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Radiator fan kicking on premature?

4K views 8 replies 6 participants last post by  GLHNSLHT2 
#1 ·
Good evening TD guys and gals. SO I have a 1987 Daytona Shelby with a TII engine. My question lies within the cooling system. I recently noticed when I warm the car up from cold, the radiator fan comes on when the temp needle hits the mark on the temp gauge thats the line below half and then runs until the temp gauge hits a few needle widths below the half. While driving, the temp gauge goes up to the 3/4 line and doesnt really get any higher maybe a needle width higher. And then while the gauge sits at the 3/4 mark and the car is stopped at a light I cant hear the fan running either. As I recall, my '88 Daytona I use to own, the fan would kick in when the gauge shot a needle width above the half mark and then run the temp back down below half. I checked the voltage of the coolant temp sensor with the chart NAJ provided(thank you) and all checks. Would this be a faulty temp sender? Thermostat? Maybe a silly question but I really don't want my car to overheat and running 3/4 hot seems a little hot to me. Any help would be great. Thank you.

And I also made sure there were no air bubbles in the cooling system. Even did the ol' 1/16 hole in the thermostat trick to make sure.
 
#3 ·
Your engine has two sensors, one that goes to the engine controller (logic module) and that one is located in the thermostat water box on the front of the head, behind the thermostat and just below it. The gauge on the dash is fed by the small cylinder head temp sensor that is over between cylinders 1 and two, just to the right of the AC bracket. The logic module uses the signal to determine when the fan should run. The fan should not run until the engine reaches operating temp and them passes the ceiling that is in the logic modules programming tables. The logic module you actually have is available in a variety of tunes and the table is/could be different. The factory fan schedule for example is not the same as a Mopar Performance fan schedule and is different than those included in the (for example) from Front Wheel Drive Performance. So that might explain the difference between what was and what is now. The dash temp gauge and the cycling of the fan are not directly tied, and the dash gague is not super accurate, and the quality of the connection and the wiring can and will make a difference.

If you have a laser thermometer, and you point it at the water box behind the thermostat, you can determine what temp the engine is really running at and at what temp the fan is cycling on and off at. That can also help you to determine when your thermostat is opening and closing if you point it at the front of the thermostat housing, you can see a notable rise in temp when the thermostat actually opens.

The fan coming on before the engine has reached normal operating temperature and actually crossed the ceiling is NOT normal. However, the fan will come on if you have the AC or defrost mode on and the compressor is running and system is charged enough to close the two switches that are in series that control power to the AC compressor.

If you checked the voltages at the CTS in the water box and they measure correct for the known temperature, then if the fan does not come on after ceiling is reached, then you could possibly have an intermittent fan relay or wiring connection condition or a fan with brushes that is about to quit working. It's good to spin the fan a bit by hand to listen for noisy bearings or for the sound of worn out brushes. The fan should come on reliably every time it has 12 volts obviously. Fan relays commonly wear out and are intermittent, and so are the connections at the relay! If the connections look like they have been hot, the connection is probably not good or the fan is drawing a lot of current, a sign of bearings going out or the fan running more than it should, like if the thermostat is stuck partially open/closed making engine run hotter than ceiling most of the time or radiator being partially clogged.
 
#6 ·
I would agree with OP that your vehicle seems to be running warmer (on the gauge) than what would be considered normal. Try this. Turn on your defrost (activating AC compressor) temp setting on max cold and see if temp gauge comes down to what you remember as normal on your previous vehicle. This bypasses the heater core assuming you still have AC bypass valve installed.
I've seen exactly the symptoms OP describes when heater core becomes partially/substantially restricted. Waterpump cavitates because of severely restricted flow through heatercore, then when bypass is activated (recirculation bypass valve engaged), vehicle returns to normal temperature, because waterpump can now actually move coolant somewhere. The fix is new heater core, or if your lucky you can sometimes clean them by reverse flushing heater core lines. This has happened on two of my P-bodies over the years.
Also you can try turning heater fan on high and control to full heat and see if this brings gauge down any. If this brings needle down, this would indicate heater core functioning properly but radiator core restriction.
After all these years, the heatercore and radiator has probably had millions of gallons of coolant circulated through them. Sort of small wonder they don't fail more often than they do.
 
#7 ·
Well thank you everyone. I will try reverse flushing the core and then taking some temp readings with my laser thermometer. As far as I know the computer is stock. I rebuilt the engine a year back but always noticed it ran in the 3/4 mark of the temp gauge. Didnt think anything of it until now. Will check all ground wires, bound to be one amuck since I had to re do all the wiring as well. Then try the A/c trick 4 L-bodies suggested which all the lines have been replaced and compressor converted to r-134a to keep the green ones happy. And the car is not in motion when this happens either. When sitting the highest the temp gauge gets is two needle widths below the half mark and then I have tried to let it set longer to see if the temp goes back up to cycle the fan again but doesn't. Only when I drive it the gauge gets to 3/4 and a little above but again when it hits this mark or before the fan should come on but doesn't. I will check the motor and relays and do a go through on her. It great that a community comes together through the power of a passion. Thanks again everyone.
 
#8 ·
I am confused... :confused:

If you checked CTS voltage and the cooling fan cycled on/off numerous times at the correct temp then the engine is not running hot, only your gauge is not reading correctly which is normal for these cars.
If you think the car is running hot while driving or under load use jumper wires so you can place your DVOM in the car with you and read actual CTS voltage while driving.

In the summertime my gauge runs to the 3/4 mark, the fan cycles on and cycles off at the 1/4 mark.
In the winter the gauge never reads above 1/4 although the engine is running at the same temp and the fan still cycles normally.

If you want to see an accurate coolant temp all of the time install a quality aftermarket electric temp gauge.
 
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