Staghound Hydramatic Transmission Question

[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

[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.