Manipulating turn on/off points for elec cooling fans

[skyman]

Does any one know of a way to change the turn on/off temps for the twin cooling fans on a 1991 eldorado? On some cars Jaguar for instance there is a switch that actually screws into the cooling system and activates when the coolant temp reaches a certain point. I searched the manual for my 91 eldo and from what I can see, they are operated by the onboard computer. Is there a way to adjust these points?

Sky

[rockfangd]

Just out of curiosity what would be the reason in modifying the fan temp activation.

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i suppose you could put a resistor inline but there is 2 seperate fans with 2 seperate kick on temps

[Cadillac Jim]

In my 1997 model year Northstar, there is an engine cooling temperature (ECT) sensor that screws into the rear head on the driver's side. The PCM uses that information, and whether the A/C is on or not, to turn the cooling fans off, on low speed (connect them in series), or high speed (full 12 Volts on each of them). There may be some way to reprogram the temperatures at which the low and high speeds are turned on with the OBD I PCM, but there are a few things working against you. First, these temperatures aren't something that would be expected to be part of tuning or programming in cars in the field, so these temperatures may not be in EPROM where the TECH II can change them. Second, these temperatures are selected for best performance in conditions that include charging onto the Authbahn with a cold engine on a hot day as well as normal urban other driving of the average Deville around the USA and Canada in all weather. Thus, there doesn't seem to be a strong reason to change these temperatures. Third, whenever the A/C is turned on the fans are switched to low speed, even with a cold engine. This is needed to pull air through the A/C condenser when the car is not moving.

If you do have a compelling reason to change the fan temperatures, there are several things you can do. You can find out what the calibration of the ECT is and change its output to the PCM, which isn't a good idea because that will change the fuel injection programming in a bad way. Or, you can disconnect the fan relays from the PCM and control them with your own circuitry, which will have some minor challenges in turning off any OBD codes that come up if the fan relays are simply unplugged, like substituting resistors for the relay coils in the wiring harness. There are three fan relays, and they are actuated in different combinations for low and high fan speeds, so a three-pole three-position switch could do it.