Are the low speed aux fans susposed for cycle on and off when the a/c is on or is it susposed to run constantly?

The specifics for that depend on the model/year..
..of course.

i have a 89 190e 2.6

The aux fan(s) are connected to two devices.
1. Low speed is controlled by the A\C high pressure switch. This cools down the condenser and should be checked by connecting gauges to your A/C high and low side. If it is 85 -90 deg outside your low speed aux fan should be running all the time. May have bad wire going to high pressure switch or bad pressure switch or bad connections to fan. Or you can simply disconnect the switch and jumper the connectors together and move the wires around to see if you have a bad wire / connection. The Hi pressure switch has 2 wires coming out of the switch vrs the low pressure switch has only 2 connectors and no wires coming out of the switch.

2. High speed for high engine temp.

There are 2 relays behind your fuse box try swaping them to check the relay. The High speed aux fan relay does not operate as much as the low.

You could also be low on freon or have a bad compressor thus your high pressure does not get high enough to active the switch or its too cold outside.

Good luck
Anthony Bridges

i was filling my a/c system yesterday and the fans came on and stayed on and last time they stayed on for longer the 10 mins the resistor burned up.

Well if that is happening that means that you have a load of some sort on your fans.

With engine off key out check to see if blade is spinning freely....

You may need to remove fan, dissasemble and clean.

I had lots of muck in mine. I took apart and cleaned with soap and water it was sooooo filthy and oily. I then used shop air to dry out and Degreaser from a spray can for fine cleaning. Then I put a drop of oil on bearings. Making sure not to get on brushes. Reassyembled and wala magic.

You could buy a new fan but pretty expensive. I would try cleaning and replacing resistor. I bought one at junk yard, with a lot of other stuff for 5.00. But if you just want the resistor might cost 20.00.

GOod luck again
Anthony Bridges

ok thanks.