Hi, One day last year, I started my 1998 C-1500 350 2WD, and surprise - the a/c blew hot air. I said a bad word. I only use the truck ...

Hi,

One day last year, I started my 1998 C-1500 350 2WD, and surprise - the a/c blew hot air. I said a bad word. I only use the truck to go to the lumber yard, so I just let it be - until today, when it was 98F and I got stuck in a 10-mile traffic jam (I said many bad words).

Dashboard switches appear normal, but a/c clutch does not engage.

Troubleshooting so far:
1) 10A fuse appeared OK, replaced it anyway - no change
2) Swapped a/c relay in fuse box with starter relay (same p/n) - no change
3) Removed connector from "low freon sensor" - no change
(Is this properly called a "cycling switch"?)
4) Installed paper clip in said connector - no change
5) Removed connector from clutch, and hooked 12v and ground to clutch - engages fine

Using a test light
6) Clutch connector - one side goes to ground, other side goes to "nothing"
7) Low freon sensor - one side goes to ground, other side goes to "nothing"

8) a/c relay socket - both front contacts had 12v
Do not know whether the fan switch was on or off when tested. Not sure what the function of the relay is - perhaps it closes when the fan switch is on? Will check tomorrow morning.

Dashboard controls are three rotary switches - fan, temp, where; and two buttons - recirc (w/indicator lamp) and a/c (w/indicator lamp).

My guess is that the problem is in the dashboard controls. Before I rip the dash apart, is there anything I overlooked checking? What will I find inside the dash? Is there anything I can test or is it just some black box "module" that I need to replace and keep my fingers crossed?

Thank you for your time,
Dave Hennessey

hi Dave, and WELCOME!
I hate to state the obvious....but could it be low on freon? If it's low enough, the compressor will not even kick in. Just a thought before you start tearing things apart.

EK,

Thanks. I only have an old R-12 gauge set, so I can't actually test the refrigerant level. But I thought bypassing the "low refrigerant sensor" (connected to a refrigerant line just behind the compressor) momentarily would tell me if that was the problem.

But maybe it's not as simple as that. Back in the 1970's and 80's, these were simple pressure transducers that closed a set of electrical contacts if the pressure was above XX psi.

Have these things changed since then?

Dave

Ooops. I missed your point #3, from the first post. Sorry!
Not sure if they have changed. I would imagine one of our resident mechanics should be stopping by this post anytime to offer some assistance. In the meantime, stay cool!

After scanning the web this morning, I found out that the sensor at the back of the compressor is really a high pressure sensor - cars didn't have these 30 years ago when I was a mechanic.

The low pressure sensor is back on the receiver-dryer near the firewall.

When I jumpered the connector at the low pressure switch, the clutch engaged, telling me that I'm low on refrigerant. So now I know which direction to go.

Dave

You could spend $40 @ Advance & get the 134 kit w/ leak sealer & a gauge kit.. Verify that there's pressure in the system before adding freon. If it bone dry... you'll have to take it to an A/C shop. Or ... skip the first part & take it to a shop..

Good luck.

I would be willing to bet it is the climate control head/switch panel my 95 did the exact same thing a couple months ago.symptoms of mine were... would only blow out at floor vents and a/c and recirc buttons would not function however fan speed switch did work .I found the factory climate control head on ebay NIB for 113 shipped