Thursday, August 7, 2014

Tecumseh TVS90 Kill Switch Operation

The kill switch on a Tecumseh TVS90 lawnmower engine resides under the engine's flywheel, where it works in conjunction with the engine brake. A closed kill switch grounds a point in the ignition system, killing engine operation. Here's a view of the switch in its closed/off/kill position. (I've whitened the switch's 'pigtail' contact to make it easier to see.)

And here's a view of the switch in its open/on/run position.

The switch itself is rugged and adjustment-free; its only real enemy is rust. (More on that later.) Following are the likely failure modes/mechanisms:

a) Engine Won't Start -- No Spark

A stretched/binding control cable can cause this when the cable develops insufficient travel to open the switch. The best solution is replacement of the control cable. In a pinch, it's possible to 'adjust' the switch to accommodate the impaired control cable by bending the switch's wire 'pigtail' contact. That's not a method that I recommend, but I have done it and gotten away with it.

b) Engine Won't Stop When Bail Is Released -- Binding Control Cable

This is an unlikely failure mode because of the switch's design -- the switch doesn't need much control cable travel in order to close. However, it is a possible failure. The only practical solution is replacement of the control cable; there's no good work-around for a cable that's binding that badly.

c) Engine Won't Stop When Bail Is Released -- Rusty Switch Contacts

I've actually seen this failure happen -- a kill switch with contacts so rusty that it couldn't make, even though it would physically 'close'. The solution was to scrape/file the mating switch contact surfaces. That got the switch working again.

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