Filed under: 1971 Datsun 240Z (The Fairlady)

Filtered Content: This photo may not be appropriate for work.

It's been a while since I last worked on the Datsun. This winter has just been too cold, and I have been busy with other things. But but last fall when I parked the Z for its long winter nap I had two problems with the cooling system:

1. The temperature switch had become unreliable.

2. The larger of my two fans had become intermittent, when it worked at all.

So now that the weather is beginning to stay above freezing once in a while, I decided that this was the time to get back to work on the car.

The first thing that I did was to select new fans. Last time I ended up buying what the local auto parts store had on hand. Those fans were marginal. They worked most of the time, but I really needed more air flow. The original fan clutch frightens me because it looks like it comes too close to the radiator. The early cars also don't have a fan shroud. The lack of a shroud makes the clutch driven fans much less effective.

The radiator in my Z is 23.5 inches wide and 14 inches tall, making it impossible to mount two 12 inch fans. The options are one 12 inch, and one 10 inch, or one 14 inch, unless you buy a different radiator.

Originally I had two Imperial brand fans, one 12 and one 10, which had a combined rated air flow of 1250 CFM and a current draw of 14 A. Derale now makes a series of two-speed fans with much higher air flow ratings. The smallest of this series is the 16212, a 12 inch fan which is rated at 1450 CFM. I pared this with a 16510, which is a more standard air flow 10 inch fan. The two combined produce 2100 CFM of air flow, but unfortunately also draw a combined 26 A. Since my existing fan circuit that I installed with the original fans only has a 20A fuse, the current draw was a problem. But for a 68% increase in airflow, I am willing to make some changes.

I modified the original circuit, which only consisted of a fuse, a temperature switch, and one relay as shown in the scan of the diagram. I left the original circuit mostly alone, except that I added a relay to prevent the fans from starting unless the ignition is ON. I added the diode shown in the diagram so that on hot days when I park the car with the fans running, they will stay on until the temperature drops below the switch cut-off. But to conserve battery power I switch the 12 inch fan to low speed once the ignition turns off.

I went to a single stage fan controller with a thread-in probe. The only place where I could easily mount a thread-in temperature probe is in the coolant line from the thermostat housing to the carburetors. The hard part is adapting the NPT pipes available in the U.S. to the BSPT threads in the housing. Fortunately I found that Mc Master Carr has a multitude of fittings for that very purpose. So I bought one of their stock number 5832T112 7/8" long nipples that is threaded 1/4" BSPT on one end and 1/4" NPT on the other.
(The Datsun uses ISO taper pipe threads, which are actually BSPT.)

The temperature switch is supposed to come on at 180 F and shut off at 165F. But when I checked the coolant temperature with a meat thermometer, the fans didn't come on until the radiator fluid had reached nearly 195 F. With the ambient air temperature below 40 F, it only took about one minute for the fans to cool the engine back down to 180 F. The fans shut off just below that, perhaps 175? I wonder if the kit that I bought had the wrong switch. Actually once I thought about it, I decided to leave it alone for now. Having the fans come on only when the temperature gets above the thermostat setting (180 F) may not be a bad thing. It reduces the load on the alternator, and when the car is moving there isn't much need for a fan anyway.

I drove the car about 30 miles and didn't lose any coolant. With the air temperature so low the fans never came on, but my temp gage on the dash was registering "normal" so I will have to wait for the weather to heat up before I make a final judgment on this setup.

Happy motoring!