How to test the anti frost switch?

[Grusvag]

Anyone know how to test the anti-frost switch for the AC system? Also, how bad would it be to bypass it by connecting the two wires together?

Thanks.
John

[Geoff Weeks]

You test with a thermometer, it should open before 32/0 deg and close before 40 deg.
It is there to prevent condinsation from freezeing on the eveaporator and preventing air flow, Why would you want to bypass it? to test, turn on the a/c and check for power on both sides of the switch. Once the evaporator cools to the set point it should open (power to only one side) and shut off the compressor until the evaporator warms enough to melt off any frost.
If the compressor is running and you don't have reduced air flow out the vents or visiable frost on the eveaporator core (look thru the air intake) it is likey ok. It must be closed for the compressor to turn on.

[Grusvag]

Where do you measure the temperature, at the vents in the cabin?

My AC cuts out sporadically and I thought if I just bypassed the anti-frost switch I could see if that was the problem.

[Jim Mesthene]

If you bypass the switch, the A/C will stop working, whether that's your problem or not. With the switch bypassed, the evaporator turns into a brick of ice in short order. Don't forget the low pressure switch, high-temperature switch, wide-open-throttle switch, and manual switch.

With just a voltmeter you could test it electrically and avoid all that wire crossing. There's a wiring diagram in the Stickies; Shop Manual section 3.2.
It's easier than guessing, especially if it's an intermittent problem.
Harbor Freight sells an adequate volt/ohm meter for about $4.00.

[Grusvag]

Thank you.
Where do you measure the temperature?

[Geoff Weeks]

In the evap core itself, right next to the switch probe.

[Grusvag]

Thanks.

Anyone know what the full troubleshooting procedure would be for an intermittent ac problem? Works fine for a while, then blows room temp air for a period then starts working again.

[Jim Mesthene]

Step 1 is to look at the evaporator when it stops working. If it's a solid block of ice, the anti-frost switch is NFG.
If it's not a block of ice, you have to test each item in the circuit. I haven't seen a c900 in years, but my best recollection is that the power from the switch goes to the anti-frost switch to the low pressure switch to the high-temperature switch to the relay. The ground for the relay goes through the wide-open-throttle switch.
I doubt that that's either very accurate or very wrong, but you could look up the wiring diagram and get it right.

[Grusvag]

Thanks everyone. Will slog through it and see if I can find the culprit.