Dead Computer surprisingly easy repair

AndrewInOz · Aug 28, 2021

Deere Computer surprisingly easy repair

A neighbour called with a JD4720 that had had repairs after rats had eaten insulation on the fwd/rev solenoid wiring. Wiring had been repaired but the computer still gave a solenoid fault.
The transmission controller was a Danfoss 1090993 / LVA12707 used on many JD models.
Given that a new replacement was big $ and time , we looked hard

at the dead computer....

Luckily it was potted with silicone so was easy to par off the potting to expose the printed circuit.

These mobile computers often, as this one , have current monitoring on outputs for example when an output to a solenoid is turned on, the output current is measured so a program can check that the solenoid is not connected / connected / shorted and take appropriate action. As it did in this case, reporting a high current, even though the solenoid resistance was ok.

By comparing the faulty output channel components with an adjacent channel, the problem was obvious... the current sensing resistor was burnt out. This component puts a small resistance in series with the output so the solenoid current generates a small voltage in proportion and is measured on an analog channel. Hi currents like in a dead short can overheat and burn out these sensing resistors. Although the software should pull this up before then, not in this case it seemed.

By measuring ( with a 4 wire lab meter ) the in circuit resistance of a good channel was about 0.25 ohm.
Out of my junk box I had 1.0 & 0.33 ohm in parallel.. pretty close. Cutting out the dead an replacing took less than an hour + couple $ in parts.

Photo shows good resistor at pencil and my larger replacements. Too big in size to fit in orig place so nearby and wired to original solder pads. Hot melt and new silicone over.

Back in the tractor and all good. Neighbour very happy.

Hope this helps .. I'm thinking lots these junked because incorrectly deemed unrepairable/ no cct diag but pretty easy in this case saving over $1000

This is a pretty trivial repair for a electronics repair person if you don't feel confident in pcb repairs, the critical point is that yr computer must have soft potting material. If its say hard epoxy then not so easy...

Does anyone know what the unmarked output transistors are? Would be good to have that out there.

k0ua · Aug 28, 2021

You did an excellent detective work and repair. I am surprised they did not pot that in epoxy.

I repaired a cruise control module out of a 1978 Ford Grenada. I had to make a dozen phone calls, but I eventually got hold of the guy that designed the circuit board at Ford Aerospace and he was very helpful. He described the theory of operation of the circuit which applied a reference voltage to a gate of a FET, and theorized that the 9.1 volt reference zener was likely the culprit.

He said he would send the schematic along in the mail. I went to radio shack and bought a 9.1 volt zener diode for $0.59 and had the unit repaired and back in the car and tested out that afternoon. True to his word the schematic arrived in the mail a few days later. But it is hard to find guys like that in the world today.

cqaigy2 · Aug 28, 2021

Well done!!

Eyes are good tools, and your nose will detect leaked magic smoke.

Schematics are nice to have. I usually didn't have schematics and used some of the methods you and James, used to troubleshot your issues. I remember working on a medical argon laser and talking to a person that worked for that medical laser manufacture, told him what i'd found and i said, nothing's wrong as far as i could tell, it should fire. He had the piece of information that i needed. The lasing tube didn't have enough argon in it to ignite. There was a method to add argon, but if too much, the tube was junk, too little it wouldn't fire.

DanMc77 · Sep 1, 2021

That is so neat seeing your repair. I don't think you'll have to worry about those big fat resistors burning up!

Doughknob · Sep 1, 2021

Cool. want to replace this chip for me?

k0ua · Sep 1, 2021

Doughknob said:
Cool. want to replace this chip for me?

It is surprisingly easier than you might think. Step one get some Chip Quick and the rosin that comes with it. Spread the rosin on all pins up close to the body of the chip. Take a flat tip, yes a flat tip on your about 40 watt iron. NOT a pointed tip as you would think, but a flat tip that will spread the heat between several of the pins at once.

Now introduce the chip quick "solder" which is a special blend of solder with a very low melting point and is slow to freeze because it is formulated with special alloys. Float it on to both sides an and do it several times until both sides are molten. Take your tweezers and pluck the chip off of the board. Now take your solder sucker and/or solder braid and clean up the board and the traces.

Very carefully place the new chip and get it perfectly aligned. using a light touch use a good lead based solder to tack down one corner pin. Flood all of the other pins with liquid rosin right up next to the chip body. Checking carefully that all pins are still aligned. Now taking the same flat tip, flow solder all of the pins while pulling the solder outward to the ends of the pins. You solder multiple pins at once, the solder will flow because of the rosin. Carefully inspect all pins with a good high power glass to make sure there are no bridges.

Reflow any solder bridges after adding more rosin, re-inspect. Take a Q tip and 91 percent isopropanol and carefully clean up all of the rosin. Carefully inspect.

That is how you do it on the cheap if you don't have a hot air rework station. Chip Quick is available at Amazon and will do several chips for the $12 kit.