1949 Desoto - Stalling on Hard Braking

Hi all, (1949 Desoto S-13)

Does anyone have an idea or experienced a MOPAR flathead six that stalls under very hard braking from say 25 or 30 MPH? This car has a flathead six, a fluid coupling, and a m6 Trans. The dashpot magnet is working, however, I cannot tell if the inside of the carburetor part of the dashpot is working.

After a day of testing...we ruled out:

1. Carburetor float bowl or sloshing.

2. Intake gasket leaking.

3. Electric Dash Pot - Anti-stall.

4. Carb switch.

5. Vacuum Booster.

6. Idle speed.

What it looks like is the Interrupter Circuit on the transmission. When I take off in high range and keep the car below 25 MPH so the governor does not kick-in...and it stays in 3rd (or 1st in low range)

...then I hit the brakes hard the car stops and the engine is running fine.

If the car is allowed to shift into high range (2nd or 4th gear) and you hit the brakes hard, the engine stalls just as you come to a stop.

Something in the interrupter circuit on deceleration is causing the interrupter circuit to ground the coil longer that a split second.

I tired two NOS switches, as well as an old beat up one. The NOS switches do it every time. The old switch, which does not always make contact, when used allows the car to stop without stalling then I get the big clunk as it downshifts very very late.

I am wondering if there is an issue with the piston or spring in the trans.

Perhaps the forward momentum with hard braking due to the power assist 4 wheel disc brakes is causing the piston and rail to push back to the rear too slow and hold the coil ground too long ?????

There is an intriguing statement on one of the manuals on the imperial web site that also makes me wonder if it could be a lack of current problem.

How the Hydraulically Operated Transmission Operates (Session 23) from the Master Technician's Service Conference

This car is a frame up with all new wires, good grounds, and the like. I do run a set of modern Russell spark plug wires and the issues of reverse current flow and high resistance in the secondary ignition system may be playing a part.

Any thoughts from M6 Trans experts welcome.

Thanks, James

These may not be your problem but they were mine.

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I had the problem of the engine dying when I stopped quickly and by increasing the idle, I had a temporary fix. This progressed and got worse to the point of just slowing down on an interstate ramp and it would die. Also the engine lost power accelerating up a hill.

The problem was the exhaust valves were not closing completely. The problem was identified by doing a compression check. Three of the 6 cylinders were very low. Had a cylinder checkdone on all 6 to find the problem was not rings but the valves.

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I had another car with the valves adjusted too close. The car ran good when cold but after a mile it lost power to the point it would not pull ahill. The exhaust valves over 2 years were burnt and had to be replaced. The valves are no longer quiet but have a light tap.

Turns out it was in fact the interrupter switch. The piston for some reason was retracting slow and holding the switch open too long thus killing the engine. After about 50 miles the problem went away. The piston must have freed up.