dr bob

For years I've been preaching that the common failure mode on the cooling fans is connections, with final stages module down the list. My own car proved me wrong today when I noticed that it was reading a little warmer than usual after the slow climb up the hill here in 90º+ ambient today with AC on. Plus the fans ran on after the engine was stopped, a very rare thing. Popped the hood and found that the passenger-side fan wasn't turning. Followed my own recommendations on troubleshooting through the fuses, then just popped in a spare final stages module for grins. Problem solved for now. So I guess I'll have a middle-sized serving of crow for a late-night snack please. The Good News is that it showed up in my own garage, I had the spare/rebuilt controller handy, and had the time and tools to repair in less than 30 minutes. Pieces are in the spares box to restore the original controller that failed. Plus I should go ahead and replace all those fan bearings, on the chance that there was a higher current draw from them.

A monster's work is never done.

Rob Edwards

I'll host the fan rebuilding clinic, if you can get a volume discount on bearings and brushes. I've got a spare pair of fans that need attention and could probably stand to do the pair in the GT as well.

928myles' great S4 fan motor writeup:

https://rennlist.com/forums/928-foru...iator-fan.html

Theo's final stage fan amp rebuild notes:

http://jenniskens.livedsl.nl/Technic...0/MyTip011.htm

Koenig-Specials 928

With the info above and the cost/age of the system that is in use to monitor the temperature and initiate cooling, I have encountered a rad fan controller that I would probably install in conjunction with the change to put the sensor in the top rad hose. Its called Drift and has good write ups in in a lot of the ricer forums, mostly in Europe & Asia.

fraggle

I need to do some system testing. I think some of my sensors are on their way out. The fans don't kick on as soon as I'd like to see, especially in traffic.

Are the EZK/LH sensors the same ones that drive the fan speeds? My car also runs rich at WFO.

jpitman2

Despite half believing the stories that the bottom rad sensor is the right place because its the water return location, controlled by the thermostat, the fan behaviour in my car with that sensor location does not track the temp gauge well at all. A friend here who tracks his car reports much better temp control with a top hose sensor.
jp 83 Euro S AT 54k

dr bob

Top hose connection makes the fans run sooner/more, moves temp control functions more to the thermostat. So more fan load, traded for more stable gauge readings. Hobson's Choice.

Rob Edwards

http://en.wikipedia.org/wiki/Hobson's_choice

dr bob

Hmmm.. Hobson's Choice: "Six of one, half-dozen of the other" Figuring there's no net benefit to moving the switch or sensor.

Rob Edwards

Ah. 'Twas a new on me, figured I'd edumacate myself and others.

jpitman2

What else but the thermostat should control the temp? I have an in cabin LED connected to the fan relay so I know when its on. And I see the fans on mostly when the temp gauge approcahes the second white line, and come off around half scale, but I also see cases where it doesnt come off until the gauge is down near the first white line, or it stays on when the gauge has been half scale for many minutes.
jp 83 Euro S AT 54k

WallyP

The function of the radiator is to lower the temperature of the coolant. If air flow from movement is sufficient to do so, the outlet temp will be low enough that the fans don't come on.

If you switch the fan based upon coolant temp entering the radiator, the fans will run even if air flow alone would be adequate to lower the coolant temp.

jpitman2

What if you choose the sensor setting such it keeps temps in the good zone, but cycles the minimum amount ? There are a large no of different ranges available these days.
jp 83 Euro S AT 54k

dr bob

The piece that's missing in the factory setup is there's no variable directly and solely related to engine load. The fan controller has a target range of coolant return temps it works towards, regardlerss of how much heat is added by the engine or extratcted by airflow through the radiator. The thermostat does take that into consideration, so it is the best thing to use for temp control. Moving the sensor to the top hose makes it a lot more sensitive to engine load/heat, but then ignores heat shed by the radiator. I'm not sure that one is better than the other, enough that it has value enough to change anyway. You'd want a different thermocouple for sure. The ideal/ultimate solution might be a way to have two sensors in series, so that fans respond to both the addition of load/heat from the engine, with a factor for return temp. Just need to make a resistor bridge, modify a pass side tank to include a port for the sensor, and a lttle extra wiring at the front of the radiator. The resistor bridge would be at the controller end next to the passenger seat.

I'm all over it, as soon as the sytem in place doesn't perform adequately.

Hilton

Here's another excellent thread on the topic of rebuilding the controller:

https://rennlist.com/forums/928-foru...ive-fixed.html

Looks like modern replacements for the original RFP25N05 mosfet run about $1-2 each. e.g. NTE2389, IRFZ34, BUZ11A

This thread reminded me I have an intermittent single-fan-running issue on my '89, usually noticed by sounding like a flymo at stop lights when one fan is running at high speed. Time to buy some mosfets

WallyP

I still have a few BUZ11a mosfets left over from killing the current drain in the final stage. $1.50 each plus actual mailing cost...