Jump to content

Thermal fan switch for us 4 bangers.

Recommended Posts



The threads of the T-stat housing bung are 3/8MPT and I could not find a sensor or switch with the correct threads that was not discontinued so I had to improvise.

Finding a suitable normally open switch with the correct temp range and suitable thread size was not easy either and apparently I'm not the first to go looking for it as seen in the link posted above.

I chose the BWD TFS8, 3/8NPT and a closed temp of 204 degrees. The pigtail is for a brake sensor from a 80s GM since I could not find the correct connector at the junk yard but this is suitable and it works.

The system works well with one exception and that is the temp will rise to about 212 degrees before turning the fan on and I have found that if I shut the engine off just before the fan cycle begins it will boil over to the reservoir. What I should have accounted for is the coolant is cooler at the stat housing than it is in the block. What I will probably try next is- http://www.summitracing.com/parts/HFM-ZFSWF/ with the expectation that it will come on earlier and cycle just before shutting the engine down.

As seen in the pics below I drilled out the bung and tapped it 3/8NPT. Another consideration may be to run the hot lead direct to the battery and allow the fan to continue it's cooling cycle after I shut the engine down. The fan had been triggered by the ignition and ran full time and I wanted to cure two problems, overcooling and the continuous draw of current on the electrical system.







Link to comment
Share on other sites

Why not just use a resistor in series (or parallel to lower) to adjust the resistance of the sensor (thermistor)? Then you could control the NO/NC thresholds precisely. Looks like it's pretty close now.


Help me to understand that better please? How would I use a resistor?

Link to comment
Share on other sites

As you know, the BWD TFS8 is a NO switch. It acts as a thermistor; when heated the resistance between the output wire and it's case ground either increases or decreases depending on it's design. Since this is a NO switch, open when cold, most likely the thermistor resistance increases based on the temp of the coolant until the switch closes, turning on the fan via it's relay. If you can't find the resistance value thresholds from BWD for this switch, you can test it in a pot of water with a thermometer, ohmmeter, and test light. Watch which way the resistance goes when it's heated (up or down) and at what resistance/temperature the test light goes out. Depending on how it goes, you might be able to change the threshold at which the temp switch closes by adding a small resistor in series with the switch output lead. Or better yet a small potentiometer so you can dial the switch closing setting in precisely.

Link to comment
Share on other sites

  • 3 weeks later...

Upgraded to the 105 amp alt today over the 74 amp.

I chose one for the 88 Caprice V6, three differences.

Need to re-clock to line up ground bolt hole to ground mount. Be careful not to strip bolt threads...ask how I know. Tap to 1/4 20 with new bolts.

Need to drill 3/8 hole above the 12oclock bolt hole in mount bracket and loosen bracket at bracket pivot to allow enough drop to align ground mount hole to bolt hole in back of alt.

Need to swap pulley.


Otherwise it is similar enough.



Image Not Found

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...