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View Full Version : Is it possible to "fry" your ammeter gauge?


Jacket
04-02-2008, 09:23 AM
So last weekend I was troubleshooting my faulty fuel gauge and pulled the instrument cluster out of my 40. The good news is that I found a short in the fuel gauge wiring and I'm pretty sure its fixed. The bad news is that I touched the hot lead on my ammeter gauge to the dash panel for a second or two, and (after a spark shower and some electrical smoke) now I think that gauge is not working. It used to be "lively", and bounce around when the truck was running. Now it just sits there and never moves.

I couldn't see anything obviously damaged, but then again, I don't remember what it looked like before "the incident."

subzali
04-02-2008, 09:33 AM
I don't think shorting out the hot lead would fry the gauge, as the power should come straight from the battery to the hot lead. I can't remember if there's a fuse (I don't think so), but you might have melted some of the wiring. Try and trace it back to the battery, or put an ohmeter on it, to check it. Mine usually jumps around when I first start the truck as the battery is being charged, but after a while it gets to the point where it doesn't move much. A quick way to check is to watch it (engine off) when you turn the headlights on and off - it should jump back and forth.

Oh and disconnect your battery next time you pull the gauge cluster ;)

Jacket
04-02-2008, 10:06 AM
:lmao: I know - but there was a lot of trial and error, and I guess I forgot one time to disconnect the battery :banghead:

DaveInDenver
04-02-2008, 10:42 AM
The ammeter in your FJ40 is really just a voltmeter in parallel with a shunt resistor. If you know the exact value of the resistor, then the voltage across it is the current. Remember Ohm's Law, V=I*R => I=V/R. IOW, 15A produces 15V across a 1 ohm resistor, 40A is 20V across a 0.5 ohm resistor, 10A is 1V across a 0.1 ohm resistor, etc.

8505

What that means is shorting the (+) side of the ammeter to ground just completely bypasses the ammeter and shunt. Most likely you melted wires and the reason your meter isn't working is a bad wire someplace. Like Subzali says, trace the wiring I bet you find one that is melted or burned up.

Jacket
04-02-2008, 12:09 PM
Figures.... I had so much fun tracing the wires for my fuel gauge, why not do it again?:rolleyes:

Thanks for the info.

DaveInDenver
04-02-2008, 12:38 PM
Figures.... I had so much fun tracing the wires for my fuel gauge, why not do it again?:rolleyes:

Thanks for the info.
It could be worse.

First, it's much simpler under the dash and hood on your 40 than the Tacoma. Second, well, it'll be the one that's black and charred...

treerootCO
04-02-2008, 12:43 PM
When I burnt my CJ5 to the ground on Holy Cross, I shorted my ammeter ;)

If you look on the high lift you can see the entire electrical system that had to be replaced. I am sure glad I brought the spool of red wire with me.

http://www.root45.com/upload/Bio/HolyCrossFire.jpg

subzali
04-02-2008, 01:02 PM
Who's Cruiser is that behind you? IIRC you sold the CJ right after you got back, and then came over from the darkside not too long after right? :p:

Hulk
04-02-2008, 01:55 PM
Nice chrome bumper there, Root. :D

subzali
04-02-2008, 01:56 PM
That's OEM on J**Ps, that's why the Cruiser gods degraded them from off the factory floor. By the time AMC caught on it was too late...

nuclearlemon
04-02-2008, 05:05 PM
Who's Cruiser is that behind you? IIRC you sold the CJ right after you got back, and then came over from the darkside not too long after right? :p:

that's zach's cruiser before he butchered it. it was a beautiful rig.

Rzeppa
04-02-2008, 05:29 PM
So last weekend I was troubleshooting my faulty fuel gauge and pulled the instrument cluster out of my 40. The good news is that I found a short in the fuel gauge wiring and I'm pretty sure its fixed. The bad news is that I touched the hot lead on my ammeter gauge to the dash panel for a second or two, and (after a spark shower and some electrical smoke) now I think that gauge is not working. It used to be "lively", and bounce around when the truck was running. Now it just sits there and never moves.

I couldn't see anything obviously damaged, but then again, I don't remember what it looked like before "the incident."

YES it is entirely possible (and not that uncommon) to fry the amp meter in precisely the manner you described. It *may* be repairable, maybe not.

For this reason, I ALWAYS advise people to disconnect the negative terminal of their battery whenever pulling the cluster on a 40.

Yes, I have also made arc welder when pulling the cluster on a 40 (only once, I learned :-). Fortunately the short was on the upstream side and did not damage the instrument.

Rzeppa
04-02-2008, 05:32 PM
What that means is shorting the (+) side of the ammeter to ground just completely bypasses the ammeter and shunt.

If the meter is buggered, then he shorted the (-) downstream side. Shorting the (+) upstream side can not damage the meter. Only the battery side feed wire or battery.

Rzeppa
04-02-2008, 05:38 PM
Most likely you melted wires and the reason your meter isn't working is a bad wire someplace.

If the between the battery and meter were melted (open) then the system gets no power. No lights no phone no motor car, not a single luxury....

The gauge is buggered. The surge overloaded and opened the windings in the meter. Which is a true amp meter, not a voltmeter. While the diagram shows a shunt which is correct, mechanical gauges are always amp gauges. Current flows through them, the windings create a magnetic field which deflects the mechanical mechanism to move the needle. In the case of an amp meter which is designed to measure large currents, most of the current flows through the shunt, and a small, calibrated fraction flows through the windings. But when the current through the shunt and windings, which are in parallel is excessive, the excessive current through the windings opens them.

DaveInDenver
04-02-2008, 05:56 PM
Oh, sure, I'd get the little Yoda for being all technical and talking about a D'Arsonval. ;-) I was told there are important details and there are not so important details. Basically waving your hands and saying it's magic apparently is OK sometimes. Who knew?

Don't have a FJ40 wiring diagram, so I just assumed that the hot side wire leading to the gage wasn't very big, but it sounds like it's a feeder for several things and so definitely listen to Jeff. If the negative side of the gage was what was shorted, that would definitely kill the movement.

corsair23
04-02-2008, 06:03 PM
Matt,

If you did indeed fry it I might have replacement for you. I'm pretty sure I have a complete cluster but I haven't looked at it since last year. Someone on here was giving it away and Subzali grabbed it for me last just before the Spooky Night run. I had delusions of going back to a stock cluster at the time :hill:

No idea of the condition of the cluster though. On a side note, I also have the air can (and possibly an air filter) from an early year 40 as well (no top though) from the same source that I can't use.

subzali
04-02-2008, 06:12 PM
Hm...now that you mention it I might have one too. I'll look, it might be at my parents' house.

Rzeppa
04-02-2008, 11:20 PM
Don't have a FJ40 wiring diagram, so I just assumed that the hot side wire leading to the gage wasn't very big, but it sounds like it's a feeder for several things and so definitely listen to Jeff. If the negative side of the gage was what was shorted, that would definitely kill the movement.

Yeah it's big, I'd guess 8 or 10 gauge or so. Basically all the current that flows into or out of the battery goes through there, except the starter motor.

DaveInDenver
04-03-2008, 05:46 AM
Yeah it's big, I'd guess 8 or 10 gauge or so. Basically all the current that flows into or out of the battery goes through there, except the starter motor.
When I got up this morning, I did pull out my FJ40 FSM and looked. I made a mistake, I thought there was a shunt external with small wires to drive the gage in the combination meter, but that is not right. The circuit comes straight off the battery, through the fusible link and into the ammeter and back out to the rest of the system. So all stuff above, yeah, that's just how the guts of the combination meter work, not so much good for troublehshooting...

See what happens when you go and try to cram too much into your head, you push other stuff out. I must have been remembering something else on my truck and now that I scratch my chin I think I know what it was. I had an MSD box on my FJ40 and with those you have to use a ballast resistor and that's probably what I was picturing.

Jacket
04-03-2008, 09:04 AM
Matt,

If you did indeed fry it I might have replacement for you. I'm pretty sure I have a complete cluster but I haven't looked at it since last year. Someone on here was giving it away and Subzali grabbed it for me last just before the Spooky Night run. I had delusions of going back to a stock cluster at the time :hill:



Hm...now that you mention it I might have one too. I'll look, it might be at my parents' house.


That'd be great. I would be much obliged.

corsair23
04-03-2008, 02:00 PM
That'd be great. I would be much obliged.

Just let me, or Subzali, know and we'll figure out a way to get one to you :thumb:

subzali
04-03-2008, 02:06 PM
looked last night before the meeting and didn't find it - will check Saturday night at my parents' house, too bad I didn't have it last night at the meeting :(