Two Electrical Issues

[tim71]

As I put my truck back together I have encountered 2 issues:

1) I replaced my original gage cluster with a new one that has a tach. The tach seems to work OK other than it bounces around temporily when you change the fan speed on the heater/ac blower motor. I'm using the original wiring harness, so tach wires are not associated with the harness at all.
2) With the AC controls in ac and temp on cold, shouldn't I be getting current at the plug that plugs to the compressor? I've been testing with a tester/light and I get nothing. Tested with fan speed in different positions.

Trying to solve any gremlins I can while it's still relatively easy to get to everything. I would appreciate any input.

[AussieinNC]

1/ Tach voltage supply is most likely in same draw circuit as fan circuit, changing the fan speed temporarily drops the tach voltage and thats why it fluctuates...run totally separate ignition on hot wire (with FUSE !!!) to supply side of tach...that usually foxes the issue...make sure you include inline fuse and also ensure the power supply si only on when Ignition is on.

2/ AC voltage to clutch circuit...I am assuming the system has all the necessary components including the vacuum components...

The biggest issue here is usually the system is not charged with gas sufficiently to enable the low gas cutoff switch...when charging a new system, it is sometimes necessary to bypass the low gas pressure switch to get the compressor to cut in...

[tim71]

I'm confused on the AC response. I thought our ac systems are a non cycling system, the compressor is either on or off. The POA valve adjusts the flow of refrigerant so the evaporator doesn't freeze up. I haven't plumbed the under hood part of the ac system yet, I was just trying to detect current at the compressor switch if the dash controls are set for the ac system to be on.

[AussieinNC]

As far as I know, there is always a low refrigerant sensor in the system that shuts down the compressor clutch feed voltage to protect the compressor from running dry...the refrigerant also has an oil load so if it loses the gas, the system can run totally dry without lube...

I will see if I can find a layout diagram...

[Steeveedee]

Yes, it is a constant run system. Is the system low on refrigerant? I'm a little hazy on when GM started using a Thermal Limiter fuse on their trucks. If your system is low, this fuse will pop. My '70 doesn't have it, and the compressor will run if the system is empty. To determine if you have this fuse, trace the compressor power wires back. If there is a three-prong gizmatchee in line, that's the fuse. You would have to replace it for the AC to work, if you have it. If not (which I think is probably the case), just trace back through the wiring until you find where you have power. Electrical problems just take persistence. I'd almost have to bet it's the switch. Mine died on the fan side awhile back. The AC would kick in but the blower fan didn't work.

[AussieinNC]

Is this an R12 or 134 system?

The low pressure switch is usually in the dryer reciever...

[Mike (Asheville)]

If the system is wired properly and has insufficient pressure (i.e., low or no freon) no voltage will be sent to the compressor. You can not test the compressor wire with an empty system.

Originally Posted by tim71

I'm confused on the AC response. I thought our ac systems are a non cycling system, the compressor is either on or off. The POA valve adjusts the flow of refrigerant so the evaporator doesn't freeze up. I haven't plumbed the under hood part of the ac system yet, I was just trying to detect current at the compressor switch if the dash controls are set for the ac system to be on.

[VetteVet]

Here's a couple of diagrams. The compressor clutch is powered by the 14 gauge dark green wire.

[tim71]

Thanks for the info. I need to study the 2nd diagram. What I was trying to accomplish was to see if the switch in the ac box below the dash was operating correctly. On the diagram it is called a thermatic switch. It is directly connected to an ac cable and lever. Maybe this switch can measure the air temperature in the cold air box, but there is no way it can detect the state of your refrigerant charge. I need to learn more about this switch.

[tim71]

The ac system will be converted to 134a. New compressor, new rec/drier, new POA valve and expansion valve both calibrated for 134a, and new hoses. Evaporator and condenser both leak checked. There was quite a bit of junk(leaves,pineneedles,dirt) in the evaporator box when I opened it up.

[VetteVet]

Maybe this will help

https://swtc.edu/Ag_Power/air_condit...thermostat.htm

[tim71]

Thanks for info. I still don't understand why the compressor switch is not showing current when the ac controls are set to cool and the thermomatic switch is currently in an environment of warm air.

[VetteVet]

One thing I noticed is that the horn relay in the upper left is used for a power junction with the gray wire being the hot wire to the AC switch. The horn relay was not used as a power junction after 1966 and you have a 1971. But the later models used relays instead of cables as shown in the top diagram. You must have power to one side of the thermostatic switch,shown as a tan wire in the second diagram.

I'm not sure what they used after 1966, apparently you have the cable to the T-switch, which closes the switch mechanically, and then the switch will control the power between the tan and dk green wire for the compressor clutch, based on the temperature of the airbox. At least I think that's how it works. You seem to know this so just follow the tan wire to the power source.

Dumb question-did you check the AC fuse in the panel and install a 25 amp fuse?

The orange wire from the fuse panel is the source wire for the relay and the fan switch.

[tim71]

Da, stupid me, problem solved. The test light wasn't grounded properly. I ended up grounding it to neg battery post. Results are: 12v current at compressor switch with ac control set to cool and blower switch
on, no current if blower off or ac control set to heat. I'm just trying to make sure everything operates correctly while truck is still blown up enough to get at everything fairly easily. After aprox 3 years I'm at this stage. Fortunately I retired last year and only work part time so my project is starting to move along quicker. Thanks for eveyones input.

[AussieinNC]

Just a thought, make sure you have a good ground strap between the cab and the chassis itself...I never rely on the incidental ground paths like throttle links etc...

Glad you worked out the issue...

That is a NICE build you have there...

I also just retired, and was hoping to get going on my 67 Longbed...but now I have a client that needs my help for six months or so...

Oh well...