[dataway]

Yep, can't imagine the dashboard control would have lasted long switching full power to the blower with little sliding contacts.

[Old Goat 67]

Wait, my 67 had the same wiring as the 68 and it has worked flawlessly for 51 years!

Only thing I did many years ago was to put an ignition condenser on the battery feed to the Master Relay to protect the contacts from arcing, same as the points in the distributor.

[Greg Reid]

Yeah, I guess mine still works too actually but it still makes a lot more sense to use the relay for the high speed. Maybe in the colder climates the hi speed setting is used more. I dunno.

[dataway]

Quite possible the 68 and earlier used a different fan speed switch that was more up to the task of switching more current than what they wanted to use in the later models.

Still looks to me like the later model wiring makes more sense. Also looks like it would allow the use of smaller gauge wire in several places that might save 5 cents per vehicle

[Pepi]

Unless I missed it. Have you tried unplugging the clutch coil with the ambient switch hooked up to see if it still blows the fuse? Again unless I missed it, have we confirmed which fuse it is blowing, the inside one or the one behind the alternator?

A lot of good information here and troubleshooting tips.

[Ben M.]

Seriously, the advice here is good and the troubleshooting is relatively simple even with no multimeter. Unplug the AC solenoid\clutch wire and ambient switch, run blower. Fuse still blow? Wiring harness issue or control head issue. If not, plug ambient switch back in. Fuse blow? Switch (or wiring to it from control head) is bad. If not, plug in clutch solenoid - if fuse blows it's the AC clutch or the wiring from the ambient switch to it. This is all precluded on the assumption that you're actually turning the AC clutch switch on in the dash as '69-72 cars have an AC switch in the middle left that you can turn the AC clutch on and off with. If you are keeping the switch on Vent and the fuse still blows, the ambient switch is a red herring as it isn't in the circuit and you've got issues in the control head or blower relay controls (once again assuming everything is hooked up correctly).

I was invited up there to visit a friend but am not sure if my invitation still stands otherwise I'd give you a hand with my multimeter as I do electronics for a living. As said earlier, a competent electrical tech could have it diagnosed in half an hour or so.

[Pepi]

Ben,

That would be my logic for trying to figure it out if it is the inside fuse blowing. The ambient switch is "in the middle of the circuit", may not be the right circuit, but agree with what you are saying, unplug it all and slowly start plugging items back in one at a time. Helps narrow the search.

[Greg Reid]

Only the inside switch is blowing.

[BN]

Originally Posted by Ben M.

Seriously, the advice here is good and the troubleshooting is relatively simple even with no multimeter. Unplug the AC solenoid\clutch wire and ambient switch, run blower. Fuse still blow? Wiring harness issue or control head issue. If not, plug ambient switch back in. Fuse blow? Switch (or wiring to it from control head) is bad. If not, plug in clutch solenoid - if fuse blows it's the AC clutch or the wiring from the ambient switch to it. This is all precluded on the assumption that you're actually turning the AC clutch switch on in the dash as '69-72 cars have an AC switch in the middle left that you can turn the AC clutch on and off with. If you are keeping the switch on Vent and the fuse still blows, the ambient switch is a red herring as it isn't in the circuit and you've got issues in the control head or blower relay controls (once again assuming everything is hooked up correctly).

I was invited up there to visit a friend but am not sure if my invitation still stands otherwise I'd give you a hand with my multimeter as I do electronics for a living. As said earlier, a competent electrical tech could have it diagnosed in half an hour or so.

I unplugged ambient switch and compressor, blower works in all four speeds. plugged in ambient switch. fuse block fuse quickly blew.
If you can tell me what to try next, that'd be great.
If you're coming u this way I'd be happy for you to name your price to take a look. Right now top is down, seats, and radio are out so it's easy access. Can't take it to a shop anytime soon as its single digits and lots of salt everywhere.
I appreciate all the interest and help.

[Pepi]

Originally Posted by BN

I unplugged ambient switch and compressor, blower works in all four speeds. plugged in ambient switch. fuse block fuse quickly blew.
If you can tell me what to try next, that'd be great.
If you're coming u this way I'd be happy for you to name your price to take a look. Right now top is down, seats, and radio are out so it's easy access. Can't take it to a shop anytime soon as its single digits and lots of salt everywhere.
I appreciate all the interest and help.

So if I understand correctly, with compressor unplugged still, it blows the fuse once you put ambient switch in.

If that is the case, I would try and use a jumper wire in place of the ambient switch just to rule out ambient switch. If is still blows, the trouble is somewhere between the ambient switch and compressor. It would have to be a pinched wire or a short in a connector.

[BN]

Originally Posted by Pepi

So if I understand correctly, with compressor unplugged still, it blows the fuse once you put ambient switch in.



If that is the case, I would try and use a jumper wire in place of the ambient switch just to rule out ambient switch. If is still blows, the trouble is somewhere between the ambient switch and compressor. It would have to be a pinched wire or a short in a connector.

That is correct. Also if I unplug ambient switch and plug compressor back in the fan works on all four speeds....


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[Old Goat 67]

Do you have a volt ohm meter?

Should be an easy thing to isolate that lead from the switch to the compressor and look for a low resistance to ground.

[BN]

Originally Posted by Old Goat 67

Do you have a volt ohm meter?

Should be an easy thing to isolate that lead from the switch to the compressor and look for a low resistance to ground.

Within the next day or two I will jumper the ambient switch and get a volt meter and will report back.


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[Old Goat 67]

Sure sounds like you have a short to ground somewhere from the sensor to the compressor coil. Could be the sensor itself or the wire or a bad coil on the clutch.

[Pepi]

Originally Posted by Old Goat 67

Sure sounds like you have a short to ground somewhere from the sensor to the compressor coil. Could be the sensor itself or the wire or a bad coil on the clutch.

That is what I am thinking, but since the compressor is still disconnected I am leaning away from that.

[Greg Reid]

Why would a short from the sensor to compressor blow the interior fuse and not the alternator fuse?

....
Also, the following is worth looking at-

Also if I unplug ambient switch and plug compressor back in the fan works on all four speeds....

Actually, looking at the print, plugging the compressor in with the ambient switch out is showing nothing at all. The compressor is still isolated from the rest of the circuit..unless I'm missing something again.

Bottom line is, if it's blowing only the interior fuse, the problem seems to be in the master relay circuit as far as I can see.
That's the only part of the circuit that pulls current through that fuse.

[Old Goat 67]

Greg,

The way this circuit was changed from 67 &
68, was the relay is feeding ONLY the Hi Speed on the fan.
Kinda hard to see, but pay attention to the fact that the relay winding is only energized when the fan speed is in HI, as the switch is drawn.

Everything else is fed by the fuse box fuse.

Now, with this schematic, everything starts to make sense.
Unplugging the sensor keeps the fuse from blowing, therefore the only suspects left (shown in Yellow) are the sensor being grounded itself,
or the wire going to the compressor clutch, or the Clutch itself.

[Pepi]

Originally Posted by Old Goat 67

Greg,

Now, with this schematic, everything starts to make sense.
Unplugging the sensor keeps the fuse from blowing, therefore the only suspects left (shown in Yellow) are the sensor being grounded itself,
or the wire going to the compressor clutch, or the Clutch itself.

I agree and if an ohm meter isn't available, that is where he could try using a piece of wire to jumper in place of the ambient switch to see if fuse still blows. if it does, that almost 99% eliminates the ambient switch. If the fuse doesn't blow, then it sounds like the ambient switch is internally shorted/grounded.

I guess I don't know where the ambient switch would be getting a ground from though. Isn't in mounted on the fiberglass case?

[Old Goat 67]

From an earlier post, it looks to be on top of the cowl???

Shorted wouldn't hurt nothing, grounded sure could.

[Pepi]

Guess I missed that. I still say disconnect and use a jumper wire or paperclip to bypass sensor. Heck, could even try removing sensor from cowl so it isn't grounding out.