School Me on Proper Stock Radiator Fan Operation
 

Hey guys, my car is a 1995 GT model with the 5.0, but I'm posting in this section because I figure you SN95 guys will be more familiar with this than the 5.0 crowd, on account of electric fans being a '94+ thing. Anyway, I was thinking about how our stock radiator fans operate, and in an attempt to help clear out any misconceptions that I may have been developing, I turned to the Ford service manual:
The manual only states that the fan motor "starts" and "stops". It doesn't say anything about under what conditions one should expect to see LOW speed or HIGH speed operation. Both circuits are shown in the wiring diagrams, but that's only good for tracing the circuit. It doesn't help determine whether things are working properly or not.

It was my understanding (and please correct me if I'm wrong) that if the coolant temperature started to get too hot (like from a lack of radiator airflow due to sitting in traffic), the fan would come on at LOW speed. And if coolant temp continued to rise past another set point, it would trigger the fan to run at HIGH speed. I was also under the impression that A/C operation triggers the HIGH speed fan. But I can't seem to confirm or correct these assumptions.

The manual also includes a note about what you should see happening on the dash:
This has NOT been my experience. My gauge tends to keep the needle near the "O" in "NORMAL", which is in the lower portion of the gauge's scale. If the car is not moving, I can hear the radiator fan kick on as it gets close to the "R", and it keeps the needle below the mid-point of the gauge.

A few months ago, it developed a tendency in traffic to get close to the top of the "normal" range before the fan would turn on. I discovered that the fan was no longer working on low speed, but high speed still worked. The new fan works just fine on both low and high speeds, and my gauge is back to staying on the cool side, near the "O" in "NORMAL".

So is there anything about my understanding of the radiator fan that's in need of correction? In my searching, I found a thread where somebody claimed that the original tune in a stock T4M0 PCM (that's what I have) turns on the fan at "208 low speed and 224 high speed". That sounds about right to me. But why doesn't the manual specify fan speeds? Again, this is the 2000-page Ford manual, not a 400-page Chilton or Haynes book.

I got my '94 GT over a year ago, in May. Soon enough, i saw the gauge go from low end all the way to high end, then fan turned on and took 10 or 15 minutes to bring it back down. I hated this. My '04 Explorer with factory fan and clutch, belt driven, holds temp. rock-steady, doesn't ever move even 1/16 inch on gauge needle after T-stat opens.

So, I tore into it. Now have a great-operating fan and clutch combination which holds temp. nice and steady. If you care to see pics, just ask.

Not what I'm looking for here, but that is very interesting. I think you should post a whole write-up of that, when you get a chance. I saw a couple of pics in one of your other posts and I gotta admit, that mod deserves its own thread. If you post up specs for the spacer and everything while all that info is still fresh in your mind, I'm sure someone will have use for it.

The main reason I like a fan clutch over an electric fan (NOT a solid, mechanical fan, though) is that the clutch can impart almost an infinite number of speed increments of the blade, from coasting, almost no air delivery, to maximum air delivery, with fan speed corrections for temperature changes which are very small, IOW, continuous control of fan speeds. The electric fan has one, or two, modes of air delivery, none, or 1 or 2 blade speeds. To match a clutch's performance, the electric fan would have to turn on and off very often, at least several times every minute. Special narrow-differential temperature switch would be needed, and the switch (relay) and fan motor would be subject to fast death.