I just finished my project (a complete rebuild from frame up of 74 T-Top). I have Dual Spal Fans and a Sanden AC Compressor. If I run the car without AC all is well. If I turn the AC on high my battery goes dead. With a 74, when you put AC on high, it operates a relay that connects fused DC directly from the HOT terminal block (on Driver side fenderwell) to the blower motor (by the way the blower motor is also new as is most everything on this vette). I wired the HOT lead for the SPAL fans directly to the battery. If I run just the FANs the Alternator puts out 40 amps. On the wire that runs from the HOT Lead Connecting block, to HOT terminal of starter I measure 27 AMP and the other 13 amps is split between two wires that go inside the car to fuse block and ignition hot terminals. I also measure the 27 amps coming in to the battery + terminal and 13 amps going out on each of the SPAL FAN HOT leads that is connected to + battery terminal.

Now when I switch AC on high, the high speed relay operates and the Blower Motor is connected directly to the HOT Terminal on driver side fender. When Altenator sees the Blower Motor, the output from the Alt output drops to Zero.
Anyone else seen this problem that installed dual Spal fans and /or single wire alt ???

 

I have not experienced you problem but try this...

Remember the battery is only there to statr you car, after that it does not really need to exist, the alt should be supplying all power and charging the battery.

1. Connect your spals the the output post on the Alt, not the batt.


2. make sure there is a connection from the hot terminal directly to the alt.

 

The one wire senses voltage at the alternator only. You've probably got a significant voltage drop along the 10g cable that connects the distibution point (drivers side horn relay?) and the battert cable (at the starter). Try wiring the fans to the distibution point and replace the 10g starter to relay wire with 8g.
Gary

 

I am about to embark on a rewiring project and Spal installation. I did a lot of research and went to a site that will definately help with your electrical questions and issues. I bought a bunch of stuff from them and they have REAL good products.

www.madelectrical.com

 

While I'm no fan of single wire Alts, he was saying he had zero output. Even a single wire isn't that bad! Something is being disconnected

 

OK, where your problem most likely lies is the alternator is not powerful enough...you need minimun 100 amps, more the merrier...and also you need to use a FULL 3 wire install, none of this one wire crap....

second off, you wire those main power leads from your fan controll relays to the alt directly, and the ground leads too....

do not pass go on this wiring requirement...you MAY get away with the other, but doubtful...you will need a full 3 wire install to have a prayer of any voltage stability....

stop reading currents, and get with voltage at various points....
current reading drive techs crazy, and for NO reason....

beleive me, it's all in the VOLTAGE readings....do your analysis from there....

you need a 3 wire hookup, and if your one wire alt has that jinxed for you , you need to get another alt, without getting too technical, which since you are asking, I have to assume you know nothing of the internals...so lets stay away from that...

OK, now the 3 wire hookup is also going to NOW include the stock 3 wire, PLUW two extra leads...one to the heavy positive post output line, to the fan realy DIRECT, no feeding through any other wires....then the fan motor ground leads into the engine directly...I use the alt frame for ease of install , but in fact any engine direct ground is fine....do not circumvent what I say.....

NOW, use a 30-60 amp fuse, depending on what the fan (s) require....your call....use a GOOD type fuse with weathertite rubber shielding...NOT some damn junk open contact corrosion prone POS>...

IF you are confused about the alt 3 wire connections, IM/Pm me....

GENE

 

Agree with everything....100 amps....the three wire alternator....connecting the fans to the alternator output, not the battery.
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(On at least early C3, the way to connect directly to the alternator output was in fact by connecting to the correct terminal on the horn relay. A lug on the horn relay functions as the main electrical power distribution point for every electrical load on the car with the exception of the starter motor. The starter motor is the only thing connected directly to the battery.)