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Old 05-03-2020, 09:25 PM
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Default Underhood signal voltage tap-off?

After years of avoiding the weight of an electric fan and heavier alternator, I've got a combo I think I can live with. (I prefer a lightweight mechanical fan setup for the simplicity, low weight, reliability, and stock look, but with my front mounted steering rack, the mechanical fan location options have been very limited, and the shroud shape modifications necessary to clear the rack have finally used up all my sense of humor.) The "new" lightweight radiator and (modest current) electric fan seem to do a good job of keeping the engine cool, and now I'm working on the fan control board (just a transistor and a few discrete parts I have laying around). For this first go-around, I'm doing a manual "turn-on" of the fan (I'll go fancy later, if necessary, but I like to keep the visuals clean of excess wires and sensors on my antique engine). I just need a voltage tap-off that I can use as a signal to tell the fan control board to turn on the fan. My first thought (subject to change or improvement) is using the heater fan control (just turn the interior fan to low to enable a voltage to the fan control board), or turn the parking lights on, or whatever. Just looking for ideas of where to get an easily accessible signal, preferably from under the hood.

Thanks!
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Old 05-04-2020, 10:07 AM
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Ahah! An electrical question about a 69. Those months of looking at 69 schematics wasn't for nothing. I think you nailed it. The blower motor is the best choice for finding underhood power switchable from the interior.
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Old 05-04-2020, 03:58 PM
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My first thought is - use the Parking Lights. Not so obtrusive, looks "normal" when driving. Then again, the fan would be on at night too- maybe not so bad. At least it is probably the shortest wire run too.

When you are ready for the "full Monte" on the circuit, I'd consider any number of the off the shelf 12 digital controllers on eBay. We have several on the boat and use them to turn the fridge and freezer on. Also used one to control a cooling fan when the compartment the compressor is in gets warm.
Have I suggested this before? Found the image in my "album.":


Using the probe on the radiator may take some "calibration time - but eazy-peasy. You could pull the 12 volts off the alternator, and the fan would shut down when the car is off. If you wanted it to run until low cool temp is reached - well then you gotta' find the 12 v lead again...

Cheers - Jim
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Last edited by phantomjock; 05-04-2020 at 05:34 PM..
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Old 05-06-2020, 04:26 PM
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Quote:
Originally Posted by BBShark View Post
Ahah! An electrical question about a 69. Those months of looking at 69 schematics wasn't for nothing. I think you nailed it. The blower motor is the best choice for finding underhood power switchable from the interior.
Well, spending a few unproductive minutes under the hood to figure out what switch position gives me a voltage at the (orange? IIRC) wire that plugs directly into the motor. But so far I can't get the damn wire unplugged, despite how firm I squeeze the rubber connector cover and pull on it. This wire hasn't been touched by human hands in over 50 years, but I suspect the problem is me, not any rust/corrosion issue.

Any helpful suggestions are appreciated (to keep me from doing something stupid ).
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Old 05-06-2020, 05:35 PM
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I'll bet it wants to stay there after 50+ years. I cut mine off to use a C4 blower. Took a look at it and my guess is that if you try to grip the rubber part tight enough to get it loose, you are also squeezing down on the terminal? Maybe if you gripped the rubber piece on the crimp it would pull off? Or maybe you could get a blade un the edge of the rubber piece and slide it back?

Not easy reaching up in there to do anything. If you mess up the connector let me know, I'll send you this one.

EDIT: Since the power to the motor goes through the resistor, and the resistor is more accessible, perhaps you could find 12 volts (switched) there?




Last edited by BBShark; 05-06-2020 at 05:42 PM.. Reason: added info
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Old 05-07-2020, 12:19 AM
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Quote:
Originally Posted by BBShark View Post
I'll bet it wants to stay there after 50+ years. I cut mine off to use a C4 blower. Took a look at it and my guess is that if you try to grip the rubber part tight enough to get it loose, you are also squeezing down on the terminal? Maybe if you gripped the rubber piece on the crimp it would pull off? Or maybe you could get a blade un the edge of the rubber piece and slide it back?

...................
Took your advice/guess, and squeezed less and pulled harder. Worked like a champ. I put my DVM on the harness wire, and have 12Voc on all three fan switch positions. Now I just have to sit down this weekend and solder up the circuit board for the radiator fan control circuit.
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Old 05-08-2020, 11:30 PM
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So 12v to the fan means that the resistor is after the fan (to ground) and that is what controls the fan speed? That seems weird but definitely not my area of expertise.

Thinking about it maybe its better to do it that way. If a wire shorts out, it will still work?
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Old 05-09-2020, 03:20 PM
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Quote:
Originally Posted by BBShark View Post
So 12v to the fan means that the resistor is after the fan (to ground) and that is what controls the fan speed? That seems weird but definitely not my area of expertise.

Thinking about it maybe its better to do it that way. If a wire shorts out, it will still work?
At this point in time I don't know where any of the fan speed resistors are. I haven't had time to figure that out, and for this/my particular application fortunately it doesn't affect things. As I mentioned earlier, I saw 12Voc (open circuit) at all three switch positions. (No attempt to insult anyone's intelligence with my descriptions here, just plain and simple explanations are my comfort zone.) Measuring the open circuit voltage at that fan power line is like measuring the voltage at the ignition ballast resistor, once that line is disconnected from the coil C+ pin. No current (or insignificant current due to the high input resistance of the DVM/VOM) means no voltage drop across the ballast resistor, hence the same voltage (12V) on both sides of the ballast.
The signal input resistance of my engine fan control circuit will be high enough (megOhms) that any resistance in the switch/harness will be insignificant in comparison. (At least that's how the circuit I drew up on the back of that old envelope says what should happen. )

Regarding a shorted wire, I'm unclear which specific scenario you're looking at.
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Old 05-09-2020, 09:38 PM
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I was expecting to see the voltage to to motor to change depending on the position of the fan switch. But that doesn't seem to be the case. I think I wired my fan to ground so I will find out when I get the heater control wired in (all speeds are high speed).

It's been a challenge to figure out (from schematics) what things do because they only show the termination, not the function of the device. So its basically a wire harness diagram, not a true electrical schematic.
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Old 05-19-2020, 07:13 PM
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Default So far, it works.

I soldered up a transistor circuit to turn the coolant fan on and off, using the heater fan switch low position as a signal input. I put in some negative feedback at the power transistor, to slow the motor turn-on rate, and keep the current surge to a reasonable level. So far things work. While I'm trying to find a small box to package things into, I'm playing around with further slowing the turn-on rate to see if there's any issues or further benefits there.
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