2 things happening right now, could they be related? When I turn the truck on, the ac only blows through the defrost vents, if I turn the ac off and back on it blows through whichever vent I select. Also, abs light and brake light are on. No codes. ABS module was sent off to Atlanta and rebuilt a few years ago. New front  brake lines 500 miles ago. GN horse trailer currently hitched up, I unplugged the trailer connector and still getting abs/brake light. 4x4 works. Any ideas on a good place to start.

 

 

edit*- took the truck/trailer down the driveway with the connector unplugged and that cleared the abs light. Ac issue still present.

Edited May 14, 20222 yr by Andyba20

More questions than answers.  

 

When and under what conditions did you first notice the ABS and brake warning light?  the abnormal AC operation?  How many times has this happened?  Are the symptoms consistent?

 

Noting that when the trailer was disconnected and the truck was driven and the ABS and brake light went out may not have any value.  When an ABS lamp is lit, it will not reset until the truck is driven a certain distance above a certain speed (usually a short distance and less than 25 mph).  If this problem is intermittent, the light could go out on its own, regardless of the trailer wiring connection.

 

43 minutes ago, Andyba20 said:

When I turn the truck on, the ac only blows through the defrost vents, if I turn the ac off and back on it blows through whichever vent I select.

 

This condition could occur from a moderate vacuum leak or a lack of an electrical signal to carry out the command.  If you turn of the engine and restart the engine two or three times, do the symptoms repeat?  Thoroughly check for vacuum leaks.  If possible, use a vacuum pump to supply the HVAC vacuum line only and see if the problem persists.

 

- John

The abs issue happened after the truck sat for 2 days with the trailer hooked up while I was cleaning the roof. Got in to move it and the lights came on. Off for now, and I’ll report back if it comes back on.

 

The AC: I started it back to back 3-4 times like you said, and it happens each time, though I will say it doesn’t even have to switch to “off” I can move it to any vent position and it will work correctly, but upon initial start it still only blows from the defrost until I switch positions. It seems like it might have done that the other day (starting a week ago), but I was distracted and not paying much attention. You’re probably spot on about the vacuum lines. I guess it’s that time.

A hand operated vacuum pump is a great tool for diagnosing vacuum leaks.   You should find a small vacuum manifold with many connections just above and nearby the top of the valve cover.  Locate the line that goes to HVAC unit.  Unplug that line and connect the vacuum pump to it.  Operate the vacuum pump.  After several pumps the vacuum should easily rise to 20" HG or more and hold when you stop pumping.  If you cannot make this happens, you probably have found your problem.

 

All vacuum operated HVAC systems default to defrost (for safety) when the vacuum supply is lost.

 

As I am writing I just realized that in my first post I mentioned there could be an electrical problem as well, but after thinking about that - it is not the case.  All door functions are controlled by vacuum actuators.  The only exception is the blend door actuator; it is controlled electrically.

 

- John

Edited May 15, 20222 yr by Tractorman

One thing I can add

 

ABS codes might not be read by some code readers, high end stuff like a Snap On Modis or similar will read which I have but I also have a SO brick and that won't

 

Also you should be able to see stored codes again with high end stuff