Key Switch and Coil Fire Problem

[RTR]

I’m curious as to what causes a problem I am having with a Massey Ferguson 135 gas tractor. I’ve also had the same problem with a Farmall Cub and a Farmall 140 tractor ......all of which are key start systems. When starting the tractor and turning over the starter with the key switch, The switch is not letting fire/battery power to the coil. When the key/starter is released, it allows fire to the coil then. Basically when the key is on the “on” position you have power to the coil but in the momentary start position it doesn’t have power.

Is this a simple wiring issue I have made a mistake at, or does this mean the key switch is bad?

 

[gene bender]

yes your wiring is wrong need to check and see why there are no volts to the coil when engine is cranking

 

[Russ from MN]

If your having the same problem with 3 tractors it sounds like it's a problem you created.

 

[Janicholson]

Disconnect the starter switch from the Key wire that makes it crank. Put a volt meter on the coil (key side) turn the key to start and see if you have voltage there. No volts, a poor conduction wire, or bad switch. Voltage there (yes) then try it while cranking for 3 or 4 revolutions. Does the voltage drop to 8 volts or so? if so it is too low to fire a 12v coil adequately. From battery cables, to connections, to excess starter draw, the issue needs analysis. Jim

 

[Paul Janke]

If the ignition switches have terminals for battery, starter, ignition, and accessory, then the problem could be that the ignition is on the accessory terminal. A lot of those switches shut off the accessories while cranking in order to reduce the load on the battery while starting.

 

[Steve@Advance]

Did this happen on it's own, or has the switch been recently replaced?

Either way, easy to test with a volt meter.

Disconnect the small wire from the starter solenoid.

Put the volt meter on the + terminal of the coil to ground.

Turn the ignition switch to the on position, should get near battery voltage on the meter.

Slowly turn the switch to start position, watch the volt meter closely. The voltage should not vary. Try it several times, hold the switch in start position and move the key around, it should maintain a steady battery voltage reading.

If it drops, every time, and there has been electrical changes made, possibly the switch is wired wrong. If the reading is erratic, or no voltage to the coil in start position, the switch is bad.

That is a common problem, especially if the tractor stays outside. Be sure to use a tractor or marine switch, as automotive switches are not water and dirt resistant.

If that test passes, try an actual cranking voltage test. You will need an analog meter.

Pull the coil wire so the engine won't start. Take a voltage reading at the coil while cranking. It will be hard to get a steady reading, but the volts should stay above 8-9 volts. If it drops below that, load test the battery. If the battery tests good, could be a bad connection. Feel for hot connections, discolored, corroded, connections. Some of those grounded the battery to the sheet metal, a common bad connection. Feel for warm cables, if any battery cables have been replaced, be sure they are big enough gauge. Many times automotive cables are too small, more insulation than wire.

Finally check the condition of the starter. If the bushings are worn it will draw too many amps, nothing left to fire the coil.

 

[Jo Bird]

Your 135 has a neutral safety switch, make sure it is working.