2nd Radiator Fan Wiring

I hesitate to update, knowing that forum members here seem strongly to prefer doing things the Hard-But-Right way instead of the Easy-But-Wrong way... But there may be a bit of useful info in here for future browsers of this thread.

I simply rewired the fans in parallel instead of in series, on the original factory circuit. Flipped the switch and WHOOOM! Full fan speed on both fans, no blown fuse. This circuit appears to run through a 16-amp (stock) fuse, so...

*Potentially Useful Info* my takeaway is that a factory X1/9 fan draws less than 8 amps, even including the initial surge when they flip on. Those suckers can really move some air, though! Take this with a grain of salt, as I know next to nothing about electricity.

It's not the Right Way, but it seems to work for now, allowing me to turn my attention to the broken heater control valve + broken radiator thermoswitch + 10-year-old coolant problem. Once that's solved, I plan to go back and wire in a second circuit/relay for the second fan. For now, though, at least everything is running thru a relay and is fused, so there's that...
 
JNewm said:
Thanks so much for the responses--super helpful! A substantial overhaul, including aluminum radiator and 10" aftermarket fans i/o the small + heavy stock units, is in my future.

Sort of a complicated story on why I didn't just replace the old, broken thermoswitch. The slider bar for the heater seems to be stuck, and it's slid all the way to the cold (or "not hot") side (to the right). Without the ability to turn on or run the heater I'm not 100% confident I could properly flush/refill the radiator fluid. As a result, I was leery of replacing the thermoswitch, despite the fact that the system very likely could use a flush and new fluid, as the car just sat for ~10 years.

Suppose I wanted a quick-and-dirty fix for now, with @Dr.Jeff's recommended method to be done down the road. Is this how I'd do it? (Again, apologies for the dumb questions--I've been a tinkerer/bolt-on type guy for awhile, but am finally trying to expand my knowledge and skillset at least a little bit.)

EDIT: I am running them serially (what I've been thinking of as "daisy-chained". @JDExSquid, that makes total sense. And it would potentially be the "quick-and-dirty fix for now." Thanks!
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If I may give my humble opinion,
I have replaced my stock rad with an all aluminum rad and at first ran a duel 8” aftermarket fan setup. I didn’t feel the 8” fans were sufficient at cooling the system so I upgraded to a 10” spal fan. I wired this fan with a “bypass” switch that allowed me to trigger the relay manually. Before installing the spal fan I did a test of performance between all three fans. Stock, 8” aftermarket and 10” spal. Surprisingly, the stock fan (and pardon the pun) blew the other two away! So, I now have dual stock fans installed on my all aluminum rad. One is wired to the factory wiring and thermo switch. The other is wired separately to a relay and switch giving me complete control of it. I should also note that if the fans are wired in “series” ie: two fans on the same circuit, they will only run at half speed.
 
I use this method on 928's. Granted, I use a large single two speed fan but this will work on any car with an electric fan.

Forgot to add that if you wire it to battery rather than the ignition, the fan will continue to run after shutdown as the radiator temps rise.
 

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lookforjoe said:
2nd relay, with it's own power supply. add ground via thermoswitch, or via existing relay ground wire.

EDIT: ignore relay on left - the two on the right would be the way to wire it. 15 = switched 30 = constant

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The WD master mechanics installed a second radiator fan in the 1974 with no AC. They ran a relay for it with a 30amp fuse. Is this overkill for the one fan, given what some other posts in this thread mention? I was wondering if that fuse could be dropped to 20amp, and wiring down to 12-14 gauge from the fuse to the battery/power source. Thoughts? The '74's stock fan runs off a 16amp fuse it shares with the horn.
 
tvmaster said:
The WD master mechanics installed a second radiator fan in the 1974 with no AC. They ran a relay for it with a 30amp fuse. Is this overkill for the one fan, given what some other posts in this thread mention? I was wondering if that fuse could be dropped to 20amp, and wiring down to 12-14 gauge from the fuse to the battery/power source. Thoughts? The '74's stock fan runs off a 16amp fuse it shares with the horn.
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Just make sure the gauge of the feed and ground are not smaller than that of the fan itself, and evaluate the distance the wiring will travel from the relay. Most aftermarket fans do not have anywhere near the output or current draw of the factory fans (esp. on startup).
 
lookforjoe said:
Just make sure the gauge of the feed and ground are not smaller than that of the fan itself, and evaluate the distance the wiring will travel from the relay. Most aftermarket fans do not have anywhere near the output or current draw of the factory fans (esp. on startup).
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When you say ‘feed’, you mean + battery power feed? I suppose wiring up a meter and measuring real amperage would be smart when I get the time. Thanks H.
 
tvmaster said:
The WD master mechanics installed a second radiator fan in the 1974 with no AC. They ran a relay for it with a 30amp fuse. Is this overkill for the one fan, given what some other posts in this thread mention? I was wondering if that fuse could be dropped to 20amp, and wiring down to 12-14 gauge from the fuse to the battery/power source. Thoughts? The '74's stock fan runs off a 16amp fuse it shares with the horn.
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Just because there is a 30 amp fuse doesn’t mean it draws 30 amps. It just means if a greater than 30 amp draw occurred due to someone grounding out the supply wire it would pop the fuse.

You can certainly put a 16 amp fuse in place of the 30 amp and no harm will come. Aside from maybe your fan draws too much amperage and pops the fuse. Then you go to a 20…

The wiring can’t be any less than 12 ga, there is no reason in the world to drop the gauge down to 14 given the distance and the draw.
 
kmead said:
Just because there is a 30 amp fuse doesn’t mean it draws 30 amps. It just means if a greater than 30 amp draw occurred due to someone grounding out the supply wire it would pop the fuse.

You can certainly put a 16 amp fuse in place of the 30 amp and no harm will come. Aside from maybe your fan draws too much amperage and pops the fuse. Then you go to a 20…

The wiring can’t be any less than 12 ga, there is no reason in the world to drop the gauge down to 14 given the distance and the draw.
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Super. last thing in my electrical rewirng week, doing something about the master mechanic‘s wonderful relay work :)
 
tvmaster said:
When you say ‘feed’, you mean + battery power feed? I suppose wiring up a meter and measuring real amperage would be smart when I get the time. Thanks H.
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I did this for forum readers in one of my posts a few years back. The initial surge of current was quite high. I'm on a train writing this so cannot search back for you, sorry.
Also see my long thread re heater matrix replacement and gaskets recently.

RDS
 
This shows two fans controlled by a single thermoswitch on an early pre79 X. It also adds a switch to be able to override aka force both fans to run.

If one used the VW type thermoswitch, each fan would come on at different temps. A switch could be used in the same way to force one or both fans to run as desired.

A later US market X is similar but had wiring in place for a second fan for AC so it is set up for a relay already with wiring in place to relate to a different switch, for those without AC it would be relatively simple to utilize what exists to add the second fan and extend control wiring.


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tvmaster said:
wiring up a meter and measuring real amperage
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typically takes more than just a meter, usually needs a shunt (a large low value precision resistor made specifically for the job.) typically 1.00 ohm, and you measure the voltage drop across it. (the 1 ohm value is to make the math easy). 1 volt x 1 ohm = 1 amp.
 
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