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

[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 the 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

[Malcolm Towrie]

Rereading your post, you have the extra port FORWARD of the adjusting screws on one tranny. That would match the vacuum port location on the Chaffee. Maybe you have a Chaffee transmission? In which case, you won't have a complicated linkage from the carb, just a tubing line to it from the intake manifold.

Malcolm

[rob love]

Malcolm: Of course it is on the transmission that is still hiding under the ammo bin/cover, but I will give it a look. The linkages appear to be the same as on the other transmission. I may put a gauge on each plug, and run up the engine and see what each shows. I'll try and get a decent shot of the transmission in question tomorrow and get your opinion.



Acklands Grainger carries the required hobbs switches, and if you get any kind of a corporate discount, the price is quite favourable. Unfortunately for me, the local Acklands Grainger was closed up a couple years back and now the nearest one is Winnipeg.



This thread turned out to be quite timely for me....sorry for the hijack.

[rob love]

Luckily for me it turned out to be the transmission that was accessible, so today I installed a 90° elbow and a 60 psi NC switch. I am now in the process of installing the second switch on the other transmission....it's a lot tighter working on that one.

For switches I used a pair of 60 psi switches from the MLVWs...they were rated for air or liquid, and have the bonus of even being adjustable if need be. NSN is 5930-00-434-5441. I modified this one for one wire, the other will remain 2 wire.

[marco]

Rob,

The "old position of sending unit" actually is the hole were the indicator rod is visible when adjusting the front band.
No oil pressure there...

Marco

[rob love]

Thanks for the confirmation Marco. I got sucked away to other projects the last day or two....hopefully tomorrow I can finish up this part and run up the vehicle to confirm the warning light operation. Then I can get back to buttoning up the panels below the turret and some items on the turret basket.

At that point, the vehicle should be able to move under it's own power to the other building, which will be all I am after at this point. I have 5 or 6 other vehicles/guns to prep for VE day. My preference is to drive them in as opposed to pushing them in....we'll see how it goes.

[Malcolm Towrie]

Your setup is the same as the Staghound, see photo. Nearest the engine is the cover for the front band adjusting indicator pin, as Marco says. The cover's missing in the photo. Then the front band adjuster, the pressure switch, the rear band adjuster, and the rear band adjusting pin.

GM had designed the Hydramatic just a few years before and, boy, did they modify it on the fly. First no band adjusting pins, which I think you said is like your other tranny. This requires removal of the tranny to adjust the bands, which was a routine task!
Then two pins to allow external adjustment of both bands.
Then they made the rear band self-adjusting, so only the front band had the pin. The Chaffee has this setup.

I don't know how far you are getting into the trannies on the M5, but the adjustment of the linkage between the carb and the tranny is critical. The tranny needs to know engine load so it can delay shifts, increase pump pressure, clamp bands and clutches tighter under load, etc.

Malcolm

[rob love]

Thanks for the explanations Malcolm. Am I OK with merely having moved the plug from the pressure port over to the hole for the pin? The pin does not end up protruding up, does it? When I was looking onto the hole, it didn't look like it could. Edited to add: instead of being so lazy, I read the manual. The Stuart manual covers both type of transmissions. The pins would come up to the level of the boss if properly adjusted, so to that end the plug I moved over from the center port will not be suitable. I am going to probably use a short 1/8 pipe and a cap.



I'll plan those linkage adjustments for when it comes back from display in about 9 months. I would like to think something is right on this vehicle, but as I mentioned before, for every job done 3 more seem to show up. As to the transmission, it was out about 1 mile ago (which is also about 7 years ago) so it would be a nice thought that it was adjusted at that time, but he is no longer around to ask. I did see blue RTV on the side pan when I removed one bolt to use as a ground for the switch.



I managed to get the last switch installed and wired up at the end of the workday. I went to try it out at the control panel, but the bulbs are all removed, so that will be done tomorrow morning.

[rob love]

Originally Posted by Malcolm Towrie

. This requires removal of the tranny to adjust the bands, which was a routine task!
Then two pins to allow external adjustment of both bands.
Then they made the rear band self-adjusting, so only the front band had the pin. The Chaffee has this setup.

Attachment 111940

The manual covers adjustment of the older transmission. It is just a matter of torqueing the screw to a certain torque (engine running for the rear band) and then backing off a certain amount of turns. It is something I'll try in 9 months when I get the tank back to here.

I ran the vehicle up this morning and both warning lamps are staying on. Before I get the pressure gauge out, I'll check the levels of both transmissions. They both leak, and the vehicle has not been fully operated in 7 years or more.

[rob love]

Man, the previous work on this vehicle is starting to annoy me. I ended up adding 3.5 quarts of fluid to the left transmission, and while the transmission was giving forward movement, the warning light would not go out. So I removed the sending unit, added a gauge, and on start up, and at idle, the transmission was reading almost 90 psi. Nothing wrong with that. That's when I noticed that even though the wire was disconnected, the warning light was still on. It turns out that on top of two wrong (and different) pressure switches being initially installed, with one of those on the wrong port, they had wired the two wires to the wrong transmissions. Well, except for the wires they left hanging, but process of elimination kind of dictated where they were going.

Had I started both engines at the same time, everything would have looked fine.

Anyway, I switched the two wires from left to right, and all is well.

I do have a bit of frothing on the left transmission, and while that panel was open, I noticed that someone had wired both the engine oil pressure switch and the temperature switch together. That will explain why the temp gauge climbs to halfway on start-up. The second wire for the oil switch must be buried in the conduit.

I am going to close up the access panels, re-install the ammo bin, and worry about the wires on the engine next week. I think I have had enough Stuart tank for one week....it's time to let all the bruised legs heal for the weekend.

[Malcolm Towrie]

Well spotted on the pin, Rob. The pin is rigidly attached to a tang on the front band, so restricting its movement would prevent full band clamping, unless the pin is sized to buckle or something. You can see the pin and the tang in the right hand window of Marco's photos.

The Staghound engines had pressure and temp senders for engine oil and coolant installed but they hadn't been wired to the gauges on the instrument panel. I can see why. The senders were modern design which are totally incompatible with the old school radium-illuminated gauges on the panel. The original temp gauges need a 25 to 250 ohm variable resistance from the senders, but modern senders work more on the 2500 to 100 ohm range. Same with oil pressure. Modern senders don't work with 1940's gauges. So I gave up on the gauges and installed warning lights on the I/P, using modern switches set at 250F for coolant, and 5 psi for oil pressure.

I figure a bright red LED light will get more attention than a gauge going off-scale high or low anyway.
Malcolm

[rob love]

Stewart Warner still makes sending units in the ranges you need for the original gauges. I was just researching the subject tonight, as I need to replace/match up some of the gauges to their sending units. They have a very good webpage showing all of their gauges and sending units. That said, their prices can be intimidating if you are not careful about where you order from.

[Malcolm Towrie]

I checked out the Stewart Warner site but it seems the temp sender info doesnt give any degree-to-ohms calibration info (at least I couldn't find it), so they can't be matched to an original gauge, just a new SW gauge. Which is still a good option.
Also, none of the pressure senders, which do give psi-ohm calibrations, match the 0-30 ohm/0-60 psi calibration I found the original gauges have.

However, I did find this good information on all the NAPA Echlin senders and switches which allowed me to find a 5 psi low oil pressure, NC, 1/8"'NPT warning switch and a 250F high temp switch.

In hindsight, maybe I could have found senders for the original gauges with this list.

Malcolm

https://s3.amazonaws.com/pageturnpro...uide112011.pdf