Calling Electrical Guru's - A/C System


I have the Retro-Air A/C system installed and have noticed that the ignition switch gets very warm (maybe even HOT) when the system is turned on for any length of time. Down here in Alabama, we want the A/C to stay on a lot!

I wired the system in accordance with this wiring diagram:

Is it correct to say that the power for the evaporator blower is coming through the ignition switch > fuse #6 and therefore the reason the ignition switch gets so hot? There are a lot of relays to figure out in there!

I am assuming that, even though all of my signal lights are L.E.D.s, it’s not advisable to simple move the white wire from the switched fuse #6 to the unswitched fuse #4, right?

Yes, probably best to install a relay but I’m having a little trouble figuring out where to install it. Any direction in that regard would be appreciated.

And all advice is always appreciated.


Not an AC guru by any means but looking at the wiring diagram the one load off fuse 6 that I can see ‘might’ pull some current is the heater blower. Can you get a reading of the amps being drawn?

It looks to me like the entire AC circuit is powered, via relay contacts, directly from the battery. That includes “AC motor” which I assume is the blower motor used by Retro-Air? The only thing powered through the ignition is the relay coil. IMHO.

Take a long, slow look at the diagram. It’s very clear. What you need to know is that the W lugs on a relay are the electromagnet winding. The C lugs are the contacts which carry the load. If you look carefully, you will find the load is connected to the B+ lug on the alternator. If you look at a factory installation, there will be a ring terminal with a 3/8” disconnect there for the purpose.

The W contacts take power from the inside of fuse six, they don’t go through the fuse. If there’s an extra lug, you can move that connection to the outside, which would be safer. But if you don’t wire the C contact correctly, you will burn out your ignition switch and risk a fire for sure.

There is a lot of current going through the switch.
The spades on the back that you clip onto are riveted to the switch and over time, the rivets loosen a bit. This creates more electrical resistance and resistance means heat is generated

I had the same problem. I took out the switch, cleaned the spade metal around the rivet until fresh copper colour, then cleaned the rivet and the landing the spade rivets to. Then I soldered the rivet/spade landing area.

Problem went away

If you post a photo of the rear of the ignition switch, I will highlight the areas that need to be cleaned before soldering

Dennis 69 OTS

The good news is the car is up, bonnet off and battery out so I can step back and re-trace everything to make sure it is wired IAW the diagram. The bad news is I’ve run out of time and it will take me a few days to get to it.

Thanks for the recurrent training and suggestions. I’ll post back when I know more.


A quick update.

I have only myself to blame. I have found that every time I “farmed” something out during the restoration*, mistakes were made that I had to “discover” and fix. I have some stories that might shock you.

In this case, I contracted with someone else (experienced?) to finish the install of the Retro Air system while the car was having the interior installed. The primary task was to finalize the installation of the evaporator unit and charge the system. I did not inspect every element of that task because the system worked. But now, looking closely at the a/c relay the ONLY wire connected to it is the NW from the alternator! Nothing else!

At this point I’m not sure exactly how the system gets it’s power but apparently it gets it through the ignition switch, therefore the heat. I will proceed to take 2 steps back, remove the evaporator and wire it correctly. It’ll take me a while but I will post an update when finished.

Dodged a bullet!


*Except engine, gearbox, rear dif, water pump, and carbs

Indeed you did. …

I have the Bosch two wire alternator install IAW your CoolCat write-up. With the A/C, it looks like I need to make it a three wire install. Just to be sure, I need to connect the NW wire to the big B+ lug on the Bosch, right?

Other than that I have restored the A/C wiring to match the diagram linked above, including proper color wire! Thanks to all the help here.

Yup. It’s a pretty ugly place for it, but that’s how they were wired at the factory. Wired this way, the A/C will not place additional load on your ignition switch. If I was designing this from scratch, I’d it run to the Positive “terminal post” behind the battery instead, just to get it out of sight. I’d also install a fusible link to protect the wiring. These early a/c systems were pretty scary.

I can’t tell from the photo, but if you’ve just twisted the wire around the stud, you should go back and do it with a ring terminal.

Got it. I haven’t terminated the NW wire yet, just stuck it there for the picture. Thanks!

To update and resurrect this thread:

I have made the A/C wiring match the wiring diagrams, with the proper relay in the proper location and proper color wires! The load is now being carried directly from the alternator B+ terminal as described above.

I have just returned from a 4 day, 3 night 860 mile road trip where the temps at the lower elevations pushed 100 F.

The ignition switch was still too warm for comfort. It must be that continuous operation of the radiator cooling fans was the cause so for that reason and to eliminate my obsessive opening the bonnet to keep the paint cooler after shutdown I’m going to make the wiring mod outlined here:

In order to confirm proper operation of the cooling fans, what is the normal expected current draw?

Dual fan S2 setup should draw about 16A.

Craig several years ago my ignition switch would get quite warm to the touch. I think it had been in there for a long time and perhaps since the car was built. One day it fried and melted the wires that go up thru the tach. I replaced it with Lucas 34680 and swapped the original key tumbler into it. It seems to be heavier construction than the one that fried.

Also my fan fuse used to get warm to the touch. When I put the Coolcat fans in my car it solved that problem. No more problems with ignition switch or fan fuse in several years. My car does not have AC and I am certainly not an electrical guru so I am just passing along some of my experiences that sound somewhat similar to what you describe.

68 E-type FHC

Thanks for the tips on the ignition switch David and Dennis. I’m going to address that when I’ve assured myself that the circuits are operating and drawing power as designed. Thanks Mike for the data point.

This is an interesting topic. If for my own peace of mind, I wanted to put a relay in to take the load off the ignition switch, how much current should it handle? 40 amp enough?

'64 S1, 3.8 FHC

For the fan circuit, 40 amps would be plenty.

I’m thinking for the whole switch, all switched amps?

Ah… sorry. 40 should be good, 50 would be an insurance policy. Just make sure if that relay is fused, it is not fused greater than the highest stock fused circuit.

As I think I understand it, this relay:!/English/parts/8814b869-be02-4e5e-b858-b21db77bc4e4
can be used as “FAN RELAY (1)” in this diagram (on the left):

(1) the resistance across W1 & W2 is approximately 76 ohms
(2) a jump wire is installed between C2 and W2 on the relay

Is that true and how do I know what the resistance is before purchase?

Reason: Someone has installed a relay in this position with 16 ohms between W1 & W2 which, as I think I understand it, increases current and may contribute to the heat in my ignition switch.(?)