Well-running engine stalls as soon as semi-auto is put into forward gear

[DeS-11]

This just came up, so I wanted to get some of your thoughts before I go further into it (and I won't have time for a little while yet, anyway); this is relating to a '47 S-11 with the semi-auto transmission. The engine turns over seemingly normally (for 6v anyway) and starts after several revolutions, idling normally. You can raise the RPM and all seems smooth, with no missing or hesitation; you can go into reverse gear, and the car moves normally. But when you go into a forward gear (specifically third in the 3/4 range), the engine quits immediately as if the ignition had been turned off, but will start back up right away. This happened twice in a row, both with the lights on, it should be noted; it hadn't happened previously, with or without the lights.

 

So what are your thoughts? I was thinking a possible issue with the idle speed being too low, so I'll see where that is (although I don't know why it wouldn't have been an issue before). Could it be a dying coil, or might a weak battery lead to this? The fact that the lights were on when it happened, and I've mostly only been doing very short drives recently (short enough to where the battery probably isn't being charged enough) led me to think that. The strange part is that it runs well in neutral, but then just cuts out altogether when the driveline load is applied--and the carb dashpot wouldn't have anything to do with this as it occurs when taking off from a stop, not coming to a stop, correct? Any ideas are welcome, especially from anyone who's experienced this with one of these transmissions.

 

Edited August 20 by DeS-11

[Doug&Deb]

How does the wiring to the transmission look? I have the later M6 transmission and bad wiring caused similar problems. I recommend the Imperial club website. They have tons of info about the transmission including the troubleshooting manual that’s available for download. Is it shutting off while the clutch is disengaged or as you engage it?

[DeS-11]

I'll have to check that wiring and see how it is, as that's something I've not done yet. It shuts off after you engage the clutch, as soon as you go to step on the accelerator--basically as soon as the forward gear engages--a frustrating problem, to be sure. Thanks for the Imperial Club service information, as well; I read through it and it should be a good resource to have regarding this transmission. The principles seem straightforward enough, but it looks like there are a lot of small moving parts to go bad, especially after nearly 80 years. 

[Doug&Deb]

The trans is pretty bulletproof. After all they used it with the hemi for a few years so our 6 cylinder shouldn’t be a problem. Problems are usually something simple. 

[Roofus]

Untimely kickdown start on the carburetor ?

[keithb7]

If I recall there is a circuit on these cars: When in top gear under 25 mph, you mash the throttle to the floor, it briefly grounds the ignition coil. This allows for the shift to take place by taking the engine load off the transmission input pinion.  I’d be looking into that portion of the wiring. Test for a possible short to ground. Check the rotating silver points switch on the transmission centrifugal weights too. 

Edited August 21 by keithb7

[DonaldSmith]

Keith is on to something. My S-11 Shop Manual has 3 pages of wiring schematics for the M-5 transmission. 

The third page is "Accelerator Pedal Kickdown and Ignition Interruption".  Hmmmm.

A wire from the grounding side of the coil goes to the PRI contact of the Kickdown Relay, where all sorts of magic happens, like momentarily grounding the coil when kicking down. 

 

Check the Shop Manual, or the info from the Imperial website.  

  

[DonaldSmith]

OH, there's also an interrupter switch in the transmission, to ground the coil momentarily during the normal shifting.  A work of engineering genius. 

But sometimes it's hard to understand genius.  

[Doug&Deb]

These things can be finicky but once sorted they’re pretty reliable. Remember they weren’t designed to last for 80 years lol.

[Eneto-55]

On 8/21/2024 at 3:10 PM, Doug&Deb said:

.... Remember they weren’t designed to last for 80 years lol.

[DeS-11]

Thanks for all of the ideas; hopefully I'll get some time soon to look into it, and I'll let you know what I find. Hopefully one day I'll be able to actually drive it for some kind of distance, as well. . . it's supposed to be fun, so I hear, and I keep reminding myself. . .