Why do I have continuity between the two disconnected terminals on my '81 secondary fan motor? Should not one be a ground and one be hot? For whatever reason, I'm suddenly completely confused and lost here.
My understanding of the circuit is 12 volts from the fuse panel straight to the fan motor and a ground from the temp sensor to the fan motor which remains open until the sensor closes at x degrees completing the engine to ground circuit. What am I missing/overlooking here? It's as if the sensor opens and closes on a hot lead and the fan is grounded to the mount. But if that be the case how/why does the sensor ground to the engine? Heck all I know is I'm confused.   

The fan switch in the cylinder head is the ground. The fan motor and mounting bracket is bolted to the fiberglass fan shroud. So there is no grounding there. 

Are you measuring the FAN itself, or the connector on the car for the fan?
You're right in that the 12v power comes from the "Fan" fuse, and the temp switch in the cylinder head provides the ground when it gets to it's pre-set temp rating...assuming the switch is functional. Have you disconnected the temp switch and checked the resistance on the connector then? If you disconnect the temp switch, and STILL see continuity between the two wires in the connector, you probably have a short between the two somewhere. If you do NOT have continuity with the switch disconnected, the switch itself may be gummed/corroded internally, causing the resistance. Check the switch itself....you should have NO continuity between the terminal and the switch body/ground. If you do, the switch needs to be replaced.


btw...if you have continuity on the ground side, the fan should be running all the time, depending on how solid the short is. Sometimes, the temp sender wires get melted on the exhaust, and do silly stuff like this.....


|UPDATED|1/22/2014 11:13:29 AM (AZT)|/UPDATED|

With no wire connected at the switch there is no continuity between either of the two terminals to switch housing, nor between the two terminals. Should there be two terminals on this switch or one?
Measuring at the fan terminals there is continuity with no wires connected. From the fan terminals there are two wires which run to a 2 pin connector located above and front of the fender near the a/c compressor. Disconnected at the fan terminals, there is no continuity between those two wires. From that connector's mate, a black wire runs directly to the fuse panel. A light blue wire runs to a six pin connector near the fire wall where it mates to another wire running to the temp switch.  With everything connected, because of the continuity between the terminals at the fan both the light blue and the black wire show 12 volts when grounded back to the chasis via the multi meter or a test light and the fan is not running. With all wires disconnected at the fan, I can run a 12v wire to one of it's terminals and a ground to the other and it will run.  I'm probably not doing a good job describing this, but it's the best I can.

Thanks sarge. I agree 238 degrees seems much too late for that fan to kick in. Though I've heard many folks say these things run better a little on the warm side. I couldn't say as I very seldom see mine below 200 on the gauge.

 

http://content.mamotorworksmedia.com/production/website/articlefiles/installfiles/601972_X.pdf

 

It looks like replacement switches have 2 wires. This is from Mid America.

If you are going to put a meter on the 2 wires coming from car to fan you would have to remove the fuse and disconnect the temp switch to isolate the circuit. to check to see if there is continuity between them or a short to ground. If their was a short to ground on the pink wire the fuse would blow if the lt blu was shorted to ground fan would run all the time, if the wires are shorted together and the temp switch closed it would burn out the contacts in the sw.

This is how I would check it.

Disc the connector to fan, disconnect connector at temp sw check for continuity between the 2 wires at the fan Con car side if shorted together (bad) look for wires melted together. Ground the lt blu wire at temp sw , now check for 12v pink wire at fan Con. If not this is the circuit that is bad and you can concentrate here. If OK check for ground on lt blue wire at fan connector. If no ground this is the circuit to follow.

 

Now what is not clear is the temp sw. if it is a 1 wire it would ground threw the sw to the block. (if it was wrapped with Teflon tape could effect that)The MA instructions say to hookup just the lt blue wire to existing lt blue, which would indicate it grounds through the sw to block. But the 81 wire diagram shows that the temp the sw makes and grounds through the sw black wire to ground  

Picked up a new sensor with a lower temp setting and with the info from you guys the fan seems to be working great now. Reading voltage on both those leads was really confusing me. I don't know if it's an age thing or what, but a lot of fundamentals seem to evade me in recent months. Oh well, with you guys here to bail me out I'm good as long as I can remember how to log on.
Thanks again for all the help. I appreciate your knowledge.





|UPDATED|1/24/2014 7:23:00 AM (AZT)|/UPDATED|

  

Glad ya got it fixed!
'Lectrical stuff can be confusing as heck at times. But....just remember that a MOTOR will show voltage on both sides of the connection until it gets switched to ground. Light sockets are the same way....until the circuit goes to ground, there is voltage on both sides. That's where switches, and relays come into play.

Glad you got it going