millermatic 250

[fred kobs]

Just got to thinking after a fairly lengthy welding session it's been a long time since I heard the cooling fan run on the
old girl. So I took off the tin work and blew the 30+ years of dust out of it and hoped that might do it but then I practiced
for quite a while with it turned up higher than I had in a long time and still no fan. The sensor appears to be tied to a
heat sink mounted to the bottom of the machine and when I was done, I could lay my hand on it. Expected it to be hotter than
it was. So my question is how hot does it have to get and where to kick the fan on? If I remember it would run sometimes when
running thin body material. I don't even want to think what a replacement welder would cost this day in age so i really don't
want to hurt it. Any input here would be appreciated. Thanks in advance. Fred

 

[MarkB_MI]

I pulled up the manual for a <a href="https://www.millerwelds.com/files/owners-manuals/o1308m_mil.pdf">Millermatic 250</a> from the Miller web site. It appears there are two different thermostats wired in parallel that can activate the fan. One (TP2), is mounted on the rectifier heat sink, while the other (TP3) is mounted on the transformer. The manual doesn't specify the temperature at which they turn on, but I'd guess it's between 100 and 200 F.

If you can touch the heat sink, I'd say it's likely you never got it hot enough to activate the fan. Silicon diodes can operate at around 150C (300F) before they fail, but obviously the heat sink needs to be much cooler to keep the diodes below that.

The two thermal switches operate the fan through a relay (CR1). It's possible the relay or fan is bad, so you might want to test them by jumpering around one of the thermal switches. If the fan turns on when the switches are jumpered, everything is probably fine.

I'll bet if you look closely at the thermostats you'll find they're marked with their temperatures. They have different part numbers, so they probably have different switch temperatures.

 

[jon f mn]

You would need to weld heavy steel like 1/4" for quite a while to turn on the fan on those. The 252 I had you had to weld for an hour on heavy plate to turn in the fan. Things like building a trailer where you weld for a bit then tack parts it would never come on.

 

[Loosehandle]

I've had a Millar 250 for 30 years in a ornamental iron shop running .035 wire can count the times the fan came on on one hand.

 

[grizz02]

I have a MM 210 and have laid hardface .035 wire on loader buckets for hours and never had the fan come on thats why you pay more for miller

 

[DoubleO7]

n

I have a white face MM200, 1980 something.
Those were built with fan always on.
The later black face versions had fan-on-demand instead.

You could always jumper over the sensors and run fan full time on yours. Then you know the fan has died when it does not work.

I prefer fan on all the time as I believe it helps blow the idle time shop dust off the internals as well as confirmation that fan is working.

 

[fred kobs]

ok thanks guys i feel better about it now. i know the fan used to come on in the summer time as i was fixing whatever 20 truckdrivers could destroy on a fleet of reefer trailers but i have burned long deep welds as of late and just hadn't heard it in forever. It doesn't owe me anything and its been thru the wringer more than i care to remember. i will try and jump a wire or 2 and see if it takes off. And you are right in the accumulated dust, i am sure the tins have never been off and other than rare shop air thru the vents it never got much attention and wow was it grungy inside! Thank you again. Fred

 

[Hobo,NC]

The fan came on mine yesterday several times while repairing a badly bent up scrape blade. I spec it took several feet of weld to make it come on then after that it did not take much for the fan to come on... I did have it cranked up : )... I did several feet of weld Saturday I don't remember it coming on...

For the most part of what I do the fan never comes on I would make a guess you have to be into it about 10/15 min for the fan to come on...