W123 85 300D with W126 OM617 installed, Condenser fan not working

[jusme]

I bought this car as a basket case from a gentleman that was having a mobile mechanic install the low mileage engine out of his 300SD which was very rusty into his 300D. He managed to get the old engine out, disassembled some of it, and never showed back up. So, I purchased it a couple years back, bolted the engine in enough to load it on a trailer, and eventually got around to finishing the job. I was able to get the A/C blowing very cold, but when my wife drives it, it's generally in town, and because the aux fan is not working, the performance of the system is suffering. I found during the swap that MANY things are different between the chassis's. Tach setup, oil filter housing, oil pressure (sender vs. mechanical), and many other items. So, I was wondering before I go into this too far, is there a party here that knows both chassis well enough to understand what if anything could be the culprit with a W126 OM617 in a W123? Temp switch setup differently? Would it be easier to just wire the fan so it turns when the AC is on? Although that would disrupt any engine protective abilities (and my wife is NOT one to watch gauges).

Any thoughts on what needs to be done with this mongrel?

Thanks in advance!!