OEM Carb Fan Controller Replacement (1 Viewer)

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May 11, 2013
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Several years ago I found out my '78 had something commonly referred to as a "carb fan" and associated thermistor and controller. I didn't notice it was there because it had never worked. A PO had replaced the fan controller's inline 5A fuse with a 15A fuse. That's likely why the controller board was scorched, diodes fried, and inoperative. I replaced a couple of burnt diodes but gave up attempting to fix the controller. I replaced the controller with a salvaged one and the system worked (and continues to work) fine. However, I thought it'd be a fun challenge to build a modern drop-in replacement. This summer I finally devoted some time to designing a circuit, programming a controller, breadboarding, testing, and prototyping.

The default firmware profile is identical to the '78 Federal OEM controller. When the ignition is switched off and the thermistor indicates over 100C, the fan relay closes and the fan runs until the temp is below 85C or 30 minutes elapses, whichever occurs first. Although I have a CA-spec engine, I do not use the CA-profile (fan on if over 120C and off below 105C) because I don't have the "thermal reactor" exhaust manifold installed.

Although it only indicates about 20mA at idle (fan off), I designed it to completely cut power to itself after the fan cycle is completed, so there is no power drain when the ignition and fan are off. It powers on when the ignition is switched on and powers off when the ignition is off and the fan profile is complete.

First pic is the circuit on a breadboard. I was operating the circuit for extended periods (24 hours) at 15V and measuring max current with the relay closed (fan on).
IMG_0354.jpg


Here's the PCB prototype. I laid out the components functionally so you can see what's what, so to speak. Ignition determines if the ignition is on or not, Power Supply provides a stable 5V and shutoff logic for the board, Relay is just that, and MCU is the micro controller unit and comm header. The shop did a nice job rounding the corners and notching it just like the OEM board. That big hole in the center is for the screw that holds the connector to the board.
IMG_0358.jpg


Here's the board undergoing some further testing using a trimpot rather than the thermistor so I could test it under varying conditions without getting the engine hot. The board is connected to the laptop via a USB-UART interface to record the data. For the mcu temp curve I recorded the thermistor resistance at various temps and computed a profile with Steinhart-Hart coefficients.
IMG_0359.jpg


And finally, on the left is my old burnt board (minus the connector) and on the right my updated controller (with the cover removed). The relay may look petite but it's a 12V automotive relay capable of 30A for over an hour, well exceeding a max 5A/30min requirement. I added a protection diode and snubber for the relay to protect it from any inductive transients. The 5V voltage regulator was also designed for automotive use and because the current requirement is so small, I was able to go with a smaller TO-92 package. The two-pin header is for data logging.
IMG_0361.jpg


Controller works great. I want to add a Bluetooth module to read the temp data wirelessly and replace the relay with a MOSFET circuit to run the fan at different speeds using PWM. Maybe a future version. It's unfortunate that it uses a connector that's no longer produced. It could be easily reprogrammed to use a different thermistor profile, a different brand thermistor, or even a single-wire temp probe.

There's lots more great info on fan controllers in this thread.
 
Great job!!

I wired my fan to a toggle switch, because I didn't want to deal with trying to get everything to work and potentially fry something else...

If you decide to market your solution, let me know how much!
 
Great job, very informative for the FJ40's that came with the carb cooling fans. Like the blue tooth set-up for collecting data.

My 40 didn't come with one, so I improvised. I purchased a carb cooling fan, SPAL temp switch and a In-Power 20 minute fixed module. Wired it up per In-Power's direction and it works very similar to the OEM system. This setup can be seen in this thread. A year later, still works as it should.
 
Excellent work and a valuable contribution to the MUD community!
I still have the original OEM and still functioning cooling fan controller, I've experimented on several occasions by disconnecting the fan to determine if the fan is actually needed and sure enough my 40 experiences starting issues when the engine is hot while the fan was not running. It's comforting to know that a replacement controller may be available in the future. I look forward with interest at future development of your project.
Thank you for your hard work!
 
:clap::clap:
 
WOW.....that's just amazing! and very informative

(at least to someone like me who can only let smoke out of the box)
 
My 40 didn't come with one, so I improvised. I purchased a carb cooling fan, SPAL temp switch and a In-Power 20 minute fixed module. Wired it up per In-Power's direction and it works very similar to the OEM system. This setup can be seen in this thread. A year later, still works as it should.
I read your thread some time ago (and every carb fan thread here) and was about to do the same type of install until I found a working controller.
 
wow, sweet ... the carb cooling fan is an ongoing issue for me not so much because its essential, but because its useful, and, its part of the truck therefore should work!!!

my US truck's original relay was missing for who knows what reason (part of the de-smog "purge", perhaps) ... bought two used 78 fj40 relays recently, one for use now and the other as a backup ... had both checked out by an electronics repair shop and one needed a burned out resistor (or whatever they're called) replaced ...

if the used OEM parts refurb doesn't work for me, GA-Architect's solution is in the back of my mind as a modern fix that is both available, proven and cheaper than creaky OEM used parts ... your solution is very attractive ... looking forward to hearing how it works out ...
 
Very nice work. What mcu? Where'd you have the board fabbed?

A couple ideas, if you were to make a few:

Require a 'core', so you would have a supply of original connectors.
Fab the board so it can use the other style of sensor. I don't remember the details, but my '76 is more like a switch, and I think open/closed is reversed compared to the '78-style thermistor.
 
It's comforting to know that a replacement controller may be available in the future. I look forward with interest at future development of your project. Thank you for your hard work!
Thanks. I want to run it in the 40 for a while to see how it performs and to measure what the controller says the temp is and what the temp actually is. So far it's working fine.
 
Very nice work. What mcu? Where'd you have the board fabbed? A couple ideas, if you were to make a few:
Require a 'core', so you would have a supply of original connectors.
PCB is from OSH. Not the least expensive, but they can provide two-week turnaround. I'm using a AVR ATtiny mcu though I did prototype it with a TI MSP430G2, a PIC12F, and a PICAXE. I halfheartedly looked at 3D printing the connector but the metal strips would have to fabbed as well. It gets into the "too hard" category for such a low-demand component.
Fab the board so it can use the other style of sensor. I don't remember the details, but my '76 is more like a switch, and I think open/closed is reversed compared to the '78-style thermistor.
I'm not familiar with more than the '78 that I have. SOR lists the following variants:

Fan Controller (Relay)
182-90A 1/76-9/77 FJ40/55 CA $146
182-90B 9/77-1/79 FJ40/55 FED $146 not available
182-90C 9/77-1/79 FJ40/55 CA $146
182-90D 1/79-8/80 FJ40/55 #1 $146
182-90E 1/79-8/80 FJ40/55 #2 $146 not available
182-90F 8/80-91 40/60/62 #1 $146 not available
182-90G 8/80-91 40/60/62 #2 $ 19
182-90H 1/90-1/98 FJ80 $ 19

Manifold Temp Sensor
182-85A 1/76-9/77 $ 35
182-85B 9/77-8/92 $120

sor-relay.png
sor-sensor.png


Based on the above, you do have a sensor different from the rest. I'd have to know exactly what it is and if it's indeed a thermistor I would only need to know the relevant resistance values. Nothing on the board would change, only the firmware would be different. However, I don't know if the metal housing dimensions or connectors are identical amongst all the non-#2 versions. Apparently the #2 versions are very different. I don't know what they look like. Any version that shares the same 5-wire (battery, ignition, ground, fan, sensor) connector and metal base would use the same PCB.
 
Mine is likely the earlier one. All my '78 parts are working.

If you want to fool with it I could send you my sensor and a controller that is known to be bad (fan stuck on).
Sure, I'll PM you my address. Do you mind posting a picture of the sensor and controller PCB? Just curious how different they are. It's unusual for a relay to "stick" in the closed (fan on) position but it is possible. The sensor could have failed as well.
 
If you could measure the resistance of the sensor at say 0C, 25C, and 100C, I could generate a NTC curve for it. Or I could do it if you want.
 

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