Thank you Carey.
When oil passes through the cooler some amount of cooling happens even when the fan is not running. It seemed to me that in some cases that could be sufficient, If the oil coming out of the cooler is below the trigger point for the fan, why run the fan?
So, I put the fan switch on the outlet (cool) side of the cooler.
My Mocal sandwich at the filter sends oil to the cooler when it is above, say, 180 deg. Then, if the oil coming out of the cooler is below 180, there is no need for the fan. If the oil coming out of the cooler is above 180 the fan comes on and stays on until the oil coming out of the cooler is below 180.
I have an oil temp. gauge with numbers on it and the oil stays near 180. I have a light on the combo. gauge that tells me when the fan is on.
I understand that this is contrary to what was said above and I think it is contrary to the Setrab instructions.
What are any issues with this scheme?
Question, if I have the fan running all the time wouldn't that manage the heat better than having it come on 180? Will it take long to bring the heat down if it comes on at 180 vs constantly cooling it?
You don't want the oil to be too cool.
@Bobby D ^^That^^ is the biggest reason, but with a slight twist:
If it's cooler/cold outside and the fan comes on "all the time", then it'll either take hours or never for the engine to properly warm up because the oil is too cold. Also, cold running like that invites water condensation in the crankcase and elsewhere. That is evident by that beige slimy stuff inside of the oil filler cap or clinging to the upper half of the dipstick or rampant in the valve covers (yuck 
). That's coagulated water/oil mix that hasn't been able to boil/evaporate the water out. And we all know that water and iron don't like each other and the water always wins.
So, let the engine oil temp climb up to 180 - 205-ish and have a happy, non-slimy engine.
Now, about that "Will it take long to bring the heat down if it comes on at 180 vs constantly cooling it?"
With a decent, fan assisted external cooler I would expect that the fan comes on at 180 and starts cooling, slowing the temp rise more and more til it levels out at whatever - 200? 205? 210? Whatever your car likes under the present conditions. This whole process takes a couple of minutes, depending on ambient temp and how you're driving - it happens faster at higher revs. Without the fan, it just keeps rising (as you saw) to the best effect of the two coolers (internal/external) til it levels off 20 - 35 degrees higher (or more).
Just remember, as you're gazing at your Dakota gauges, that an aircooled engine under varying loads (speed changes, uphill/downhill, outside temps, etc) is going to show a varying oil temp. Head temp will change faster, but expect your oil temp to swing between 180+ and close to 201 under "normal" conditions. Climb a hill it goes up. Go down the other side, it goes down. All in "A day in the life" of aircooled driving.
@Michael McKelvey, Fear not! I did the same thing with my cooler fan switch - it's watching the output side of the cooler. I assumed (without much input from the SOC back then) that I didn't really want the fan coming on if the unassisted cooler was handling things, so my fan comes on when the cooler output creeps over 180 and needs additional help. I figured thats a big margin (20 degrees) to where I wanted it at 200 and it seems to be nicely managing oil temp on days from 70 up to mid/upper 90's out (I don't drive much over that).
MOnitoring the heat output side is a carry-over from my computer days. We didn't give much of a rat's pitutie for the input temp to the card cage (it was usually 55F coming in) but we sure as heck wanted to know if the output started to climb over 90F and would start to take automatic action.
.
Again, I'm not seriously recommending that anyone do what I do (switching the fan on and off manually), as that is a defiantly 'bronze age' approach, but it can teach a lot about what stresses these engines - and what doesn't.
You'd think climbing a long, steep hill would drive the oil to its hottest, but that's often not the case. I usually downshift and sometimes let the revs climb to the high 3000's on steep grades. Leaving the oil cooler fan off, the oil temp will often drop as the engine fan spins way up all the way to the top of the hill. The sainted German engineers did understand a lot.
Also, just noodling around in traffic at slow speeds on flat terrain - with the engine sounding very unstressed - will sometimes start oil temps climbing. If the fan were on a thermostatic switch, you might miss subtle changes like that.
Driving this car is a kind of tap dance for me. The engine is always happiest in one particular gear, and the gearbox seems to prefer careful rev matching more than most modern ones, so driving smoothly becomes a sort of game. Paying attention to what the engine needs means constantly watching the tach and temp gauge, so switching the fan on as required becomes just another part of the dance.
And it's not like that is a constant task. On most days, the fan gets turned on at some point and stays mostly on after that.
I got this car because I wanted something that wasn't a Toyota and, on balance, that has worked out pretty well.
.
My fan sometimes comes on when I am waiting in line at the gas pump. The engine isn't working at all but the revs are low and the engine fan isn't doing much.
My thermostat for the fan is on the input side of my Mesa 96 plate cooler. I did that because it is my only cooler, no oil cooler under the 911 shroud. That way the fan comes on immediately if needed. If I had two coolers, I'd put the switch on the output side of the cooler like Michael and Gordon did.
I also have an oil thermostat that loops the oil back to the engine/dry sump tank until it hits 190. Engine sump through suction pump to thermostat(cooler) back to the top of the oil tank. Bottom of tank goes to pressure pump, oil filter, then full-flow input on the case. The fan only comes on in traffic, after shutdown, or hot idling on a hot day after the engine is fully warm. I never hear the fan underway, but the temp needle never reads over 190. Things might be different in the desert...
I like Michael's idea of the light, that would be the only way I'd know the fan was running when moving above 30-40 mph.
Danny
Where did you place the 96 plate cooler? Seems like that would be too big to fit in the wheel well.
I have a Spyder, so plenty of room under the clamshell. Sorry to muddy your waters.