Ammeter Fuse size?

[Randy and Easton]

Have ammeter in gauge cluster and does not work. Previous threads show 2 fuses, we can find only one and it’s blown and broken off in old holder. What size fuse does it need? Going to replace it with a new bladed fuse holder just need size. If there is another fuse any hide and seek information is appreciated as well.
Very small wire, like 14gauge so I’m thinking not much? 3 or 5 amp? What previous post seem to indicate

All help appreciated

Randy and E

 

[Goldie Driver]

Have ammeter in gauge cluster and does not work. Previous threads show 2 fuses, we can find only one and it’s blown and broken off in old holder. What size fuse does it need? Going to replace it with a new bladed fuse holder just need size. If there is another fuse any hide and seek information is appreciated as well.
Very small wire, like 14gauge so I’m thinking not much? 3 or 5 amp? What previous post seem to indicate

All help appreciated

Randy and E

Wow - been a while since seeing 1 of those !

Seems like it is a short 3 Amp fuse, but ...


That's from a 32 or so year old memory.

 

[Randy and Easton]

So cool to just replace with a blade style 3 amp? Is there a second fuse everyone talks about. We found nothing.

 

[RustyPile]

The ammeter circuit is a 2 part parallel circuit, called a shut.. One part is a calculated length of the main power distribution wire.. The other part is also calculated length and made up of two small gauge (I think 18 ga.) wires, one going to the ammeter, and the other coming from it. That black wire you have in the picture is one of those two wires.. The other one is located on the engine side of the core support near the left headlight.. Both the wires have a 3 amp fuse that is only to protect the ammeter.. The length of the wires making up this circuit are calculated precisely in length.. Do not alter the length in any way. Simply remove the pieces of the fuses and replace with 3 amp fuses.. All the fuse designations are listed in the service manual and wiring diagram..

 

[Randy and Easton]

Thank you @RustyPile. That answers many questions actually. Ammeter never worked. AND we had this black wire that was cut off and bare by PO sometime. It’s hot 12V when we tested it and never knew where it went. It is right next to the black and white wire in same spot that has the fuse holder we found. So I guess we figure that second 3amp fuse holder is gone and that’s why 1 meter does not work and 2 what the heck that wire is now.

Is there a way to figure how long the wire has to be and just add a blade style inline fuse holder for that wire?

I guess another strange question is where does it go? Or is it a certain length with a fuse?

 

[RustyPile]

Both ammeter wires will check "hot" because they both lead back to the main power distribution wire, although the points where they connect are several feet apart on that wire. There may be a formula for calculating all the dimensions of the wires but I don't know that formula... One of those black wires has a white strip on it, but both wires run from their respective connection points on the main distribution wire, back to the the two terminals on the ammeter .. Who knows what has been hacked and jerry rigged on your truck in the past ~ 50 years.. Over the years, the resistance values of the circuit have changed and the accuracy of the ammeter has become questionable. It's quite possible the meter itself is now non-functional.. Replace the broken fuse, and, maintaining the length as near as possible, replace the missing fuse holder with the same 3 amp glass fuse type.. The important purpose of this entire circuit is to know if the charging system is working.. The ammeter is not graduated with amperage rates.. It simply indicates the magnitude and direction of the current flow in the charging system.. In summary, since the circuit has already been butchered, replace missing parts and "hope for the best"...

 

[Randy and Easton]

@RustyPile , gotcha. Will do. The black and white wire is the one that is intact and had broken fuse. The solid black wire is the one cut at fuse block.
We will get another glass fuse holder and run it up to the area of the front left headlight and terminate it there and hope for the best!
The ammeter is new when we rebuilt the cluster a year ago or so.

Thank you again sir.

Randy and Easton

 

[RustyPile]

@RustyPile , gotcha. Will do. The black and white wire is the one that is intact and had broken fuse. The solid black wire is the one cut at fuse block.
We will get another glass fuse holder and run it up to the area of the front left headlight and terminate it there and hope for the best!
The ammeter is new when we rebuilt the cluster a year ago or so.

Thank you again sir.

Randy and Easton

Maybe I didn't explain it clear enough.. Do not relocate the point where both the black wires connect to the main power wire. repair them as good as possible, keeping their lengths as near stock as possible, but maintain the same connection points for the black wires. A shunt works off a certain amount of resistance.. Every wire has some resistance.. The size and length determine just how much resistance is present. The voltage exhibited at the points where those black wires connect is slightly different depending on the amount of resistance between those two points. The ammeter reads the voltage at those two points and determines whether the battery is being charged or discharged.. Changing the location of either of the black wires connection point changes the resistance and upsets the accuracy of the meter. The engine bay is a somewhat hostile environment, so if at all possible, use the same weather tight fuse holders..

 

[Randy and Easton]

Nick, I am causing too much trouble trying to figure this out but so appreciate all your help. The only wire that is intact is the black/white one in the picture, it has the fuse holder. The solid black coming out of the fuse block that is cut is just there, no length (it may be 2” at the point its cut, connected to nothing and that it where I am stuck trying to figure out what to do to complete the circuit.

I was frustrated and tried to find a volt meter and do the conversion but everyone is backordered. I have to put new fuel gauge in next week and thought I would do both when I had it out but now I am back to trying to at least get amp meter workin.

 

[RustyPile]

Nick, I am causing too much trouble trying to figure this out but so appreciate all your help. The only wire that is intact is the black/white one in the picture, it has the fuse holder. The solid black coming out of the fuse block that is cut is just there, no length (it may be 2” at the point its cut, connected to nothing and that it where I am stuck trying to figure out what to do to complete the circuit.

I was frustrated and tried to find a volt meter and do the conversion but everyone is backordered. I have to put new fuel gauge in next week and thought I would do both when I had it out but now I am back to trying to at least get amp meter workin.

I sent you a PM..

 

[75gmck25]

I replaced the ammeter fuses in my ‘75 and the ammeter still didn’t give any decent reading. I then did the conversion to a voltmeter and moved on. The only glitch is that the new voltmeter has much brighter orange on the needle than the old worn gauges.

Using a shunt based ammeter is common to avoid running high amp wiring up to the dash, but it’s only accurate if the shunt has a predictable resistance. Since this in-dash ammeter uses all the old wiring as a shunt, it’s not going to be predictable.

 

[DoubleDingo]

If it were me I'd just switch to voltmeter and be done with it. Of course the voltmeter has to be wired up correctly and is different than an ammeter.

 

[Randy and Easton]

Got to replace fuel gauge Thursday so wanted to only take dash and cluster out once. Can’t find a voltmeter in time as everyone is back ordered. So I think to save my brain and quit troubling y’all so much, I’ll wait for the voltmeter to come in and just pull cluster over the winter and do the conversion. Thanks @RustyPile and everyone for the input so helpful as always.

Last night at dinner Easton asked if he could put a big block in it… are kids ever satisfied?

 

[Radiohead]

Last night at dinner Easton asked if he could put a big block in it… are kids ever satisfied?

Lol, I can tell he's a good kid. Didn't ask about an LS swap, went right to BB.

 

[75gmck25]

IMHO, a big block swap is usually not worth the trouble unless you have the money to build it correctly. Swapping in a relatively stock BB gives you bragging rights, but it’s primarily going to give you good torque for towing or hauling. Getting high HP out of a big block will cost money.