regarding the two pressure taps on the one Hydramatic, I suspect the one you are seeing that's close to the engine, ie not between the two band adjustment screws, is not a pressure tap at all but the 1/8" NPT cover on an indicating pin that allows adjustment of the front band without removing the side or bottom pans. Check down the hole with a mirror. If you see the top of a 1/8" pin, that's what's it is. This is not a source of tranny pressure, so having a switch on there does nothing.

On the Chaffee, there is another pressure tap on the transfer case side of the two adjusting screws. I think it is unique to the Chaffee. It's an intake manifold vacuum source to the tranny, used as a source of engine load for modifying shift points. I think the M5 uses a linkage to the valve body from the carb throttle plate, same as the Staghound does.

Malcolm