A/C fan oddity

[TJScott]

2003 TJ sport 4.0 5sp manual soft top.
Not even sure how to describe this. It's so odd.
The last couple of days, it's been hot. So, I'm running the A/C of course. Fan all the way high or a step down from max. It seems when I hit 60, the fan speed drops to about 25% of what it was, I can still feel cold air coming from the vents. It comes back to full when my speed drops to about 50. Not immediately to 100%, it slowly comes back up. I tried turning off the A/C and just running the fan, same thing. Recirculated air or outside air, it's the same at about 60. It's impossible to listen to the fan with a soft top at that speed. I haven't done anything yet as far as troubleshooting. My first thought would be to hook up my DMM and monitor the voltage throughout the event on the plug to the fan. Actually, I guess my first thought is to ask here. Any thoughts?
Thank you for any assistance you can provide.


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[Rob5589]

Does the fan slow down or does the air volume decrease? If it decreases, there is likely a vacuum leak. Does it also start coming more out of the floor?

 

[TJScott]

Really tough to tell about the fan speed. Too loud inside at those speeds to hear the fan. I did just read someone talking about vacuum issues causing the vents to be changed from open to closed, or something along those lines. I can't tell if there was an increase in floor air. I think in recirculate it does vent and floor. Not sure where the vacuum is, I'll have to look. Too hot out at the moment though.


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[Rob5589]

You will find the vacuum line along the top of the firewall. One line will run into the firewall around the ac lines. It terminates under the right fender well into a vacuum module. Mine did the same thing, I found a few spots where the hard line had rubbed through causing a leak. I bought some vacuum line and spliced it together. You can buy a new vacuum line set up, if you can find one, and they aren't cheap. My fix was about 5 bucks.

 

[TJScott]

I went out this morning to have a quick look. And it was quick. I found the line. I couldn't find any obvious signs of a hole, or an area where it had been rubbing. There is a check valve at the junction into the firewall. Pulling the line from that check valve leading into the firewall, I heard the pressure leave. The connections look really permanent on the other sides. It's possible that vacuum increases at higher speeds and it can't maintain it. But, the truck sat since Friday and didn't lose pressure, that tells me it doesn't have a leak. Or at least one that's obvious. It appears I will have to drive it and see if the air flow to the floor increases as the vents lose flow. Thank you for your help.
Edit: I took it for a drive today. I ran it up to 75, ready to check the floor vents for added air flow. Of course it didn't do it. I'll see over the week how it does.

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