Countax blades won’t engage?

Diagnosing and fixing the case of an intermittent blade drive fault on a Countax C300H

I had a call from the owner of a Countax C300H – “…the Countax blades won’t engage.” The story went on to reveal the classic intermittent fault, where sometimes they would work fine and other times they wouldn’t turn on. I found the diagnosis and repair quite satisfying so I thought I’d write about it.

The blades are driven by a belt and pulley connected to the engine crankshaft via an electromagnetic clutch. This enables electronic control of the blades along with all the necessary safety cut offs.

Picture showing test driving the Countax C300

Working out why the Countax blades won’t engage

What could it be?

Thinking through the potential causes, this could be a problem with:

  • Electromagnetic clutch.
  • Wiring.
  • Safety cut off system (e.g. seat sensor).
  • Circuit board.
  • Blade drive switch.

I decided to start at the blade drive clutch and work backwards. Why? Well the blade clutch and its wiring are underneath the tractor, amongst all the grass, twigs etc, so I thought they were the most susceptible to damage.

Visual inspection

I like to start by having a good look round things. The clutch was caked in grass, as were the 2 wires going to it. Despite this it all looked intact.

Picture of Countax C300H electromagnetic blade clutch

Next I checked the wiring to the safety cut off switch under the seat. This is designed to stop the blades spinning if the driver gets off the seat. This also looked fine and the switch seemed to be working.

You might be wondering about fuses. There are only 2 on this tractor, one for the charging circuit and one for the ignition, lights and safety mechanisms. If a fuse had blown, we’d expect to see more problems than just the blade drive.

Checking the voltage

With no obviously loose wires at the blade clutch end, I decided to measure the voltage across the 2 wires, at the connectors to the clutch. With the ignition on, but the engine not running, the blade switch can be moved to the ‘on’ position and if all is well, you can hear the click of the electromagnetic clutch engaging.

If you wanted, you could further verify this by applying battery voltage directly to the clutch and listening for it engaging.

The voltage was all over the place, as we might expect for an intermittent fault.

More wiring checks

Having found this variable voltage, I traced the wires all the way back to the control board, giving them a good wiggle as I went, to see if I could influence the voltage reading at the clutch. When I got to the circuit board behind the ‘dashboard’ I did start to see some relationship between my wiggling and the voltage changing.

Picture showing the circuit board location on the Countax C300H

Inspecting the circuit board and connector

I felt I was onto something so had a close look at the circuit board and connector. I found that the white wire at the far end of the connector was under more tension than was ideal. This had caused it to come loose in the connector.

BUT…that was not the only problem. There was also a dry solder joint where the connector socket was soldered to the circuit board. Repairing the wire may have improved things, but it wouldn’t completely fix the problem.

Picture of Countax C300H circuit board connector showing why the Countax blades won't engage

Fixing the cause of the Countax blades not turning on

Refitting the wire to the connector

As you’ll see from the picture, the connector is a ‘punch down’ type, where each wire is pushed down into the connector and metal contacts cut through the insulation to make contact with the copper inside. Unfortunately it wasn’t quite as simple as just punching the wire down again. The end of the wire was somewhat corroded, so that section needed cutting off. This would make the wire too short to reach the connector.

I chopped out the corroded section of the wire and soldered in a new (and slightly longer) piece. This enabled me to make a new joint into the circuit board connector.

Resoldering the dry PCB connection

This was quite a simple process. I just fired up my soldering iron and resoldered the dry joint on the circuit board.

Have these repairs fixed the Countax blade drive problem?

Yes they have! I tested it again without the engine running first. The voltage at the blade clutch was much more stable and much closer to battery voltage. More importantly, there was a reassuring and reproducible clunk from the electromagnetic clutch with each operation of the blade drive switch.

Next I put everything back together and tested it again for real. All was well, with the blades engaging first time, every time. And no cutting out whilst mowing, as there had been before the repair.

All in all this was a very satisfying repair, with the only costs being some solder and a ~5 cm piece of wire. I hope it helps if your Countax blades won’t engage!

I’ve also written about servicing one of these tractors, and replacing the grass collector brushes.

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