[dataway]

I just did a full power bench test of my entire dash harness and new engine and AC harness to check for hot spots and make sure all the dash lights were working.

While it was operating I kept an eye on voltage, total amps and amps through the various components. Of particular concern to me was the blower motor harness since mine, and so many originals show signs of burning up.

If anyone is curious the decent condition OEM blower motor draws 8-9 amps on low, and 22-23 amps on high. The Master Relay which energizes the blower motor is itself energized by the Master Switch .. which is a reed switch with two connections on the AC/Heat control head. What become obvious is that if the blower speed switch is on high, the Master Relay is switching about 23 amps, if the blower speed switch is on low the Master Relay is only switching about 9 amps. It occurred to me that it would be very good practice for me, and probably anyone with a 68, to make sure they always start the AC/Heat/Defrost etc. in LOW speed, then move it to high speed after the motor is running.

If you leave the speed switch in HIGH, then every time you slide the right hand vertical slider up from the OFF position it's going to cause the Master Relay to switch the maximum 23 amps. Which can't be good for the contacts I wouldn't think on an old OEM relay.

Anyway, not sure if anyone cares, but it could probably increase the life of the Master Relay considerably. Also, I have no idea if this same situation applies to 69 cars since quite a few changes were behind the dash that year.

Total amp draw with dash lights on (LED), blower motor on high and cigarette light pushed in was about 30-31 amps. That's all I could really hook up on the bench at this point. Throw in a full set of incandescent taillights, headlights, AC clutch engaged, radio blasting and jeez, quite the load on the alternator ( I'll be running LED headlights and taillights).

[gto4ben]

I did a similar load test for my 67 GTO using a clamp-on DC meter in the car. I was surprised how much current the AC and high beams drew. The most common problem I had was the master relay connector melting due to excessive heat from the Black/Double Orange power wire crimp contact resistance. I needed to solder the crimp and wire together to reduce the resistance and that helped.

[dataway]

Yep, very similar to my results. Used the same DC amp clamp method on the positive battery lead, and then on the individual leads to various components. Had a 10amp battery charger on the battery to maintain voltage.

I'm still use the original (might not be OEM) Master Relay ... cleaned the connections very well, used a new harness and anti-corrosive electrical contact grease.

Yep the common problem on original cars is everything connecting the Black/Dbl Orange melting.

[Old Goat 67]

As just a point to keep the heat out of there.

My 67 GTO was only about 3 years old when the master relay failed. In studying the relay contacts inside I found that they were pitted severely. That was caused by the current flow trying to "jump the gap" when the contacts released was my diagnosis.
So, cleaned the contacts very well, found a old Ign points capacitor and wired in into the power lead coming to the relay and put it back into service without spending the $35 GM wanted at the time.

Still working fine to this day, same original relay. Yeah, I know "Not Original".

[Half-Inch Stud]

Always kept on lowest speed unless needed for defrost.

Yet, i wondered why i found those relays/harness plugs cooked on some cars.

[dataway]

OG67 .. so when you wire in a condenser like that ... the positive lead of the condenser goes to the incoming power supply connection and the case of the condenser to ground?

[Old Goat 67]

Originally Posted by dataway

OG67 .. so when you wire in a condenser like that ... the positive lead of the condenser goes to the incoming power supply connection and the case of the condenser to ground?

Yes. Just like in the distributor.

It allows the "surge" current to be absorbed into the capacitor and protects the contacts in precisely the same way.

You can see it here where I mounted it under one of the relay screws way back when.
Protects all the time, no matter who is driving the car.

[dataway]

Think I might throw one on mine. Seems like a good theory, and certainly won't hurt anything. Imagine it will look plenty original considering the various other condensers and radio suppression equipment sprinkled around a stock vehicle.

[hgerhardt]

@dataway: one thing you forgot to mention was measure the temperature of various components under load. What I've found in my '66 GTO, the relay gets hot with the blower on med-hi and especially on hi. And it's an NOS relay with virtually no time on it. Other hot spots are the blower spade terminal, the spade terminals on the relay and fan speed switch. The resistor block connector stays cool enough.

I've checked the condition of all those various terminals and they all look perfect, but obviously they have too-high resistance. Next step is to replace the A/C harness with a new one to hopefully alleviate the high resistance in the terminals. Just not sure the aftermarket harnesses are of sufficiently high quality...

Another trouble spot is that ridiculously low-quality inline fuseholder behind the alternator that only A/C cars have. I replaced mine with a modern OEM GM inline fuseholder which holds an ATO fuse in a sealed housing.

[dataway]

I almost bought a thermal camera just for that purpose .... something I've wanted a long time but can't find enough reasons to buy

I'm using a new AC harness, and new Engine harness, quality seems as good as OEM, but that's not saying a lot.

Was just noticing last night how cheesy the 30 amp fuse setup is. At this point I've thoroughly cleaned any original terminals, applied some anti-oxidizing grease, checked the resistance as much as possible, going to have to keep my eyes and nose ready for signs of trouble. Obviously these system were sorely under-engineered, even most low miles cars show signs these connections were burning up long before they became "vintage".

[fatindian]

Thanks for the info!

[dataway]

I wonder if it would be advisable to just go ahead and solder the high current, accessible spade connections? I mean if you have to change out the master relay, the blower motor etc. it would just be an additional few moments to de solder the connections. Unfortunately that's not an option for the components in the AC/Heater control in the instrument panel ... you'd have to remove the whole panel to get to the them.

But then if you are replacing something on the control head you'd have to pull the instrument panel anyway.

[Old Goat 67]

Why?

If the connectors are compromised, replace them or clean them.

[Half-Inch Stud]

soldering the old crimp is difficult unless degreased, alcohol'd, scrubbed, then electricalfluxsoldered. I inspect, scrub re-tension and re-use, or replace the old wire with a much nicer stock old wire from a scrap harness.

I think the condenser helps with inductive spikes that occur when the contacts open. Thinking there is a shock when the fan is still rotating, or not.

[68pongto]

I completely re-wired one of my 68 A/C cars so the high speed does not go through the speed switch anymore, but only through the master relay like 69's are. I closely studied the two diagrams from 68 and 69 and then just rearranged wires in the terminals. It was obviously a problem back then too, that is why in 69 they completely changed the way they were wired. I lost one of the lower speeds in the process, but I can live with that. You pretty much have to run them on high all the time anyway to get any kind of air movement out of the ducts. I made some color-coded diagrams, I'll see if I can locate them.

[dataway]

68pongto ... would love to see the diagrams if you find them. Sounds like a good idea, I'll check the 69 assembly manual to see if it has anything on the 69 wiring. It's craze to run that 23 amps or so through the little switch at the control head. I can certainly live with losing LOW speed.

[68pongto]

Here are the wiring diagrams I did. The original 68 diagram, the improved 68 diagram, and the 69 diagram that I modeled the improvements off of.

[dataway]

Thank you very much. That will save me a ton of time if I make the modification.

Pretty clever piece of work there. It's beyond me why they would run full amperage from the Master Relay all the way backwards to the control head and then to the blower when it could run straight to the blower. And why did they run the lower speeds through the Master Relay when they are switching them through the blower speed switch anyway? Somebody dropped the ball there.

From your Improved schematic it looks like you do have the low setting? Looks like when the master switch is closed it would send power directly to the low speed connector on the resistor block without going through the speed switch? Then at settings above low the speed switch takes over and applies power to the additional resistor block connections?

Only downside I see is that the Master Switch is providing all the power to the lower speeds rather than just energizing the coil on the Master Relay. On the 68 that's a pretty wimpy switch, I bet on the 69 it's more robust.

[68pongto]

The car that I did the modification to is put away in storage, so I really can't look at it right now. I think I either lost a low speed or all the way down actually turns the fan off now, I can't remember. The 69 switches are also made by INDAK and use pretty much the same engineering in the switch. I had a couple of original 69 A/C looms laid out by the 68 loom when I did it and the 69 looms didn't show any signs of excessive heating like all the 68 looms I have seen.