Heater only kicks on at 90*

[Sgt_Turpin]

Hello everyone I a new to this site. I was hoping one of you fellows could help me. With a problem. I have a 2007 Tahoe LTZ 160,000 runs great. But I have noticed that the heater is not working. The blower works but cold air. But the twist is this. It's digital of course. So when I start raising the temp from 60 degrees all the way to 89 degrees it's the same thing cold air. Now here is the kickers. When I put it ok 90 degrees (the max) it starts bowling hot air. Someone said it might need to be backflushed because when it hits 90 degrees it opens it all the way. Please is anyone has any help I would appreciate it.

 

[Sgt_Turpin]

Sorry for the grammar.

 

[Murder n Mayhem]

On auto units won't come on & blow heat until it senses engine is warm enough to put out the temp requested.

There is a common controller that goes out in these rigs (most realize it when rear blows only cold no matter what).

There is a controller box in very rear passenger side (part #15-75306) that is common to fail. Parts under $50 & simply installs by (remove HVAC fuse under hood first) removing black tie down\cover knob in very rear, 1 small bolt & a clip pop in, then working up rear plastic floor kick panel about 1\2 way across then popping out lower part of pillar (upper back corner). You can then prop up the side panel (careful of pulling off cig lighter outlet wires) plenty to get access to remove the controller (2 screws & 2 plugs) then simply install new. Leaving the fuse out start the vehicle & let run for 60 seconds then shut off, replace fuse under hood & start again. Give truck few minutes to warm up & you're good to go. You can remove the whole rear panel to get full access but not required.

Just did this last week, took 15 minutes in driveway & fixed.

 

[Sgt_Turpin]

Question

It's still strange that it kicks on with it is a max hear temp 90 degrees. Someone told me that it may need to be back flushed. Because when you turns be heater on max it opens up something that allows a lot of heat to get through. I love working on cars but I am just now learning. So u think that might be it. Also side not. Last year I slide truck into a ditch and busted the a/c lines so I ordered s block off kit and had it fixed. Would that have anything to do with the heater only working at 90 degrees

 

[Murder n Mayhem]

It could be but I know many of the 2000+ models have\had that exact same problem (including me). Heater works @ 90 but if even drop to 89 or below stops & in majority of cases it is the part listed above.

Before dig too deep into mechanicals & your wallet I would diagnose electrical as much as can. Here are few ways can do that starting with quickest & easiest.

1. Pull the ECAS/HVAC fuse out from underneath the hood. Wait 10 seconds then install the fuse and allow the system to re calibrate. Then start the*engine*and allow 2 to 4 minutes for the system to complete the recalibration. If doesn't fix try step 2.

2. If you have rear climate (which sure you do) turn heater to 90 so blows heat up front & see if blows warm out of rear vents or if always cool (that is normally one of the first signs of Control Module going bad). If they blow cool replace the module. If blows warm try step 3.

3. Underneath the dash, just to the right of center above the transmission hump, there is the actuator held in place with two small screws. This actuator is only for the driver side. (You'll need to remove the lower hush panel which is held one with some 7 mm screws) Take the actuator off after unplugging it and turn the shaft all the way to heat. See if the heat works properly. If it does then it's the Control Module. If not chances are it is mechanical.

 

[Sgt_Turpin]

Once again thank you so much for the advice. I love working on cars just don't know to ouch right now. But I am learning.

 

[MO Viet Vet]

The broken a/c lines will not effect the heater. I also don't buy the flush causing/fixing the problem. 89 degrees and no work, 90 degrees and blows hot, no way. I think Murder is on the ball. here.

 

[Murder n Mayhem]

Once again thank you so much for the advice. I love working on cars just don't know to ouch right now. But I am learning.

No worries. You're doing the smart thing & picking peoples brain who have worked on the same rig. Saves time, money & most importantly damaging your ride.