How to fix your Fan Control Amplifier (updated)

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Skibane
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Here's a write-up on how to trouble-shoot and repair the HVAC "Fan Amplifier" module, as used in G50 (1990-1996) and FY33 (1997-2001) versions of the Infiniti Q45, the A32 (1995-1999) version of the Nissan Maxima, and probably other models as well. This thread is the result of repairing dead fan amplifiers from IrvingQ45 and goody94q45, which they most graciously loaned me for autopsies. Total cost of the repair parts is around $4.50.

PROBLEM DESCRIPTION: When the Fan Control Amplifier module fails, either one of two things happen:

1. The HVAC blower doesn't work on any speed setting, or
2. The blower runs at full speed all the time, and only shuts off when the ignition key is turned off.

Here's a couple schematics that show both the G50 and Y33 versions of the Fan Control Amplifier module, what's inside it, and how it connects to the blower and "AC Auto Amp":

Image
Image

HOW IT WORKS: The Fan Control Amp receives a control voltage from Pin 35 of the AC Auto Amp; this voltage varies depending on how much blower motor speed is desired. The control voltage varies the conductivity of a 2SD1297 Darlington Bipolar Junction Transistor (G50 version), or a 2SK2500 MOSFET Power Transistor (Y33 version) which is connected in series with the blower motor - When the control voltage is high, this transistor conducts strongly, and full battery current is allowed to flow through the blower motor, then through the transistor, and finally to ground.

When the control voltage is low, the transistor conducts weakly, allowing only a small current to flow through the blower motor. In this manner, the blower speed is adjusted by adjusting the control voltage. The AC Auto Amp continuously measures the blower speed, via a connection between the negative side of the blower motor and Pin 34 on the AC auto Amp.

WHAT THE FAN CONTROL AMP LOOKS LIKE:

Image
(Y33 version shown, G50 version similar)

HOW TO GET TO IT FOR REMOVAL OR REPLACEMENT:
(Y33 version shown, G50 version is vaguely similar)

After unsnapping and pulling out the passenger side dashboard kick panel, remove these screws from the bottom of the glove box...

Image

...and then remove these screws from the top of the glove box:

Image

Pull the glove compartment away from the dashboard, and you'll see where the Fan Amp is mounted behind it on the HVAC blower enclosure:

Image

Remove the 2 screws that secure it, pull it out of the blower enclosure, and unplug it from its electrical connector.

WHY THE FAN CONTROL AMP FAILS: In looking at the schematic shown above, it becomes obvious that control voltage from the A/C Auto Amp can only reach the transistor through a thermal cutout. If the cutout blows (due to temperatures in excess of 140 degrees C.), no control voltage is applied to the base (or gate) of the transistor, and the blower never comes on.

Or, if the transistor fails, it typically develops a permanent short circuit between the Collector & Emitter (G50 version) or Drain and Source (Y33 version) terminals, causing the blower motor to run at full speed whenever the ignition switch is turned on.

HOW TO TROUBLE-SHOOT IT: If the Thermal Cutout has blown, you will measure infinite resistance between the two printed-circuit pads shown at the top of this photo:

Image

If the Cutout is still good, you will measure approximately zero ohms across these same two pads.

If the Thermal Cutout is still good, the transistor has probably blown. In some cases, this will result in the same very low resistance being measured across terminals 35 and G when probing in both directions.

HOW TO REPAIR IT:

If the Cutout has blown, a quick-and-dirty fix would be to solder a jumper wire across both printed circuit pads, thereby bypassing the cutout. However, this defeats the safety feature of having the cutout. A better option is to replace the cutout with a new one (which only costs around 45 cents).

In order to replace either the Cutout or transistor, the printed circuit board must be unsoldered at the 5 points shown in the photo above. A roll of solder wick makes the unsoldering job much easier:

Image

Use a soldering iron to heat both the printed circuit pad and the solder wick, until the molten solder "wicks up" into the solder wick's copper braid. Gradually pull the braid across the pad so that fresh braid is continuously exposed to the pad and soldering iron, until all the solder is removed from the pad:

Image

Printed circuit board removed:
Image

Unscrew the transistor mounting screw to remove the Thermal Cutout:
Image

On the G50, the 2SD1297 Darlington transistor (originally manufactured by NEC) is apparently no longer being manufactured, and no suitable substitute was found. This transistor has an unusually high continuous collector rating (25 amps), which doesn't seem to be available in any currently-manufactured transistor which uses the same TO-218/TO3PF package. However, the original transistor may be available on the internet - I bought 5 of them from Match-A-Knob, which was all they had in stock. Four of these are for sale, at my cost, if any NICO member wants them.

On the Y33, the 2SK2500 Power MOSFET is not available as a replacement part, but the Fairchild Semiconductor FDH5500 makes an excellent substitute. This MOSFET is rated to carry 75 amps, and has a very low on-resistance of 0.007 ohms. It costs around $4.00 in single-piece quantities from Mouser Electronics (Mouser Part No. 512-FDH5500).

The original Thermal Cutout is rated to open at 135 degrees C. (G50 version) or 140 degrees C (Y33 version). A replacement with a slightly higher 145 degree C. temperature is available from Mouser (Part No. 447-XYP2BN143-RC), for around 45 cents in single-piece quantities. I also bought a few extra Thermal Cutouts and FDH5500 Power MOSFET Transistors. If anyone one wants one, let me know.

Original and replacement parts, prior to bending the leads on the new parts:
Image

(NOTE: Apply a little bit of silicone thermal grease to the bottom of the new transistor before installing it. Permatex aluminum-based anti-seize compound can also be used in lieu of thermal grease).

New parts installed:
Image

Printed circuit re-soldered:
Image

Cut off the ends of the wire leads, re-install the Blower Amp in your vehicle, and you're done.


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MinisterofDOOM
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Nice writeup! Very useful! I have 3 or 4 used fan amps laying around, so now I can fix 'em like new and be ready for the next failure.

maxnix
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I wonder if the G50 has the same layout for its controller. I bet it doesn't.

Nice for us FGY33 guys.

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MinisterofDOOM
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I didn't even realize it was for the Q41. They may still be the same. Lots of Nissans shared the same part in the early-90s...don't see why Nissan would have changed it up for one car.

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Skibane
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Quote »I wonder if the G50 has the same layout for its controller.[/quote]

If someone's willing to loan me a junker G50 controller, we'll all know in short order.

NightRiderQ45
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I'm going to print this off right now! I think that you killed two birds with one stone. This is a great detailed with pictures of how to change the cabin air filter for my Y33! This is exactly what I need and its better than any FSM!! In your 4th picture, that is the cabin air filter located to the left of your two red arrows correct???

maxnix
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NightRiderQ45 wrote: In your 4th picture, that is the cabin air filter located to the left of your two red arrows correct???
Bingo! Those are they.

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Q451990
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Awesome writeup!! This is article-tastic for sure.

Heath

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paranoidjack
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Skibane Great writeup!!!

Definitely article-worthy!

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elwesso
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Very nice!!! I'll put this in the articles section this weekend!

checksplay
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Very nice writeup. The mosfet leads look like they are very close to the housing. I would put some kapton tape in between to insulate. Not under the body of the part, just where the leads are.

David

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goody90q45
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Super write up. It took you a lot of time to put it together. Talk to Wes about a custom title, maybe "Darlington Resister".

IrvingQ45
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Skibane, You have done a wonderful job to restore mine! You save me a couple of hundred $$! I am really appreciate it.. Thanks a bunch!!

Very happy!!

IrvingQ45

slfalco
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Hi, just replaced this amp thing on a 99 Maxima, fan was dead. OK for a few days, but now it happened again. But this time it just started working again after a few mins, so it appears to be intermittent. Could that be that thermal cutout was tripped, but then reset itself? Maybe there is a problem with the blower motor drawing excessive current? This could become really annoying really fast if it persists.

Q45tech
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I would always replace the fan motor simultaneously with replacing the fan speed controller.

AS the speed contoller could last a few hundred years without the motor overheating it.

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Skibane
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slfalco wrote:Hi, just replaced this amp thing on a 99 Maxima, fan was dead. OK for a few days, but now it happened again. But this time it just started working again after a few mins, so it appears to be intermittent. Could that be that thermal cutout was tripped, but then reset itself?

Maybe there is a problem with the blower motor drawing excessive current?
Yes, a bad blower motor can cause the fan amp's thermal cutout to blow, disabling the fan amp.

The fan amp is continuously being adjusted to maintain a constant blower motor speed - no matter how much current the motor draws. So, if the motor is drawing excessive current, the fan amp will pass whatever amount of current is necessary to maintain the desired blower speed - and this higher current will cause it to run hotter in the process.

However, the thermal cutout is like a fuse - Once it blows, it stays blown. So, it's not likely to be what is causing your particular fan to run intermittently.

BTW, the fan control amp dissipates the most power (and thus gets hottest) when it is running the blower at less than full speed. At full speed, the fan amp is biased so that the MOSFET is turned completely on (fully saturated), and thus has almost no voltage drop across it. No voltage drop = No resistance = No power dissipated in it = No heat generated in it.

Naturally, running the blower at max speed also helps cool the fan amp by forcing more air across it.

slfalco
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Thanks for the insight. Slamming the glovebox door seems to get it working when it quits. I did notice a little burning (brown marks) on the connector when I replaced the defective amp, but the connections looked OK to me. I remember seeing a very small gauge wire in there, so perhaps it is damaged... I am going to go ahead and recheck all the connections, and also change the blower out as well... I also ordered some parts to try and repair the bad one for a spare, I'll get to use my electronic tech traning from many moons ago....

qship96
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Brushes are worn down on blower motor if it occasionally works by slamming on glovebox door or any other impact that would reach motor.....usually is intermittant, then completely fails to function at all.

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Skibane
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That's a good point - You might try whacking the motor housing with a wrench to see if you can get the problem to repeat itself.

qship96
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When mine began to fail {in a cold winter} I would start car and fan wouldnt operate until I hit a bump in the road.....this continued for a week or so until it died permanently- when replaced, I took old motor apart and brushes were worn down soo low they wouldnt make electrical contact- probably cheaper to take old motor to a "electric motor repair shop" in town and have new brushes put in?????

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Infinitiguy19
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I did that, But Q45tech says there are other things in the motor that can also fail because of its age and use time.

Q45tech
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With new brushes, cleaning and polishing commutator, the rebuilt motor can probably last at least half as long as a new one.

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Skibane
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There's also bearings to consider - If you can't get fresh oil on them, they won't last much longer.

dix
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Hi,

Thanks for the incredibly clear and detailed description! I have a '94 Q45. It's having very erratic overheating episodes. We've changed the thermostat and temp sender. I'm wondering about the electric cooling fan. The way it's operating doesn't make sense to me. The only time it is consistently on is when I start the car with the AC turned on. Otherwise it's hard to predict. In fact when the engine gets warm or hot I've never seen the fan running, AC or no AC. Of course, I can only check on the this when I stop the car and pop the hood. It's possible that while I'm driving and the temp gauge starts going up and then coming back down the fan is going on and off. I just don't know.

Could this be the Fan Control Amplifier? If not, any ideas?

Thanks again.

d!ck

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Skibane
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d!ck, the Fan Control Amp only controls the in-dash blower for the air conditioning and heater - It doesn't affect the radiator-mounted electric fans at all.

(To keep this thread from veering off-topic, I sent you an e-mail with some comments on what might be causing your overheating problem).
Modified by Skibane at 11:21 PM 7/10/2009

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Skibane, I can send you one of my junkyard G50 amplifiers if you want to do a comparison.

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Skibane
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Please do - Maybe we can also save G50 owners a little money!

(shipping address sent via e-mail)

skrems
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Great article. I have a 1995 Q45 and the fan amp is slightly different but the parts are still there. I was able to confirm the thermal cut-off is still ok as I get no resistance across the connections. Do you know what the output voltage/amps should be on high/low?The fan won't start unless I twist it to start. When I lower the speed down it doesn't slow down. It really seems like the fan amp is the issue but I am wondering why the fan blower needs a world war 1 bi-plane sort of start...Thanks.

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Jesda
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You, sir, are a gentleman and a scholar!

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goody90q45
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Skibane wrote:Please do - Maybe we can also save G50 owners a little money! (shipping address sent via e-mail)
Skibane- Were you able to determine that the fix and the P/N's are the same for the G50 fan control amp? I've got a broken one that I'd like to give a try at repairing.


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