Ahhhh!! 1975 Sierra Classic lost all electric power...

[fotoboy]

Hey folks,
First post here, having an electrical problem.
I have a '75 GMC  C2500 (I think that's what it is.) that I drive daily, well the other day while driving home, it just died, and I have ZERO power.  Battery reads 13v, so it is fine... I checked all the fuses and relays that I can find, nothing.  I am exasperated... It seems like it should be something simple, but not so simple to find... sigh.

Any help would be greatly appreciated.

[VileZambonie]

Check your fusible links

[fotoboy]

Fusable links??  Can you elaborate?

[Rapid Roy]

A fusible link is a short piece of insulated low-voltage cable within an automotive wiring harness that is designed to protect the harness in applications where a fuse is unsuitable.

I would start with wiring from battery toward firewall, checking wiring continuity with a meter about every 12". Be sure to disconnect battery cables. If there is no continuity at any location, it is probably a blown fusible link. You can remove from wire and replace with same fusible link.

This may or may not be your problem, but it is a start for you.

[blazer74]

No power anywhere? Check near the starter for the fusible link, looks like a really big but splice. Could be melted.
There is another one up near the firewall above the a/c heater box at the junction block.

[philo_beddoe]

Exact same thing happened to my 79 camaro. The problem was   several melted wires,,,too close to the exh. manifold.

[fotoboy]

Thanks for the crash course... so:
I should have continuity on both sides of the link... I'll just stab the wire on both sides of the link with my multimeter.  Is this terminal on the firewall a type of fusible link also? if so, how do I replace that one (already tested post to post with no continuity)

thx

[75gmck25]

I would crawl under the truck and check the connections on the large bolt on the starter.  This is where the battery power connects to the wires for the alternator charge and power to the fuse box.  If that  nut loosens up you will have no power anywhere.

The firewall junction in your picture has the other end of the wire coming from the start lug.  Its the wire on the lower right. It has a fusible link in the upper section of the wire, and it will soften and burn through if there is too much amperage. 

Bruce

[blazer74]

The junction block on the firewall is just a connection point not a fusible link. Should have power on both sides of it.

[fotoboy]

Well,
I have checked the cable to the starter, its solid tight.  Checked most all of the fusible links I could find (like 4) and there was one that I could not get a reading off of. Maybe my probes didnt stab into the copper??? Stabbed into my finger just fine! 

So that one is BLACK and heading into or from the cab.  I'm assuming that auto parts stores have new ones.

Are there any fusebox fuses that can present this problem if blown? (They all look fine, havent put the MM on them yet, guaranteed crook in the neck getting under there)

I have nothing from post to post on the junction box, but on mine (the photo earlier in the post was generic) there is nothing even on the left post.

and... how the heck do I know what the amperage is on the fusible link?  I have the schematics and they dont mention it.  ??
thx

[fotoboy]

Anybody??

[fotoboy]

So... the fusible link that I thought was bad was not, it was fine after I jammed some serious metal thru that insulation...

[bd]

If ALL electrical appliances quit simultaneously, perform voltage drop tests across the battery and ground cables while attempting to crank the engine.  Ideally, there should no more than ~0.3 volt measured across each cable individually.  Battery cables can fail internally, usually due to hidden corrosion, yet appear outwardly in good condition.

--------------------------------------------

Responding to your earlier questions, refer to the factory GM, 1975 Wiring Manual as needed.

Automotive fusible links are 6" in length with their gauge size determined by adding 4 to the gauge size of the wiring they protect.  So 14-gauge wire is protected by a 6" long, 18-gauge fusible link; 12-ga wire is protected by a 6" long, 16-gauge fusible link, and so on.  See, How to make a Fusible Link in our Technical Pages.

Don't use an ohmmeter to test fusible links!  It's possible for a fusible link to fail in such a way that it can measure continuity with an ohmmeter, yet won't pass adequate voltage to power its connected circuit.  An ohmmeter can convince you that the links are in good condition when they really aren't.  Instead, use an incandescent test light or a voltmeter while attempting to crank the engine, or with the headlamps and other appliances switched on (even if they won't function due to no power).  The current demand of a simple incandescent test light, or the vehicle's electrical appliances while attempting to operate, will effectively load a fusible link to pinpoint it when it has failed.

[fotoboy]

Hey BD, thanks for the info.. couple of clarification questions...

 ""perform voltage drop tests across the battery and ground cables while attempting to crank the engine""  do you mean cable to cable?? or +battery to -cable, not sure I quite understand...  Also, this did happen suddenly, with no weak starting before hand, and the engine DIED while I was driving, which would not even involve the battery, if I'm not mistaken...correct?

not sure what I would be looking for with the VM on the fusible links, do you mean voltage on each side? (one lead to ground?)
thx

[bd]

Ideally, 100% of the voltage (B+) supplied by an electrical system is available to power the connected electrical accessories.  Occasionally, the voltage supplied to an accessory is less than 100% and the effectiveness of the accessory drops accordingly.  Voltage losses occur in a circuit anytime unwanted electrical resistance develops.  Excessive electrical resistance can result from loose or improper connections, burned switches, broken wire strands, insufficiently sized wire, incorrect use of terminals, poor installation, corrosion, partial melting, and so on.  The unwanted added resistance consumes (decreases, drops) voltage, leaving less voltage available to operate the connected electrical appliances - an appliance is any device powered by the vehicle's electrical system.  Unwanted voltage losses (actually, unintended voltage reapportionment or voltage being wasted on the wrong part of the circuit) can be discovered using voltage drop tests.

Measuring voltage drop across any wire is accomplished by probing one end of the wire using the positive probe of the voltmeter, while simultaneously probing the opposite end of the same wire using the negative probe of the voltmeter.  Generally, the positive voltmeter probe connects into the circuit at the positive-most end of the wire or wire section under test (closest to B+), and the negative voltmeter meter probe connects into the circuit at the negative-most end of the wire or wire section under test (closest to B-).   Applying this general description, you can test any wire or section thereof for undue loss of voltage.  However, the caveat is that a circuit must be energized even if not working properly while performing a voltage drop test.

In practice, to check the positive battery cable, set the voltmeter to <20 VDC and connect the positive probe of the meter to the positive post of the battery and the negative probe of the meter to the 3/8" battery cable stud on the starter solenoid.  Have an assistant attempt to crank the engine while you watch the voltmeter and record the measurement.  Repeat the test on the ground cable by connecting the meter negative probe to the battery negative post and the positive probe to shiny bare metal on the starter case.  Note that you need to connect to the battery posts, starter solenoid stud and starter frame, respectively.  This will test the complete B+ feed and ground paths between the battery, primary circuit junction and engine ground.

Regarding the fusible links, with the headlamps switched on, probe for voltage at the firewall junction block and then at the alternator, all measured with the negative probe connected to a good clean ground.