Fan blower speeds

[bfbass]

I recently bought a 94 Q45 in great condition. One thing that has just started happening is the A/C fan blower only has 2 speeds, HI, and the next speed down. So out of 4 speeds (other than "OFF") I have 3 and 4 only. I have checked the solenoids under the hood, and also under the dash on passenger side. Any ideas?

Also, after futzing with the solenoids, now the Air Bag light on the dash is blinking. How this happened I don't know. Any ideas would be appreciated.

Thanks, Brian

[Q45tech]

The Q fan has an infinte number of speeds.......well at least 10-12 plus high as the voltage is totally variable........the display just shows a few symbols.

The fan speeds are controlled by measuring the internal temperature and sunload try raising the base temperature setting to 75-80F doesn't the fan speed drop?

Ideally if the AC is up to snuff the 71F setting can get down to 2 symbols after 15 minutes of 60 mph in 90F ambient.

High fan speed is an indicator of weak AC charge!

[bfbass]

Thats the problem, even in maual mode, when I try to turn the fan down, it only goes down 1 notch from HI (blowin my brains out). When I change the temp to 80, it still does it.

Even when I manually turn the system off, the blower is still blowing as hard as it can.

A/C cools really well, so I don't think that is a problem.

[Q45tech]

The highest speed is achieved by a seperate relay applying full battery voltage to the fan motor all other speeds are achieved thru a power transistor whose conductance is controlled via a bias on the transisters base terminal.

In the shop we would just substitute a known good HVAC controller and see if that fixes it or substitute the fan controller assembly [but the usuall failure mode for this unit is to fail open leaving only the highest speed available].

Sure you have TWO {2} high speeds not one?

[bfbass]

Yeah, I found the relay that provides full power, and 1 step down from full power. That is a blue relay right next to the fan blower motor under the passenger side dash. I can get really really HI speed, or 1 notch down when using the manual adjustment. When I take that relay out of the circuit, it wont blow at the really really HI setting. (Hope this is making sense.)When I adjust manually (via dashboad settings), the indicator on the dashboard changes. (The little fan blade icon changes from 4 to 3 to 2 then to 1 blade filled in.) But the blower speed only changes on the 4 blades to 3 blades setting. When is goes to 2 or 1, it is still blowing as hard as when is was at 3, which makes sense now, based on your previous reply. Also, when I hit the off button, it keeps blowing.

Sounds like it is the HVAC controller. Where is that located? Is it easy enough to replace myself?

Thanks so much for the ongoing help. You have already been a wealth of info!!!!

[Q45tech]

The HVAC controller is the assembly with the display [around $550]............the transistor is installed inside the air box [for cooling] from the front, check for a varying Base [like in Collector/emittor/base] voltage on the plug in connector......the power transistor might have failed or changed parameters [they usually fail open] from the OLD motor drawing too much current and spinning slower.

[bfbass]

Is there an easy way to get to the transistor in the air box??No quite sure what you mean by checking for varying Base voltage. I'm guessing that you mean the voltage variance to the transistor?

Thanks,Brian

[Q45tech]

The 3 terminals of common transistors are called base, emitter and collector. The base voltage [negative to zero to 1-2 volts] can be thought of as a gate or valve or faucet valve, a little bit of voltage [on base from HVAC controller] can affect the collector current [going to fan motor]greatly thus the beta or gain of the transistor. Multiple transitor can be connected in series or darlington configuration to further amplify the input base changes or to vary LARGE currents like motors.

Transistors were invented over 40 years ago, IT amazes me that since everything electronic has them, people don't know how they work....or how to test them........my 1967 High School Physics Book had a chapter on them.

No wonder the Chinese turn out 10 times the number of US engineers.

............next week we will discuss varactor diodes which change capacitance based on the bias voltage across the diode......how radio tuners work. Or Digital to analog convertors how speedometer/tach works.

[bfbass]

I understand transistors, and how they work.My main question was... how do I get to it???

[Q45tech]

Pull the glove box and it is stareing you in face [the transistors and heat sink] bolt into air box leaving only the assembly terminal exposed to a cable connector.......probe the terminals or just substitute the unit.

Your service manual has great pictures and a schematic of the 3 terminal device....fan control amp.............over 137 pages of tests and info.

[bfbass]

Thanks for the help.I have ordered the service manual, but have not received it yet.

[bfbass]

Thanks. Replaced it and it works great now

[pretzelboy]

O.k.---jokes on me....on the 94 all you have to do is drop the kick panel(with little feet light)----it just pops of....10mm socket--there are 4 bolts(2 each side) on bottom and 2 in glove box....comes out in one piece....of course I didn't discover this till I had the glove box in a million pieces.....so I pulled out that electrical thing---it has what looks like-stalagtites(you know-what you would see in a cave)on the back side...very crude looking....with that thing disconnected I can put the two 15a fuses back in and turn off the fan function with the switch---but I'm limited to only high useage...so-how much is this little part?
Modified by pretzelboy at 8:25 PM 3/8/2005

[pretzelboy]

ordered part from Joe---$57 plus $6 shipping.....works great