Look here:
https://pics.tdiclub.com/data/517/View38.pdf
Each fan has one ground (T3) and two "hots" (T1 and T2); one (T1) goes directly to the motor, the other (T2) goes through a resistor first. If you measure RESISTANCE (do NOT jump voltage between these unprotected as that will get VERY hot VERY fast and burn up!) between the two hots (T1 and T2) with the plug disconnected (that is, you measure the
FAN WIRE -- not the one from the chassis -- between the hots you should see a resistance there of about an ohm and NOT a dead short.) If there is
no resistance then the fan's internal dropping resistor is shorted and the fan is borked. If you see an
open then I don't see how your fans run on low at all because as you can see T1 and T2 are indeed connected with the dropping resistor between them. What you've reported so far is that the resistance between EITHER of the two drive pins (T1 or T2) and ground (T3) is identical.
That is not correct; there should be about double the resistance to ground on the low speed pin compared to the high with the fan unplugged and testing the fan only at its connector.
The values you've posted for current drain also strongly imply those resistors are both shorted. The usual failure for a wire-wound resistor is open but a short is not impossible. A 1 ohm resistor across 12V will draw 12A and dissipate 12 watts (and get VERY hot!) Of course it isn't a dead short because the motor has resistance too but on the low tap the resistor is in series with the motor windings. You're not reporting that; you are reporting
essentially identical resistance between EITHER tap and ground.
If the resistors are shorted inside the fan case then the only difference between "low" and "high" is the forward current drop in the FCM and the wiring. The HIGH circuit is designed to run on a 40A circuit (because it does) but trying to run a 40A load through a 30A fuse is going to pop it quite reliably. Note that if BOTH are energized some current DOES go down each but the total draw is effectively identical to that with the 30A circuit out-of-service as the motor, running directly off the supply, limits its current draw and speed by the commutator and effective phase lead/lag from the magnetic fields in the motor itself (and of course the drag generated by the fan moving air.) The 40A fusible link is sized to permit the fans to run on high with the engine OFF (entirely off) if required and called for by the thermoswitch in the radiator.
The values you posted for low range are too high; between those two fans on "low" the current levels you reported show you're pulling VERY close to 30A and quite-probably under some conditions over that. "Best guess" is you should be seeing somewhere around ~6-8A for each fan individually running on low.
When the coolant switch is closed the other (40A) fuse is used for high and your values are within that tolerance (and as-designed.) Thus your 40A fusible link has not blown.
Low is not a "somewhat slower" -- "low" is, well, quite low. Essentially doubling the resistance of the windings of a commutated DC motor will result in the motor running at about 25% of its full-voltage speed. It takes a few seconds for the fans to come up to speed on low and they're not loud at all with the key on -- you might not even hear them in the cabin. HIGH is unmistakable and sound like two box fans blasting away as you might expect. Its not a subtle difference. The only reason "low" exists at all is that the A/C condenser requires airflow through it as the compressor has no means of heat rejection other than through the condense and without airflow through it you'll cook the A/C compressor oil; in many conditions the radiator doesn't actually require airflow at all as the waste heat rejection of a diesel can be very low, particularly under low load (e.g. at idle.) This is why making sure "low" works is important as otherwise over time you're rather likely to destroy the A/C compressor.
What you've posted STRONGLY implies the dropping resistors in both fans are shorted. This can be verified by checking ohms on each fan between T1 and T2. If the dropping resistors are shorted (and I strongly suspect they are) then the fans are borked and should be replaced. You COULD put an external 1 ohm / 100W resistor in series with the "Low" lead on each fan instead but you'd have to figure out how to mount and get airflow to them lest they burn up which appears to me to be sort of silly considering that they're probably $10+ each and if the resistors in the fans are shorted I bet the motors have significant heat damage and don't have much longer to live.
