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post #1 of (permalink) Old 01-10-2005, 04:51 PM Thread Starter
 
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Testing fuses the easy way.

I see threads (and hear it said) all the time "pull" the fuses and check them.

Don't know why more people don't check them the easy way.

Hook up a test light and test both sides of them.
If it is hot on one side and not the other - you have a blown fuse. Hot on both sides the fuse is good.

With a test light, you can check a whole fuse box in the time it takes to pull and look at a couple fuses.

Plus - I have seen MANY fuses that "looked" good, but were actually open in the end cap or had a hair-line open and would not pass current.

Been doing it this way for over...

ahem - well a long time.

[img]images/graemlins/codger.gif[/img]

Can't imagine how many hours, and broken fuses, and skined knuckles I have saved.

As an addded bonus, I don't have to go find my darned glasses to see a testlight.

[img]images/graemlins/smirk.gif[/img]
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post #2 of (permalink) Old 01-10-2005, 05:07 PM
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Re: Testing fuses the easy way.

Q. Does this work?

A. Yes

Q. Would I do this?

A. No

Q. Why not?

A. Fuses can be flaky and can fail when removed. They can also fail under load, and self heal when the load is removed. Fuse holders are flaky and can fail when you pull or replace a fuse.

IMHO, it's best to remove the fuse, inspect/test it, inspect/test the fuse holder contacts and replace the fuse in the holder.
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post #3 of (permalink) Old 01-10-2005, 05:19 PM
 
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Re: Testing fuses the easy way.

it is also not good for checkign corrosion at the fuse terminals in the under hood fuse boxes.
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post #4 of (permalink) Old 01-10-2005, 05:34 PM Thread Starter
 
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Re: Testing fuses the easy way.

[ QUOTE ]

A. Fuses can be flaky and can fail when removed.


[/ QUOTE ]

Agreed. Another good reason to NOT remove them.

[ QUOTE ]

They can also fail under load, and self heal when the load is removed.


[/ QUOTE ]

Agreed again. Which you can sometimes spot when you test them "in situ" with the power on. Not sure how you would test for this out of the circuit though.

A good example of this was the 70's and 80's GM high fan circuit. There was a connector that used to corrode causing the high speed on the fan to fail. The circuit would test good with power off, but would open under load. I have seen this quite a few times in other applications, but this example sticks in my mind, because it beat me to death the first time I had to diagnose it.

[ QUOTE ]

Fuse holders are flaky and can fail when you pull or replace a fuse.

IMHO, it's best to remove the fuse, inspect/test it, inspect/test the fuse holder contacts and replace the fuse in the holder.

[/ QUOTE ]

Still agreeing with you to a point.
I would agree with you on a fuse holder (in-line fuses rot) but not on a fuse box.

For my enlightenment, would you, in the normal course of events, test the fuse holder in a factory fuse box and if so - how?

Thanks.
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post #5 of (permalink) Old 01-10-2005, 05:57 PM
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Re: Testing fuses the easy way.

I'll throw another wrench in there

Transformers

They will show both sides of a fuse good only to find the fuse bad.

I have 16 mechanics and can't get it in thier heads to pull fuses to check them.

test lights are great for quick fast checks but when you get stumped pull them

Clint
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post #6 of (permalink) Old 01-10-2005, 06:01 PM
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Re: Testing fuses the easy way.

[ QUOTE ]
For my enlightenment, would you, in the normal course of events, test the fuse holder in a factory fuse box and if so - how?

[/ QUOTE ]

To do a good test, remove power, clip leads to the the contacts. Set the Ohm meter to Beep, if it has that function...Then Ohm out from the lead to the wiring harness feed point, while shaking the heck outta the contact with pliers if needed. Often contacts are two piece contacts held together via a brass rivet at it's base. The rivet connects the contact to the buss feed. This joint is a point of faiure, especially in higher current circuits like fans and lights. The voltage may get to one side of the rivet/contact, but not to the other side. Shaking, tweeking the contact may restore function, or break the circuit, depending on the corrosion or loosness of the rivet. It's a crap shoot.

I've always felt it's better to remove a possible bad fuse, have it break for good than put back a fuse I can't depend on.

I know how to use wire strands to simulate fuses when needed, so pulling a bad fuse doesn't disturb me. Also, I carry a full set of extra fuses, so it's a rare day I have to depend on gum wrappers and stranded wire for fuse material. Never leave home without 'em.
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post #7 of (permalink) Old 01-11-2005, 08:54 AM
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Re: Testing fuses the easy way.

There are always these too. They have an LED that lights to tell you the fuse is blown.


I can test a fuse, but they would be nice for a S.O.'s car to give peace of mind.
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post #8 of (permalink) Old 01-11-2005, 11:36 AM
 
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Re: Testing fuses the easy way.

Well if something is dead I think you have to do a little logical thinking to do some fast trouble shooting.

If you do the test light check and both sides of the fuse check out OK but you don't have any power at the connector for the DUT then you have to assume you have a broken wire or a bad rivit in the fuse box.

So I think the test is still valid. Just needs to take it one step further to determine the problem.

Now that my Gauge diagnostic page is 99% finished, I'm working on another artical on how to do a ton of troubleshooting with just a test light.

Two other ways to test a clip would be with a Temperature meter or a volt meter. You can use a temp probe to measure an increase in temperature a bad contact will make in the fuse box. or a volt meter to measure the voltage drop across the clip and the buss were it's rivited. Most DVMs have a Millivolt range to read in this range.

I should grab my old glass fuse box and run some test. It's loaded with oxidized connections and loose rivits. [img]images/graemlins/smile.gif[/img]
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post #9 of (permalink) Old 01-11-2005, 12:28 PM
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Re: Testing fuses the easy way.

He He -- years ago I was manager at an electronics manufacturer. The final test department found a problem -
The new amplifiers were blowing fuses like crazy - every time a new one was turned on it didn't work - blown fuse.

The tech would replace the fuse and it would work fine. But we couldn't ship it that way - something must be wrong.
It held up shipping and production till we found the cause.

It ended up the girl in "Incoming Quality Control" tested 100% of the fuses per procedure - "yup, they all blew at 10 amps just like they should." She tested them (destructive testing) then put them back in the fuse holder. Production was installing pre-blown fuses!

She was supposed to test only a very small sample - like 1 in 10,000. DUH!
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post #10 of (permalink) Old 01-11-2005, 12:34 PM
 
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Re: Testing fuses the easy way.

On two of my Jeeps I found a 20 AMP(?) Circuit breaker were the fuse for the wiper motor is suppose to be. Any one else have a CJ with circuit breakers for the wipers instead of a regular fuse?

Was this a regional thing or with an option? There were used in the fuse box with the plastic fuses.
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