I have just installed my rear electrical harness on my 75 and sorted out the dash harness (tagging all the connectors and wires, this was left in the car like a bowl of spaghetti). Is there any way to test the harnesses to see if there are any problems before putting the dash back together. This is a complete body and now that I am getting closer to installing the interior I would like to check out all the lights and gauges. Thanks for the help.

 

If you have access to a lab power supply that has about 10 amps with two *****, one that controls voltage (set at 12V) and a current limit **** (set at zero), also a current display. Pull the fuses. Hook this up to the battery terminals, then slowly crank up the current limit **** and watch the current draw. Should be zero with no shorts.

Then put one fuse back, shut the switches off for whatever is on the fuse,and again crank up the current limit and check for zero current. Then you can turn on whatever as long as it doesn't draw more than 10 amps (the power supply limit).

Keep checking one fuse at a time.

Headlights and motors will need more than 10 amps but your lab power supply should automatically cut out at 10 amps or whatever it's capability is.

This is the way that I thought was the safest for checking out circuits particularity if there is harnesses just hanging there.

With the lab power supply, you always know what the current draw is..that's the advantage of doing it this way.

Others may have better ideas.

I did this for my 73 after a body off with harness hanging out, but before hooking up a battery.

Steve L
73 coupe since new

 

Thanks for the feedback Steve,
All I have is a 120v ac to 12v dc converter and a volt meter. however the harnesses are all hanging, just started to fit the a/c ducts to see how they fit. can these be used effectively?

Bill

 

If the harnesses are all hanging you should be good to go with the DC converter and volt meter. The current and voltage rating of your power supply wont really matter because you should not have a load. There should be little if any current draw. If you don't get 12V at the end of your harness you have a short. I did something similar with my harnesses. I connected the negative of the power supply to the negative battery cable. Then stuck a dental pick into the cabin end of the harness. I was able to measure voltage at the far end of the harness. The power supply I used was good for .25 Amps. When I had a short the circuit breaker on the PS would trip quickly. The disadvantage of a high current PS is that you may burn a wire before you can troubleshoot the short.

Also I checked all of the pins at the far end of the harness, in case the wires were shorted to each other. You should only have voltage on the wire that you are applying voltage.

 

Decent power supplies have a current limit ****. You keep the current limit **** at zero amps, hook the PS up and slowly crank the current **** up. With no short and all switches off, you should get zero current draw as you crank the **** up. If you start seeing current draw, stop and check where the current is going.

Steve L
73 coupe since new.

 

We use a Power Probe brand automotive power probe. It is by far, one of the most useful tools we have in our shop for electrical work. It is very easy to use and if you accidentally short something or misapply voltage or ground to a circuit, it has an instantaneous built-in circuit breaker.

Best $130 bucks I ever spent (get the III, that's what we have, not the II). It hooks up to the battery and has a very long cord and a built in ground clip. You can apply 12V or Ground with the rocker switch (remember, all the dash lights are always hot and it takes putting a ground to the outer metal ring, to make them come on). It also tells you, whether or not, if you even have a circuit to begin with! All you have to do is touch the probe and the green led will come on, telling you that you have a circuit.

Oh, and it also has two white LED's to light up the area your working in. If you have a problem with let's say a tail light, it takes seconds to put power to it, and then go back until you find where the voltage stops. Easy! Checking fuses? Just brush the probe tip across the fuses and you'll get a test tone and a light on the good ones and nothing on a blown fuse. Takes five to ten seconds. Love, love, love my Power Probe and if you don't have one, you should get one. When I installed my dash harness, that's what I used to check it, as I went along as well as checking the dash lights, right before I buttoned it up.

Prepare to leave your other tools on the bench. Once you connect the Power Probe III to your car battery you will have access to an amazing variety of diagnostic functions.

With the press of a switch you'll have an instant hot lead or ground lead. Use the probe tip and ground lead simultaneously to power up components right in your hand. Instantly check components for continuity as well as identify positive, negative and open circuits without re-polarizing hook-up clips. Locate shorts without wasting fuses.

The 20ft. lead will allow you to test bumper-to-bumper without jumper leads. Audio tone indicates positive with a high pitched tone and negative with a low pitched tone. The audio feature can be disabled when testing relays. The Power Probe III has 2 cool white LEDs that flood the work area with bright light. The housing is sleek and feels great in your hand.

Features:
•Power Up Components
•Audio Tone
•Voltmeter
•12 - 24 Volt Range
•20ft. Jumper Lead Set
•Bad Ground Indicator
•Continuity Tester
•Short Circuit Indicator
•Relay and Component Tester





http://www.powerprobe.com/pp3.php

 

Acquriam,
You can use your 12 volt power supply and your meter in series with the 10 amp setting. But there is no automatic current limiting with this set up.
To get around this, what you can do is put a 1 amp sacrificial fuse in series as well.

So the hook up is +battery cable, 1 amp fuse, voltmeter set at amps, PS, then neg. battery cable.

This then gets you current limiting by burning out the 1 amp sacrificial fuse and you can see if you have any current draw with the voltmeter.

This is a lot of steps since you want to do this 1 car fuse at a time, but in the end you will be safe to hook up the car battery without frying anything.

With your power supply you should be able to run fan motors etc. by using a higher amp sacrificial fuse.

Don't fry your voltmeter amp setting since most of the better voltmeters only go to 10 amps, cheaper ones are less. Any higher current and you will fry this ammeter setting. the rest of the voltmeter will still work. There is a small wire inside the voltmeter where the current goes thru but it can only handle the max current and will burn out. The ammeter reading is really only a voltage reading across this wire.

Also check any loose wire ends particularily unmated plugs or light sockets that have exposed copper connection strips-tape these up.

I don't know of any other way of safely checking out harness that have been hanging out or rodents chewing on them. But I would be interested if someone has a better idea.

Steve L
73 coupe since new

 

Dude, why would you be suggesting a highly complex Laboratory Power Supply set up? Most of the Automotive Techs, Alarm Techs and Stereo Installers use the Power Probe. It's gonna take him hours to check all this stuff out with a Lab Power Supply (I was an Electronics Technician some years ago). I could see it if you want to check current in highly complex situations, but for a car? No way!

I was able to power up my 30 amp fan, easily with the Power Probe.

http://vid205.photobucket.com/albums...psa9fef975.mp4

 

For auto application, your gizmo looks better.

I never suggested to go buy a lab power supply.The are pricey. Some people have one hanging around for other projects. But again for the home restorer, I wouldn't buy a $130 test instrument either, just for a one time check of wiring harness. For shop use $130 is cheap considering it's versatility and it's continuous use.

If I didn't have a lab power supply, I would have used the sacrificial fuse approach mentioned above.

Steve L

coupe since new

 

Steve, for anyone who owns a C3 or other vintage vehicle, the Power Probe is nearly a 'must have'. Believe me, you aren't going to be using it 'for a one time check', because these cars, always come up with something electrical, sooner or later.

Here's a thought. I always factor in, what I call a "Time and Frustration" factor into a vehicle. Electrical has to be one of the most frustrating, difficult things to check on a vehicle and if I spend $80, $90 or even a $130 on a tool, like the Power Probe, that will save me hours of agony, then I'm all for it. I'd rather have the tool, than not.

Different strokes, for different folks. A multimeter will do it, but it's far easier and more intuitive, with the Power Probe!

 

I'm gonna have to look at getting one of these. Thanks for your info.
Steve L
73 coupe since new

 

Thanks for all the great advise
...might just have to make the investment of 130. If its that useful in sure I'll be using it often. I need to set up the radio also. Steve thanks for the wiring set up info.

 

I would like to again thank everyone for the reply's and tips, had time this past weekend to check the harness and had power to the end of all the connections from front to rear! key switch even worked as well as the a/c, blower and heater switches...now to begin putting it all back together...the 75 will ride once more.