Eng-1 20a Circuit Troubleshooting

wh1-t · August 1, 2010

Hi guys, I am starting a new thread from an old problem. Some of you may recall the issue I had with this fuse blowing (old thread was "power source for maf and purge solenoid?"). I am now trying to diagnose the wiring. Why is it that in the fuse panel I have continuity to ground all of the time, but the fuse does not blow right when I plug it in? This seems odd to me.

I have much of the wiring uncovered under the hood, and have not found any shorts yet.

This is for the 2001 Savana, by the way

gmtech4 · August 2, 2010

do you have any manual to go by ?? wiring schmatics? electrical experience?

wh1-t · August 2, 2010

Yes.Yes.Yes.

minibike132 · August 4, 2010

Hi guys, I am starting a new thread from an old problem. Some of you may recall the issue I had with this fuse blowing (old thread was "power source for maf and purge solenoid?"). I am now trying to diagnose the wiring. Why is it that in the fuse panel I have continuity to ground all of the time, but the fuse does not blow right when I plug it in? This seems odd to me.
I have much of the wiring uncovered under the hood, and have not found any shorts yet.

This is for the 2001 Savana, by the way

How are you checking this continuity to ground. If you are checking with an ohm meter through the fuse the reason is simple but might be hard to understand. I'll use a simple bulb circuit with a B+ to fuse to switch to bulb to ground circuit as the example, though the thought is the same for almost all circuits.

A bulb is a wire, it's just one that glows when current is passed across it, just as a fuse is a wire that burns when too much current is passed across. If you check a simple bulb circuit for a short by putting an ohm meter on the power side of the bulb and the other end of the meter to ground you will see almost no resistance. That is because the meter only puts out enough amperage to push the voltage it is suppllying to ground. That isn't enough amperage to light the bulb so there is not much voltage drop there for no real ohm value (or resistance). I have a note on the face of my electrical box that says just that "a bulb is a wire". The reason that note is there is that when I was an apprentice I chased a short in a headlight circuit only to find out there wasn't a real "short". The circuit was fine, my understanding of the circuit was the problem.

The circuit you are working with goes through modules and without knowing that you have the right tools and have the knowledge to use them the right way I'm not going to tell you to use tools that could cause dammage.

Ohm's law is simple, it's application is not. lf this fuse isn't blowing a soon as you put one in then you are not going to find it by checking any components unless you have direct knowledge of what each component draws under a normal operating condition. Those values are only known by someone that would check them for future testing knowledge. I don't have them.

If someone did have those values they wouldn't have static values (which is what is being tested with an ohm meter) they would have dynamic values that are obtained during voltage drop testing or amperage draw testing.

wh1-t · August 5, 2010

Thanks for your explanation Sparks. I have since decided to abandon the original wiring. Right out of the fuse box, I ran new wiring to the MAF and the purge solenoid (the 2 codes I got when the fuse blew). I left a lower amperage fuse in place for everything downstream of the new splice. The 15 blew, but the new stuff is fine (no more codes). Now I will run new power to all that is downstream (like O2 heaters). I basically got tired of opening wire looms (vans suck for access), trying to find a short. Thanks again Sparks!

digitalembrace · August 5, 2010

Thanks for your explanation Sparks. I have since decided to abandon the original wiring. Right out of the fuse box, I ran new wiring to the MAF and the purge solenoid (the 2 codes I got when the fuse blew). I left a lower amperage fuse in place for everything downstream of the new splice. The 15 blew, but the new stuff is fine (no more codes). Now I will run new power to all that is downstream (like O2 heaters). I basically got tired of opening wire looms (vans suck for access), trying to find a short. Thanks again Sparks!

After reading all your threads including the other old postings an idea came to mind. Not sure how similar the wiring is in the two different models but may be worth mentioning. In my 2004 silverado one of the oxygen sensor leads was rubbing where it went over the frame, bared the insulation and ended burning off and separating the wire and giving me a code that indicated a bad heater wire circuit for the 02 sensor. The fuse did not blow. As your short is not constant is it not possible yours could be shorting there? I did not see the break untill I removed the sensor.

Just by looking at the fuse you can pretty well determine if a fuse has failed because of an overload- the link simply opens a bit, or a short- where the fuse violently blows metal about from the link and sometimes blackens the viewing window. When sparks was talking about the light bulb circuit, an idea I had learned 30 years ago from an oldtimer came to mind. As a trained electrician if the fuse was blackened I used to use his trick in the old fuse panels, rather than wasting money on fuses. As simply as screwing in a light bulb, the lamp itself would light and itself regulate the current and protect the wiring from burning up from overcurrent or by using too large of a fuse. If the short was there the light would glow brightly and as soon as the short was removed the light would go out.

I do not think it would do any damage to put in a lamp instead of a fuse, may be a little voltage drop, but most electronics will operate ok from about 10 to 15 volts. The lamp would glow under some load but would be full bright when the short occured.

wh1-t · August 5, 2010

Yeah, I have no doubts now that it is a short, and just rubbing from vibrations, potholes, etc. The fuses were always blackened as you described. I did think awhile back that I found it- an O2 sensor wire that goes over the frame and a heat shield, that has kind of a sharp edge to it, but that was not the case.

I have since checked all of the O2 wiring with no luck. I never have gotten a code for them, but you also have the newer electrical system. I guess I don't know if there is a code on the older system for just the O2 heaters themselves. I will continue down the line replacing wiring, I kinda got fed up with looking for it.

Thanks Digital!