Dwayne 76

Here is what I'm doing, tell me if this isn't correct: I've connected the two terminals of my Volt meter across the two posts where the fuse goes in the fuse panel, with the fuse removed, or blown. If the circuit is not shorted there should be no voltage, correct? If, however there is voltage then the circuit is shorted?

I tried that on one of the shorted circuits, the heater and blower control. With the ignition on, and the fan selector moved to any postion, other then off, I got 12.50 volts. If its moved off I get no volts. So, I disconnect the blower motor in the engine compartment, try the test again, this time no voltage. Is that to suggest the motor itself is shorting out??

I'm also troubleshooting the lighter circuit. I've tested the back light which is on that circuit, no voltage, as well as the driver side light, no voltage there either (I'm connecting my ground to the chassis. I havn't tried to test the clock yet, as I don't want to tear that out if my method is faulty.

Jvette73

When you connect the vom across the fuse contacts you are basically completeing the circut with the vom. So seeing 12v does not nessisarly indicate a problem. Some circuts are live only with the key on. Some are live all the time. So if you test a circut such as wipers, while the key is off, and you get a reading, then that would suggest a problem since the wipers should only be hot when the key is on. Likewise, getting no reading on a circut that should be hot all the time, such as the cig lighter, would indicate a problem.

bloodvette80

If your testing for short circuits, your better off measuring from the wire to ground in "Ohms" mode on the meter. If you get 0ohms, you know the wire is shorted to gnd, if you get infinite or _.___ then you have an open cct to gnd (which is good for a power wire, bad for a gnd wire). Keep in mind if your measureing the resistance from something like your heater fan to ground, there is continuity through the motor and you might see a few kilo-ohms from the power wire to gnd. But disconnect your battery, your ohmeter sends out a few microamps uA and measures the current and analyzes into an resistance reading. If you have residual current already flowing in the cct, your meter will read wonky. either very high or very high negative. gone for a smoke

LAvetteman

:iagree: with both of the above.

It is normal that you got a 12 v reading at the fuse holder when your blower motor is on because it is a rather large item. Smaller items will give you a different reading.

Basic electricity is a wonderful thing to know, it can save you lots of money and maybe even save your life one day. Learn more about it :)

bloodvette80

the reading that you are taking dwayne, is probably not what you're trying to find. I'll explain a little of the electrical theory. If your heater switch is on, which means your heater fan is getting 12V, the car's ignition is on, and you pull the fuse, of course the motor will stop becuase the current can no longer flow. BUT if you measure the voltage across the contacts where the fuse WAS, then you will see 12V. Right now your thinking how can there be 12V with the motor in series with my voltmeter. Becasue there is no current flow, there is no voltage drop. so if you put the +12V to a motor, then measure from the motor's ground wire to the battery, you see 12V. Anotehr example, you take a 9V battery, and a resistor that is about 10 trillion ohms. put one side of the resistor to the battery positive. Then measure the voltage from the other side of the resistor to the battery negative. exactly 9V, as if the resistor is not there. becuase theres no current, theres no voltage Drop. Voltage=Current*Resistance. Current = 0, so the voltage across the resistor is 0V. as soon as you conenct that other end of the resistor to the battery negative, you'll see a current of Current = 9V/10trillion. all this sums up to mean that measuring across the fuse contacts and reading 12V is not bad, doesn't mean its good either, don't feel like typing any more and explaining that. Most of your readings with your voltmeter should be from a wire to Ground, or flip it into Current mode and measure current inseries with the line.

Dwayne 76

I've always had problems dealing with electrical circuits. I was thinking that by using the Volt meter, I'd be able to measure the potential difference, between ground and the switch. I guess I should have taken the DC course that was offered, but I'm a programmer :lol: I only see in 1's and 0's :rofl:

Diode

OK guys, here's the deal..Dwayne, the test that you did tells you that the blower motor is not blown (open) and all of the wiring is intact. Otherwise you would have read 0 volts. What you need to do to find out why the fuse is blowing is measure current (amps), not voltage. Since we don't know for sure how much current the blower is supposed to draw, a good place to start would be taking some measurements on a friends car. Hook up your VOM the same way you had it (across the fuse holder, fuse out) and put it on "Amps". Now be careful doing this because it's going to draw a few amps in the "High" position and you don't want to damage your meter or blow its internal fuse. Make sure that it's capable of reading that high. Maybe start off on the 10 amp scale if it goes that high. You need to take 3 readings (low med & high speed). Compare these readings to your car. They should be pretty close. If your car reads significantly higher, you blower motor may have some shorted windings (happens). I'm saying that you should take all 3 readings just in case one of the resistors has somehow shorted to ground. This way you will be testing both the motor and the resistor assembly. This is really the best way to trouble shoot your problem. If you try to find the problem by measuring resistance (ohms) you're going to have a hard time because a motor is low impedence (low ohms) so it's always going to look shorted even if it's fine. Except of course when you have the switch on low or med because that puts resistors in series with the motor. If you have a repair manual for this car that has a wiring diagram and you can scan and email or fax it to me, I can send you back a diagram of just the fan circuit. That might help you isolate the problem. The only diagram I have is for my 68 and that uses a high fan speed relay and a "low blow" switch in parrallel with the regular fan switch. This may be a way different set up than yours. Let me know how you make out.

Dwayne 76

Diode, thanks for all that info. I tried measuring Ampers but didn't register anything, though I don't think I did it the same way that I was doing the volt meter test. I have an assembly manual for the '76, but is has no wiring diagrams. Thank you very much for the offer, I will give a look sometime this week for a shop manual. While I was using the volt meter, it registers 12.5 volts for low med, and high, though I guess that would be correct. I'll try the Amp test tomorrow, and see what I get, since I'll have a few cars here tomorrow, I'll just test theirs and see if it comes close. Again thanks a bunch.
:cheers:

Diode

Dwayne, the thing to remember when taking these measurements is that when you're measuring current, the ammeter needs to be in series in the circuit. That's why you want to use it in place of the fuse (take the fuse out). If the fan won't run (and the meter is reading zero) in this configuration, the fuse inside your meter is blown. Happens often when people forget that the meter is set for "Amps" instead of "Volts" when they put across the battery...POP! :nono:

7t2vette

Dwayne, I am a licensed master electrician, and if you are ever out my way, I would gladly sort this problem out for you.