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Need help with short circuit

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J

John Bryan

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Hi guys. I need some practical advice on finding a short that keeps blowing a fuse on my 71 TR6. It is the circuit that controls the temp and fuel gauges, as well as turn signals and brakes, so it is important. I suspect, for reasons I won't go into, the right rear brake light. Last summer, I cleaned every electrical connection I could find under the hood and put dielectric grease on them, which solved a number of issues. But this one is new. What is the real life way to do this? I understand the theory, but have never really tried to trace a circuit. Thanks, John
 
Hello John:

Does the fuse blow right away as soon as you replace it & connect the battery? If not activate, "turn on" one thing at a time; i.e. step on brakes, check fuse, directionals, one at a time, check fuse, etc. You can also try an ohm meter with all light bulbs out check between the "hot" inside socket & ground to see if there is a short.

I too lose my mind on some electrial stuff, but luckily these cars don't have that much "stuff" and number of circuits as the new cars.

When I get stuck I usually fax a copy of a wiring diagram to my dad (a former elect. eng.) & he walks me through it long distance!

Good luck & remember try & isolate the cause in an organized way.
 
If you don't have a volt/ohm meter already I highly recommend you get one. They really make electrical troubleshooting much easier.

In the old days they were expensive and only serious mechanics or total geeks (that would be me) could justify the cost. Nowadays you can get a digital multimeter for less than twenty bucks.


PC.
 
Good suggestion, PC. And speaking of digital multimeters, can those of you who are familiar please tell us which is the best to buy?

There seem to be several kinds with differing readout displays. Does it matter significantly what kind of $20 meter we buy for LBC use, or just any old digital multimeter suffice?
 
For LBC purposes, just about any meter will do. Ohms, volts, and continuity will be the handiest functions. Capacitance can be handy once in a while also, but now your talking about alot more $$.
 
As a minimum for old car work all you need is a meter with dc voltage and resistance scales. They all have those so basically any meter will work and a really cheap one is fine. Beyond that you'll pay more for higher accuracy, better construction and more ranges/features so it's up to personal usage and taste.

I couldn't survive without my continuity beep. It's really nice to not have to look at the thing while checking if stuff is connected.

Many very cheap meters don't have removable leads. If you're prone to mangling wires replaceable leads are a good thing. I like to be able to plug in longer or different tipped leads anyway but I rarely need to on cars.

A sturdy case is good. Many industrial models have protective covers (rugged rubber baby buggy bumpers).

Some meters are now coming with thermocouple temperature probes and even IR thermometers. They're pricier than plain meters but cheaper than buying those capabilities separately.


PC.

some examples:
RadioShack
Amazon
Technika

[ 02-05-2004: Message edited by: PC ]</p>
 
the only problem i've had with digital multimeters is trying to use them in cold weather.
 
Finding shorts is not so difficult if you break the car into sections. In your case remove all of the bulbs in the circuit, and then begin replacing them one at a time. When the area is discovered, then find a nearby area where the harness can be disconnected and work your way back. Then it's only a matter of looking closely at a short section of wire or fixture.

As to meters, both have their advantages. If you're working on a modern car with computers, only use digital. Otherwise, it's your choice.

One word on measuring resistance though. It can mislead you, and I always suggest measuring voltage drop instead (although don't always do that.) But consider the positive wire to the starter. Measure the resistance along the length of it. Then peel back the insulation somewhere in the middle and cut all but one strand and measure the resistance again. You'll get the same reading. Now put it back on the car and send your daughter off to college Sunday afternoon. Far better to measure the voltage at the starter when under load.
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by PC:
...I couldn't survive without my continuity beep. It's really nice to not have to look at the thing while checking if stuff is connected...<hr></blockquote>

Continuity beep is also great when setting static timing... lets you watch the timing mark while listening for the beep to stop the moment the points open.
 
Just a FYI: If you have a Harbor Freight Tools nearby (or go to www.harborfreight.com) they sell a digital multimeter for $5...I've actually seen it during the holidays for $2.99!!!
 
I can recommend Fluke meters- yes they are not the cheapy ones, and maybe more than most need for LBC work- but if you have been a good boy or girl maybe Santa will bring you one for Christmas...

One other important feature on whichever meter you buy is an amp meter as well. This will tell you how much current draw is on a circuit, and can be utilised many ways.

In the short circumstance
you could put the ampmeter on the feed wire of that circuit , and as you utilise each item seperately on that cicuit , watch what happens to the amp draw.
A helper may be needed to monitor the meter.

Good Luck
 
I agree with James. All my digital meters are Fluke. If you do a lot of electrical, or electronic work, they are good investments, as is any good tool. For the weekend warrior, they may be overkill.
Jeff
 
I agree with James and Jeff about Fluke meters, however, if you like Fluke, take a look at www.ueitest.com, they look amazingly similar, and they cost less. (hmm, I wonder who makes what for who here...)
 
I use the Boss' Fluke meter at work, but am pleased with my Craftsman model 82139 that I've got here at home. I also ordered the temp probe for it and am looking forward to better weather to play with it.
 
John,

Just wondering if you ever found your short. I've had the same problem in the past with - coincidentally - my '71 TR6.

Thanks,
 
Yo
The fuse is only going to blow on a dicient current draw.
I would suggest that you check under the dash to the VOLTAGE stablizer for the gauges & the 2 flasher units ( 1-hazard lights,1-indercators)
& make sure that these items have not gotten jammed or are rubbing against metal.
The other thing is to remove the flasher units 1 at a time as 1 might have developed a short.
Sound a bit on the sad side but it does happen
IF this fails
try a gallon of ave gas 7 ! match
Have a good one
Tnoy A
 
John, there is good reason to suspect the rear portion of the harness. The way it is threaded through the rear bodywork can cause trouble over time(lots of that has happened).Any dents or repairs in the area can be suspected of harming these old wires , which after all want to spill their smoke!!!!!
It shouldn't take too long to inspect the lomb in this area and retape it as needed.
good luck
MD(mad dog)
 
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