Repair of '79 - '80 carb cooler fan controller circuit board

Joined
Oct 7, 2011
Messages
956
Location
Austin TX
My '79 40 didn't have the blower for the carb when I got it about 6 years ago. Until recently I was ignoring my hard-starts after the engine was warm and then sat for a while. I started reading the LONG threads about guys on MUD getting their carb coolers working and having much success with ending their hot start issues.
I ordered a blower from the classified ads and installed it on the existing bracket at the louver on the driver's side. Thinking everything else was good, I soon found that the squirrel cage motor would not operate on the truck, but worked when directly on 12 volts. (By the way, I initially hooked up the motor to the battery and thought the output was pretty anemic. I tried reversing the polarity and made a huge difference.) Anyway it turns out that I had the usual problems, blown 5 amp fuse with the yellow tape around it behind my instrument cluster, also had a bad M3 relay (same as still used on fairly recent Lexus car headlights), and the controller module was not functioning.
Following the suggestions of earlier postings, I pried open the controller box and re-flowed the solder on all of the pins on the green plug socket. This socket takes a lot of stress when plugging and unplugging and mine had several cracks around the solder points. Put it all back together and still didn't work. Took the board apart again and checked the diodes, all checked good enough. Really getting desperate, I started looking at the copper traces and found two cracks on the traces near the socket. One was lengthwise on the trace and not a problem, but one crack completely isolated several components from the socket. The attached photos show the crack before and after I scraped the coating off the trace and ran some solder to complete the circuit. Success! It worked just fine after this. I just wanted to pass on the possibility of cracks in the board being an easily repaired solution.

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Joined
Apr 20, 2012
Messages
125
Location
Gautier, MS
Love the pictures and write up. Found this on a search. My problem is everytime my fj40 gets rained on the fan randomly turns on. Was hoping to get some pointers on where to look for the short. I tried unplugging the fan and I can still hear the clicking. Doesn’t seem to take much rin to cause it and everything under the dash looks dry.
 
Joined
Oct 7, 2011
Messages
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Austin TX
Yeah, I reflowed several spots that didn't show up in that early photo, including another cracked circuit trace that I noticed after I took the photo. I had to put some reading glasses on to see all the tiny circular cracks. Thanks for checking. : )
 
Joined
Oct 7, 2011
Messages
956
Location
Austin TX
Love the pictures and write up. Found this on a search. My problem is everytime my fj40 gets rained on the fan randomly turns on. Was hoping to get some pointers on where to look for the short. I tried unplugging the fan and I can still hear the clicking. Doesn’t seem to take much rin to cause it and everything under the dash looks dry.
Your signature doesn't say what year 40 you have. Prior to '78 or '79 (not sure which), the controller had the relays on the main circuit board. By '79, the relay had been moved off the circuit board and was replaced with an M3 type relay and the whole shebang was moved from the passenger side to the driver's side near the steering column.
Whichever you have, rain would have no effect on either the blower motor or the sensor under the hood in regard to turning the blower on. Rainwater isn't really a very good conductor of low voltage, so I would have to guess that the water is getting to the logic pins on the controller board and creating corrosion on the traces that might become conductors and shorting. Take the control box off the firewall and open it up and look for any corrosion on the circuit traces, especially around the integrated circuits (IC's). The controllers have both constant 12v and switched 12 volts going to them. The fan should only come on after the engine has gotten to about 175 degrees, and you turn off the ignition. Take a photo of your controller box circuit board front and backside and post it here....and maybe we can see what the issue is.
Doug
 
Joined
Apr 20, 2012
Messages
125
Location
Gautier, MS
The truck is a Feb 1980. There was some dirt that fell out of the connector of the relay but I didn’t see anything upon my initial look see. I may tear into it further after the tropical storm rolls through

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Joined
Oct 7, 2011
Messages
956
Location
Austin TX
Something is going on in the upper right of the photo of the circuit board, Looks like maybe water or corrosion in that area. Can you scrub it lightly with a brass brush (toothbrush style) to clean up the smudge? Also, it looks like one of the solder joints on the green socket may be cracked. Probably best to go ahead and reflow all the socket pins with your soldering iron. Add rosin core solder if you need to. After you use the brass brush, you can recoat the area with clear rattle can paint to help protect the circuit.
 
Joined
Oct 7, 2011
Messages
956
Location
Austin TX
Spider web.... well that was an easy cleanup ....however, even something like a spider web, if spanning the contacts of an integrated circuit, when wet, can cause the IC to act erratically. Other than the one cold solder joint on the plug pin (at least it looks like there is a ring around one of the joints) I can't see anything else to explain the fan coming on intermittently. If you want to change the M3 relay to be on the safe side, they are readily available for about $10 on Ebay, but I tend to think something on the board is tripping your relay.
 
Joined
Jan 19, 2010
Messages
78
When I repaired the board on my 79 I found the usual cold joint fractures but also the electrolytic capacitors dead or weak. I would recommend as previously stated to clean your board then replace the caps and check again all the joints- i like to use a microscope. Clean the board again and as dougautintx said to paint for protection. I also would use caps of a higher voltage rating as they have more life cycles but probably is not very critical in this application, still for a few pennies.
 
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