Bypass for Electric Cooling Fan

Hello Nation !!!

As I 've read thru some treads many outthere have gone and just put a switch in and ativate such fan mannualy.

At what point do I splice and run my cable inside?. I don't care about ground , just the pos. + Should I throw an in-line fuse also ?

this truck runs idle in traffic sometimes over an hour and I Do not what it to go over.

For the time being I'm not going with a bigger Rad.

Thanks for the feedback.
Keep trail ridin.

I assume this is an 88 due to your usename. I have not direct wired a Renix, but you should be able to use alligator clips and drect connect them to your battery until you find which wires to use before you run your wires and switch inside the cab.---make sure the connections you get make the fan pull air-not push...a reverse connection could make the fan turn the wrong way.

Indeed it is an 88 ;0)>
So Renix has a different wire set up than newer ones.
So after finding the right rotation . You suggest to get the juice directly to the battery?

Sorry meant " From the battery" ( cut and paste ttypo)

Unlike the 91 and newer systems, the pre 91, Renix has a thermal switch in the drivers side of the radiator. Why do you want to bypass it? It is set to turn on at about 185 F and off at about 65 to 170 F. It reads the colder side of the radiator so do not compare it to the gauge temperature in the block which might read 190 to 230 F, while it is seeing 165 to 190 F.

If you want to bypass it one option is to wire the battery and switch to the harness where that radiator fan switch goes. If the wiring in the harness is toast you might try going straight from the fan to the battery with aligator clamps and a toggle switch ( I actually have this as a temporary fix on my deisel nisssan engine / 85 XJ Jeep) Eventually I plan to put the switch on the dash console. I only use it for running the A/C on mine, but that is another story.

The easiest way is to find the two wires coming off of the thermal switch (lower left side of the radiator) follow them back through the connector and into the harness and either piggy back solder two wires or use a set of click type blind connectors and run two wires back to your switch, through the firewall. Took me less than an hour the last time I did it and I actually got a little fancy and installed a switch with a built in indicator light.
Don't cut the wires, just peel back a little insulation and solder a wire on there. The thermal switch or your switch will activate the relay.
The added switch, jumping/bypassing the thermo switch, closes the fan relay. Simple fast and uncomplicated. Everything is original, except the two wires and the added switch. The thermo switch is still functional, the system is fused and the relay still functions.
I soldered into the wires between the relay and the thermo switch (just because it was an easy spot to reach). I actually ran the added wires back down the harness towards the firewall, inside of the plastic sleeve for the harness.
Material list is about ten feet of wire and most any simple switch.
This is only for the Renix.
Most anytime I've been idling for more than a few minutes on a hot day, I turn the electric fan on. I haven't boiled over in years, even before I replaced my fan clutch and temperatures would creep up fairly quickly..

Yep, 8Mud's got it right!

MJ88SD: so what's wrong that you want to bypass? I also have an '88 (still with the closed system). My elec fan works fine. Temp goes up too high, fan kicks in. Once temps drop, fan goes off. Happens around 220ish on my gage.

Saudade said:

Yep, 8Mud's got it right!

MJ88SD: so what's wrong that you want to bypass? I also have an '88 (still with the closed system). My elec fan works fine. Temp goes up too high, fan kicks in. Once temps drop, fan goes off. Happens around 220ish on my gage.

Click to expand...

:repair: A couple of times the Temp. gauge has redlined and the fan never activated.

Already checked the fan motor and it works.. now that Eco mike 8mud and ur self clear things about a thermaswitch I assume that its dead.
So i'll add a line w/ a mannual switch W/O cutting any orig wiring . and look for a replacement thermalswitch .
How does that sound ??


"Keep trail ridin'"

You can disconnect the sensor from the harness and simply jump across the 2 terminals in the harness end. This should turn on the fan. If it does, then the sensor is most likely bad. You can also use an ohm meter across the 2 terminals in the sensor end. When cold, it should show open (infinite resistance). When the engine gets hot enough, it should show continuity.

Now, I had this problem once where it wouldn't turn on the fan and the test above showed the sensor was bad. When I removed the sensor to replace it, no coolant came out of the hole. Turns out I had a big air bubble trapped in the rad so the hot coolant never hit the sensor. After a thorough burping, it worked fine. I put the new sensor in anyway but I still have the old one.

The test for the thermal switch is to put it in a metal pan partially filled with water. Not enough water to cover the electrical connectors, but enough to cover the sensor end. Just before or shortly after the water boils the switch will close.
If you want to get fancy you can tie (copper wire) the sensor end of an electronic thermometer to the sensor end of the switch and get a more exact reading.
The book says the switch closes at a lower temperature than I've observed. In most of my tests it has switched just below the boiling point of water.
The sensor end of the thermal switch sometimes gets covered in calcium deposits and gets slow to react.
You can also go on a junk yard hunt and find a variety of thermal sensors. The switching temperature is often stamped into the sensor part of the switch. Various models of car (mostly foreign), use the same threads, but are often a different thread and sensor end length.
I found one (many years ago) that switched ten degrees lower than the OEM Renix, but was just a little shorter. I'm still kicking myself in the rear for not saving the part number.

Ok. So I attached a line for a manual switch . I'll test that thermo w/ an ohmeter. T, hank for all the feedback keeps me on my toes as i get to know the 4.0L realm better. ;0)>

Keep trail ridin'.

MJ88SB said:

:repair: A couple of times the Temp. gauge has redlined and the fan never activated.

Already checked the fan motor and it works.. now that Eco mike 8mud and ur self clear things about a thermaswitch I assume that its dead.
So i'll add a line w/ a mannual switch W/O cutting any orig wiring . and look for a replacement thermalswitch .
How does that sound ??

"Keep trail ridin'"

Click to expand...

Not necessarily a thermal switch problem, they rarely go bad, but do occasionally switch at odd times or temperatures.
A partial air blockage can trap, or slow the flow of really hot coolant in the top of the motor, while the radiator outlet (and thermal switch) remain fairly cool. Air messes with the flow and heat exchange.
The temperature down low in the system may not get hot enough to switch the fan on. That's likely the reason in later models they moved the sensor to the top of the motor in the thermostat housing.
A partial blockage of the radiator can also mess with the flow volume and may cause an imbalance in the system. Cooler near the thermal switch and hotter at the top of the motor near the temperature gage sensor.
Test the thermal switch and add a manual switch. Make sure you have no coolant leaks and the system pressure stays up. Check your radiator for a buildup of calcium and carbonates. Use any trick you can to bleed the system and repeat. Make sure the coolant level stays above the bottom of the surge tank, even when the motor is stone cold. And don't fill the surge tank any higher than around the seam.

I tested my switch recently (87 renix) and it turned on at 185 F and off at 165 F.

Can't you just run a wire from the battery (w/a inline fuse) to a toggle switch, then a wire from the toggle to the power wire of the two lines that go into the electric fan, and ground out the toggle?? You should be set then, correct? That way it would still activate w/the thermal switch and w/the toggle..?

mrtosh said:

Can't you just run a wire from the battery (w/a inline fuse) to a toggle switch, then a wire from the toggle to the power wire of the two lines that go into the electric fan, and ground out the toggle?? You should be set then, correct? That way it would still activate w/the thermal switch and w/the toggle..?

Click to expand...

Really no large reason why not, but then again why? Any extra wire you lay is subject to rub or whatever. The power to the fan relay comes from a fusible link, affectively straight from the battery, no reason to do it twice.
The switch setup I recommended, the switch activates the relay coil, effectively a fraction of an amp.
There is a reason they put a relay in there, the fan is relatively high amperage and switches burn up or flash burn eventually. Most relays are rated at 30 amps, most switches 16 or less.
Theoretically you could put power to the orange wire at the relay through a switch and bypass the diode pack, with no adverse affects. If the hot wire was a constant hot, you could even turn the fan on, with the ignition off.
There can be some benefits to being able to run the fan with the ignition off. On hot days the temperature sometimes spikes, shortly after shutdown. Before the convection action of the system starts to work well.
I've actually thought about cutting the yellow wire from the ignition switch to the thermal switch and hooking it to the red wire at the relay (directly from the fusible link). Many cars are wired to run the fan if the temp. spikes after shutdown or the electric fan is already running, as you shut down. The down side to this is if you messing around in the engine compartment and the fan turns itself on and the fan may be sucking juice you need for the next start.

^^^^K, which relay is it? Are you saying to basically run the toggle to the relay?

Haven't checked the thermal sw , but Yesterday was a relative hot day here in So.Cal and I got stuck in Bumper to Bumper for around 90 mins.
I turned that switch on and my temp gage never went over 210 ...
Ahh peace of mind........... ;0)>