https://www.jaguarforums.com/forum/a...p?albumid=1575

Here is what the guts of my original fan control box look like. The replacement from Welsh is now in my car and working well so far.
This one shown fails to produce any voltage to the fans under severe load. It works fine on light load. My plan is to maybe replace the five capacitors which are $2 items each, and start searching for the transistors.
One of them cross references to a NTE6090 $9 "schottky barrier dual power rectifier" but the numbers on the other three do not cross to anything.
So at bare minimum, the cost to rebuild the critical parts of this device are going to be $55 plus tax and shipping. In my own experience repairing old electronica, I have found that the capacitors fail first, then the big diodes or transistors, then maybe a stray resistor and lastly the microprocessor. What do the engineers among us think?

 

Have you tried looking at just using a relay to turn the fans on when the EU calls for it? On XJ6's Jaguar rewired the fans to just run all the time. It may not be efficient, but it does no harm to run the fans.

 

Funny that my furnace fan was failing and of course repairs costs by "technicians" felt it was the fan and wanted near $500 to fix. I too looked at the capacitor, but because it was an obvious full wave rectifier circuit (AC to DC) and the Capacitor was not functioning... $10 later and a bit of soldering my furnace was function perfectly again ... still in this case what function does the capacitor serve? Maybe for short bursts of power to a standard DC fan.... regardless it will be interesting to know if the replacement of these parts fixes the control box... my fans are not functioning as well either, so I might try the same...

 

Somewhere I read that this system of "soft" turn-on and off is done to keep spikes and transients out of the wiring grid. A hard bang relay could ripple undesirable bumps back into sensitive microprocessor devices all over the car. As for running fans all the time, I would say the shorter life of the fan motors themselves under that condition would cost money sooner than later.
One of our members is looking into cross-referencing those big diodes visible in one of the album images. I found one and am wondering why they are not all the same? FWIW, nothing on my board looks blown or burned and it tests static the same as the "new" one which dynamically functions better. Thank you for response and I am not surprised your furnace repairs were easier and cheaper than what the "expert" was pushing. That is all I am trying to accomplish here: doing it ourselves and saving money.

 

I'm thinking about taking my non-working unit to the Electronics Dept of our community college here and see what they can figure out about it. After retiring from the Navy I attended this school's automotive course and have kept in contact with them since. Several years ago I took the electroluminescence instrument cluster out of my '67 Charger to them and they got it working again. They charged me nothing, the instructor said he got about 3 hours of instructional time out of the project and said to bring any real world problem projects by any time in the future. So maybe it's time to see them again.

 

Green Machine: I think this is a TOTALLY great idea. (And not just because I rebuilt the electroluminescence inverter out of my 63 Imperial on my dining room table!)
Another member here has determined exactly what the four active semiconductors are which had me stumped. The rebuild of this device would be a worthwhile project of significant value to the class and to you and I daresay the forum. Let me know if you want the details on those parts or if you think that chasing that should be part of the exercise! --TH

 

Looking at the pics, I don't think the capacitor is to balance intermittent fluctuations of voltage to the chip (if that is what you're suggesting), this should be easily done by the step down transformer off the 12V circuit... It would seem more likely that the chip controls the transistors to switch the voltage to the fans to make them spin at different rates according to the temp sensor of the radiator... the capacitors would add the necessary temporary "step up" power to spin up the fans. Still my comments are completely blind to the circuit, really I am thinking only how I would circuit a similar system... Oh... really the motors rarely go since it would take a really bad situation to burn them out, more likely the failing is a bad sensor, or programming, or other components mentioned than the motor. Also there are two fans, so how are they activated? What are the chances that both motors are blown?.... Anyways as I said I'm thinking out-loud.

 

Somewhere, maybe in the JTIS, I recall reading that the main ECU develops a varying pulse width signal which is proportional to the cooling demands as ascertained by the ECU and the AC control box. I put a DC VTVM on that line and found it to be anywhere from 3V to 11+ depending on the situation. My failure mode was at the upper end of that. At low demand or narrow pulse width I guess that would be, the fans ran fine albeit slow. When the pulse width widened (and the VTVM rectifiers averaged it out to show 11V), the fans would not go.
This box would seem to be not much more that a frequency counter and big rectifier. It would make high current, low impedance pure DC out of a tiny high impedance stairstep signal. Or is that oversimplifying it?

 

Tom, you are correct in the fact that the ECM uses a varying pulse width to control the fans. THe capacitors are there to take these short pulses and make them into a longer signal to make the fans move. Thing about what a capacitor does: it attempts to maintain a constant voltage. In the case of the short pulse widths, as the pulse is felt, the capacitor is going to take in that pulse, charge up the capacitor, resulting in a lower voltage for the pulse. But, once the pulse goes away, the capacitor is going to discharge, attempting to maintain the voltage. As the pulse gets wider, the capacitor can charge up to a higher voltage since it is not having to discharge as much. Since a DC motor's speed is a function of the DC voltage applied to it, as the voltage goes up (output of the capacitor), the DC motor (fan) will be able to spin faster.

If you need me to explain it a different way, I can do that too.