Staghound Hydramatic Transmission Question

[Malcolm Towrie]

I'm working on a Staghound. Both Hydramatic transmissions have a fitting on top of the transmission that provides a pressure tap for the output of the two oil pumps. The fitting is located between the two indicating pins for band adjustment.
These taps are each connected to a pressure switch with a set point of 60 psi. This is identical to our Chaffee tank, where the pressure switches are used to drive warning lights on the instrument panel which light if the transmission line pressure drops below 60 psi. They also prevent each engine start switch from working on the basis that if the pressure switch contacts are open, ie pressure is greater than 60 psi, the engine is already running.

However, my problem is the only manual I have, TM9-741 (I can't find any manuals specific to engine or transmission overhaul) makes no mention of these pressure switches, and there are no warning lights on the instrument panel for low transmission pressure. And there's no evidence of added panels with warning lights.
Was this a later modification? What am I missing here?

Perhaps the pressure switches' only job was to prevent starting an already running engine?
The wiring on this Staghound is rough so it could be the warning lights were in a separate panel that no longer exists.
Thanks,
Malcolm

[tankbarrell]

Malcolm, you need TM9-1741B which covers transmission, transfer case and steering.
Copies available here.
https://www.greenmachinesurplus.com/...ering-83-p.asp

[marco]

Hello Malcolm,

The transmission oil pressure warning lights were used in Staghounds build after mid-April 1943, the first one being Ordnance Serial 554.
Before that, these warning lights were not used.
The lights only have a warning function and are not wired in other electrical circuits like in the Chaffee.

In the Staghound the lights are useful however when the car has to be reversed.
Reversing a Staghound with only one engine running will damage the transmission of the stalled engine.
With a headset on, it is difficult to hear if both engines are running but the lights enable the driver to visually check that both engines are running before shifting in reverse and actually starting to reverse the car.

Stag engine idling speed should not be above 600rpm, preferably 500rpm, otherwise shifting the transmissions in reverse at least produces some frightening noises or just isn’t possible at all.
This low engine idling speed gives an extra risk of a stalling engine, especially when not warmed up thoroughly.

Marco

[Malcolm Towrie]

Thanks for the manual source, Adrian.

And thanks for the story behind the pressure switches, Marco. Very interesting. I'll make sure the switches are hooked up to warning lights for the driver.

I'm also going to install two of those cheap eBay tachometers that sense the HT signal from a plug wire on each engine, as it seems ridiculous to me the driver had no way of monitoring the throttle synchronization of the individual engines, or the synchronization of the shifts between the two Hydramatics. It was seeing the huge shifting rpm discrepancies on the installed tachs on the Chaffee that tipped us off that at least one of the trannies was bad.

Malcolm

[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