1999 Silverado 1500. Ac works fine when cooler but when weather is 75+ the a/c stops works after about 30 minutes. Noticed the radiator fan speed doesn’t increase when ac is on or ever. It seems to stay at the same slow speed. Replaced the fan clutch. Checked the relay by swapping it out with I believe what was the headlight relay with the same part number and seems ok. Any ideas what to check next?

Also, no engine cooling problem.

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With a mechanical fan, the fan speed will never exceed the engine speed. Theoretically it will never match the engine speed except in rare circumstances and when the fan clutch has failed and seized. This is usually more than enough to draw air through the condenser to keep the AC cooling. With yours, I believe you have an additional fan for the condenser, right?

If so, I believe it will come on when the compressor cycles on. It is a 2-wire unit so you probably get either one or two speeds out of it. Since it has one relay, I'd guess it only runs at one speed unless you've heard it operating faster.

​​​​​​​​​​​​​​Sounds like the AC needs to be recharged with some R-134. It probably has a slow leak. The issue could also be the low-pressure switch going bad.

You have a mechanical fan. What relay are you replacing?

I was wondering that too. I looked it up on Rockauto and there is a condenser fan listed for this vehicle. Maybe GM did a supplementary fan for some reason . . . ?

My 2003 YukonXL doesn't have that, and does have a lot more space to cool, so maybe they figured out a better way after this truck was built.

Does that have a separate condenser and fan? If there'* a condenser fan there will be a relay for it.

It looks like a pusher fan, because it has a grille and no shroud:



It'* 2-wire, so I'm guessing it runs at a single speed. I don't think they were doing any PWM variable speed magic in 1999.