I've posted about subject this before, but now I have some data to back it up.

Note: I have both of my fans wired in parallel for ecm control, so I'm referring to both fans instead of just the main fan as would normally be the case in the following description.

By raising the a/c off fan on/off temperatures above the a/c on fan on/off temperature settings you can turn the fans off at speeds above 11 mph.

At speeds below 11 mph, the a/c on fan settings have control even if the a/c is off which can be confusing when tuning.

When drving with the a/c off:

When coming down from speed with hot coolant, the fans turn on as you hit 11 mph.

Once the fan is already on, there is a 15 second delay that prevents rapid switching off after the vehicle exceeds 11 mph. It will turn off 15 seconds later. I will be shortening this delay considerably in my next version.

Here are my current settings to achieve this effect:

Fan 1 Turn On (AC On) 183.07 Deg F
Fan 1 Turn Off (AC On) 174.68 Deg F
Fan 2 Turn On (AC Off) 216.62 Deg F
Fan 2 Turn Off (AC Off) 209.63 Deg F
Fan mph to use AC on cals 11.00 mph
Fan minimum on time 15.00 seconds

Normally both settings would have the same values 226 F on and 220 F off.

The intention here is to allow the car to stay cool while staging or in traffic and then cut the fans quickly once underway to reduce alternator load.

 

Interesting, these fan values don't seem to be correlated to the stock thermostat, is that correct.

 

Sorry I forgot to mention the T-stat.

All of my settings listed above are intended to work with my 160 stat.

The original 226 on/220 off settings were intended for the 195 factory stat.

 

Neat stuff, wish I had a good grasp of this stuff as you do.

 

I just cut the minimum fan on time down to 2 seconds.

This should have the desired effect as long as I don't spend alot of time between 10-12 mph resulting in rapid on/off switching.

I haven't had a chance to test the 2 second delay it in the car yet, maybe later on today if I can find an excuse to go for a drive.

Once the weather warms up, I'll have a chance to refine the on/off temps for best results.

For now I wanted a large window where the fans would turn off at speed (< 210 F), and still provide some additional high temperature protection (back on above 216 F). I'm still lower than the factory settings, so it should never really get too hot.

For drag strip use, I'll probably bring the a/c off temp down to 160 F or below, to bring the coolant temp right down to the t stat opening point for launch. But for now I don't want the fans on constantly at low speed. Its nice to have this flexiblity.

 

To recap the situation for my own understanding.

The car under discussion is a 1988 L98 with the optional pusher
fan in front of the radiator.

The OEM wiring has been changed so that, instead of the front
pusher fan being wired in a stand-alone circuit and activated on
demand by the thermal switch mounted in the head, this fan
is now wired in parallel in the circuit for the main puller fan
mounted behind the rad.

Consequently, power to both fans is now controlled by the thermal
sensor mounted in the head and the values set in the BIN
read by the ECM. However, the secondary pusher fan still activates
at a temperature determined by the thermal switch.

The ECM evaluates the AC On/Off status, road road speed and coolant
temperature when determining whether to activate the primary radiator
fan circuit.

Below 11 MPH (AC Switch On/Off position does not matter)
At or above 11 MPH (AC Switch OFF)
At or above 11 MPH (AC Switch ON)
Is this interpretation correct?

Also, is it correct to observe that there a vulnerability if a relay
or fuse for the primary fan circuit fails?

.

 

In case anyone is interested, Echlin "Coolant (Thermo) Fan Switches"
with various activation temps are (or were) available from NAPA as follows:

ON ............... OFF .... P/N#
204-220º ....... N/L -- FS113
213-229º ....... N/L -- FS112
224-236º ....... 212º - FS110
229-245º ....... N/L -- FS111

The On/Off activation points of the secondary pusher fan can be
changed by chosing a replacement fan switch with the desired values.
The F/S thread is 3/8" x 18 NPTF.

Fan operation has been handled differently through the 84-96 C4 model run.
CFI-EFI posted the following observations back in 2006-05

 

Slalom.

Nice summary.

Sounds like you got it right, except that my aux (pusher) fan is no longer connected to the thermal switch in the driver's side head. The connector is simply unplugged.

I can run in either configuration depending if I plug the switch in or not.

If I plug it back in, this would permit either the ecm or the thermal switch control over both fans. If the ecm provides ground: both fans run. If the switch provides ground: both fans run. Similar to a logical or condition.

Both relays are still used and power supply circuits are maintained, only the ground wire is in parallel between the ecm and the thermal switch connector.

Before I got into ecm programming I used a 200 on/180 off switch to control both fans in parallel with the original ecm programming. The 200 on/180 off switch became dominant since it grounded first which was my intention at the time.

Thanks for posting the fan switch info. I haven't seen this before.

Here's my edited version of your summary table.

Below 11 MPH (AC Switch On/Off position does not matter)

Coolant Temp < 174.68 Deg F - Both Fans Turn Off
Coolant Temp > 183.07 Deg F - Both Fans Turn On

At or above 11 MPH (AC Switch OFF)

Coolant Temp < 209.63 Deg F - Both Fans Turn Off
Coolant Temp > 216.62 Deg F - Both Fans Turn On

At or above 11 MPH (AC Switch ON)

Coolant Temp < 174.68 Deg F - Both Fans Turn Off
Coolant Temp > 183.07 Deg F - Both Fans Turn On

Now there is an additional twist available which I haven't adjusted yet....

There is the possibility for adjusting the FAN duty cycle as a function of coolant temperature, but this may be hard on the relays. There is a constant duty cycle value for a/c on and a/c pressure high under 35 mph as well as a coolant temp dependent variable table for all other conditions. I'm sure I'll be messing with the duty cycle parameters soon. Right now its just not warm enough in Michigan to really evaluate.

 

Thanks for the clairification that your Fans 1 & 2 are
currently configured to activate simultaneously at temperatures
set in the BIN.

I puzzled over how Fan 2 could activate at higher temps
than Fan 1 if they were on the same circuit and I could
only think that #2 was grounding through the OEM temp switch.

After thinking further about operation of an ECM/thermal switch
circuit, I believe my earlier post is wrong about which fan operates
when. The revison below seems more appropriate

Assume the following
ECM Temp Low: 175 off/ 183 on
ECM Temp high: 210 off/ 217 on
Thermal Switch: 212 off/ 224 on (FS110)
Fan 1 controlled by the ECM. Fan 2 runs on condition that
a) Fan 1 has been commanded by the ECM AND
b) the coolant temp exceeds the Fan 2 Temp Switch activation point.

As for Fan duty cycle and the relay, I suppose you could
check the current draw of the two fans and evaluate whether
this exceeds the capacity of the relay (not to mention the
conductor wire and fuse capacity for the fan circuit.)
A larger capacity relay and adequate wire will not be too
big a challange for a guy who managed to change his cam
in the chassis without pulling his intake.

As for it not being warm enough for evaluation. I'm afraid I can not
help much from this end, either.

Weather for Leonard, MI
45°F & Overcast
Wind: E at 10 mph
Humidity: 56%

Sat - 56°/42°
Sun - 66°/42°
Mon - 57°/39°
Tue - 66°/35°

Weather for Edmonton, AB
30°F & Light Showers/Snow
Wind: W at 14 mph
Humidity: 0%

Sat - 50°/15°
Sun - 35°/12°
Mon - 22°/15°
Tue - 26°/17°

.

 

Additional clarification:

Fan 1 and Fan 2 are simply labels from the editor program (TunerPro xdf file) for the a/c on and a/c off settings. These lables are misleading since both sets of values really only apply to the (ecm controlled) main fan. I should have edited this in the original post.

In my car, since the aux fan has been wired to be active whenever the ecm activates the main fan, both fans will be activated together in all cases. The thermal switch is not currently being used in the circuit.

Regarding the weather, it was in the mid 40s yesterday. I had to drive around in 3rd gear at 3500 rpm for awhile to get the coolant temp hot enough just to activate the fans in the first place. It was tricky keeping the coolant temp high enough to see if the fans would turn off above the 11 mph threshold.

Fan operation was verifed via scan data observing the fan duty cycle sigal.