I looked through my 89 FSM and couldn’t find the answer. I skimmed the entire section on the auto C68 AC/Heater control. I was trying to find the criteria for turn-on. It SEEMS like mine is taking a bit longer than necessary before the blower activates in cold weather. Basically, it doesn’t seem to turn on until temps are in the 160’s – by the dash display. That could me 150 as seen by the ECM.

I thought I remember it turning on closer to 110-120 in past years. Am I mistaken? Also, it sometimes seems to default to 00 for fan speed. With it set on auto, I thought it would always jump to 10 for rapid warm-up.

I’ve taken my entire controller apart, cleaned a reassembled a couple of times. I also did the re-solder of joints on the controller by the go-pedal. Am I missing something?

In general, everything else works perfectly.


Note: This time of year, it's normally set on 90. I just lower the fan or turn it off when warm.

 

EXCELLENT question, and for awhile now I've wanted to know the answer to this this myself. I look forward to seeing if anyone comes up with an official "documented" answer to this. I've looked for the answer to this question in my '87 documentation (both the FSM and the Electrical Manual), and came up empty handed there as well.

Funny this would come up today though...I pulled the car out today and decided to let it run a few minutes to put a little charge on the battery, and while sitting there killing time, I just happened to note the temp the blower came on in my '87---it started cycling up at 106° on the dash readout.

I'll be standing by to see if anyone can produce the official answer, and where it's located in print...because this info just has to be located somewhere.

 

Did you look in the FSM C68 section in the back of the book? Section 8A-64-xxx. A lot more details than the section in the front.

My default fan speed in AUTO mode is 03. I have my temperature set for 70°.

Usually I run mine in ECON. If it's hot I set it to AUTO. If it's really cold I set it to HTR.

 

Yes, and I especially focused on the Blower Controls section (8A-66-1), and the Temperature Control section (8A-68-1), read circuit operations, studied each of the circuits, looked at the inputs, etc...and nowhere could I find where it explains (or shows inputs) for what engine coolant temp the blower will start to run when the when: The engine is cold, the C68 system is set to AUTO, and the temp is set to 90°.

Is there a 5V output signal from the ECM to the controller that the controller uses to determine when to allow initial blower operation in AUTO? Could it be that the ECM does not send the 5V signal to the Controller until engine temp reaches XXX°, thus inhibiting blower operation until that time? Of course, this would only matter with the controller set to AUTO...and it would also explain why initial blower operation is delayed when the engine is COLD.

*** As a side note, the electronic climate controller functions exactly the same way in my 2003 Suburban---On initial COLD start-up, with the controller set to AUTO, the heat to 90°, the blower will not start to run until the coolant has reached a certain temp (or X-Time has elapsed allowing the coolant to reach a certain temp).

 

That's normal.

If you have the cabin/vent temps set over the ambient, OR the ambient is wayyyyyyyy below the desired cabin temp setting, and the engine is stone cold as well, the opt-68 does hesitate before allowing auto-fan to initiate as it WAITS for some heat from the core.
I do not know for sure where it gets its info, but I DO know that each of the major components of this control system are all crossed linked and much communicate with each other to make things happen.

You CAN run the fan up to speed manually, BUT you will NOT have any warm air.....because there isn't any as of yet.
The ambient temp sensor in the nose is part of this as well as the cabin temp sensor in the B pillar.
If you want to experiment, start-up with temps set to 90, auto heat. There will be NO fan....so hold the lower temp button IN and run the cabin temp DOWN to the default max of 60. As you get below 75 or so, the fan starts and will run at high when you hit the default of 60 which is max fan run. Run the temp back UP and the fan will slow, UNLESS your coolant temp has reached a temp sufficient to provide warm/hot water to the heater core.

Where/how the a/c control system gets coolant temp...I am not certain...there must be a link in the ECM since there ARE ECM circuits that are simply marked a/c with no other detail. They look like compressor or fan circuits but may be involved in cabin heater permissions as well.

It IS aggravating to start up and have to look at that 00 for the next 5 minutes....

What I would do, IF I had the stuff....

hook up scanner at cold start and observe the various temp senders, note which is the slowest to reach approx. the same temp as the time when the a/c gets permission for fan. That should give a clue as to where the signal originates. Gregg, if yours is really waiting until 160....I would have to believe that its a dirty connection somewhere that's created some resistance in the circuit that's making the ECM see a lower temp.

OR
if this is strictly a timer in the a/c control head that's triggered whenever the ambient (easy) is less than the SET temp, it could be tired and failing. These timers/delays do get old and slow down, a lot like the gas gauge that shows more than whats there until you cycle the key off/on again when it will show the real level...a slow dampener may be to blame.

I dunno...
was fun thinking (trying)this early though..TY

 

I wonder if the default fan speed is programmable? Seems like mine has changed to 00.

 

There is no input from Coolant Temp for this Year. The only inputs are the Inside and Outside Sensors. Brief delay for any Auto setting to purge moisture with air blown out of the heater ducts so it doesn't fog up the windshield. That's less than a minute at best.

If the Fan is reading 00 at Start up it's losing Memory or entering Diagnostics when it's turned off and probably needs a new Control Panel. The delay is because it's relearning door position. Pull the Blower Module and look inside after it's sat to see if the door is moving around.

 

As SunCr said, there is no coolant temperature input on the C68 system. That didn't start until 1990, when they did a complete redesign of the interior.

I leave my C68 in Auto fan mode all the time. The lowest the fan will go is 3 (while in Auto fan mode). I can go down to 0 manually.

I used to have a 1990 Supra that had automatic AC. It wouldn't turn on the heater fan until the coolant reached 150°. I really hated that. At least give me SOME heat -- anything is better than nothing.