woodmizer lt40HD that intermittently takes off in forward

school of hard knocks · October 16, 2014, 10:52:02 AM

Hi, I've got a 1987 Wood-mizer LT40HD that wants to take off at faster that full speed all by itself. It doesn't seem to matter where the variable speed switch is set when it does this. I recently replaced the variable speed switch but suspect that it might be a faulty switch.

terrifictimbersllc · October 16, 2014, 11:37:32 AM

Hi School, welcome to FF. No idea, sorry. We have a cat that does that. :D I'd call WM electrical support. They're great.

drobertson · October 16, 2014, 12:25:27 PM

not knowing much about your mill, I would too suspect the pot.

Chuck White · October 16, 2014, 12:44:27 PM

Welcome to the Forestry Forum, school of hard knocks.

I would suspect the new switch is bad.

uler3161 · October 16, 2014, 01:23:53 PM

I fixed one on an '88 mill that was doing the same thing. The feed had been upgraded to have a mosfet circuit board in addition to the controller board and both happened to be bad. If I recall, it wouldn't move at all initially. I think the first thing I did was swap mosfet boards with my '89 mill and then it would move but was either no movement or full speed. So I swapped the feed board and it fixed that problem.

WM might be able to help you test it. It sounds like you replaced the pot, so I'd be inclined to think it was the feed board. But if you're thinking the new pot is also bad, I think you can do a resistance check with a multimeter while you turn the knob. I don't recall which two wires to connect to the meter though.

Magicman · October 16, 2014, 02:11:55 PM

Welcome to the Forestry Forum, school of hard knocks.

No way would a new pot do exactly what the old one was doing, so do not throw parts at the problem. Give WM tech support a call.

drobertson · October 16, 2014, 07:09:02 PM

My first thought as well, but, there are issues with wiring and harnesses. It is close to the first link to the control voltage.

MartyParsons · October 16, 2014, 08:15:04 PM

Hello,
1987 was the first year for the hydraulic mill from WM.

You you suggested check the contacts of the drum switch. While the saw is moving forward move the drum switch a little but not enough to disengage the contacts. If you can get it to change speed then remove the cover and check the fingers contacts and the drum where they connect. ( they should be lubed every 50 hours with electric grease). If the contact are worn then you may want to replace the switch.

Also there are brushes in the motor that could be worn and not making good contact.

The power feed speed control is done buy pulsing the ground in forward. The more you turn the pot or power feed control knob the more ground you are giving the power feed motor. So any loose connection would cause the power feed to change speeds with out having any control.

Reverse on the drum switch the current is reversed and the motor will run at full speed on return. The current goes from the drum switch to the power feed motor. Nothing else
So if it works ok in reverse then the motor may not be the issue.

I hope this helps.

Marty

school of hard knocks · October 17, 2014, 08:39:12 AM

Thanks all for your input. The new variable speed switch does function properly most of the time and the old one was definitely hooped so there was some progress with replacing the switch. I have gone over all the grounds and cleaned every connection and have bench tested the motor which works fine. The contacts in the forward/ reverse switch look ok and I did grease the contacts. I will try Perhaps there is something in the feed board that is causing problems but it seems strange to me that the saw can take off at full speed forward regardless of where the variable speed control is set. I was busy yesterday and will be this morning but hopefully will get a chance to call wood-mizer this afternoon.

Knute · October 17, 2014, 09:25:13 AM

I will be interested to know more since I have been having the same problem with my 2010 LT28. I will check my connections as Marty suggested. Mine works fine in reverse also.

sparks · October 17, 2014, 03:36:33 PM

school of hard knocks, Since it is intermittant it will be hard to find but here is a way to eliminate components.

On the main circuit board there is a terminal block with a black, yellow and green wire, Remove the green wire from the circuit board. With a test light, not a voltmeter, clip your test light to ground on put the other end on the screw where the green wire was. With the key on and the drum switch in forward, turn the variable speed switch and the test light should get brighter as you go faster and dimmer as you go slower. If it does not replace the board.

The is a diode in the bottom of the box. It has green wires soldered on one end and a red wire on the other. Remove the red wire from the diode and disconnect the green wire from the drum switch and the power feed motor. Using a digital voltmeter, set for ohms, take a reading from the green soldered wires to the stud where the red wire was. Then reverse your leads and take another reading. You should only read continuity one direction. If you read anything else replace the diode and the Mosfet board. The mosfet is the smaller one that hase black and brown wires connected to it. If the diode check good replace just the mosfet board.

Hope this helps.