50 amp ammeter shunt

[73tlcv8]

Started working on the shunt for my new 50 amp ammeter. I read the threads on this and found that Toyota used the fusible link as the shunt and it is not very accurate. Did some math and found 2.7 cm of 2mm diameter stainless steel wire should give roughly 0.006 ohms needed for the ammeter shunt. 2mm wire should carry 50 amps without trouble. I just need to figure out how to mount it cleanly. I'm thinking about using one of these on each end and putting it in a 50 amp fuse holder. These are 2.1mm ID with a set screw, OD is about 1/4" so it should fit in a 3AG fuse holder. I just don't know if the connection resistances will overshadow the shunt value over time.

After cleaning the surface of the wire and my test leads really well, and offsetting the lead resistance:

 

[rkymtnflyfisher]

I see the connections being the weak link in that situation, I love your thinking here, this is awesome.

Maybe some sort of crimp connection or even a soldered joint. Although I don’t know how feasible it is to solder the stainless wire.

 

[LAMBCRUSHER]

For 60 series, not sure if helpful

 

[S.CarolinaFZJ80]

I think @Coolerman came up with a solution to this.

 

[73tlcv8]

Yes it can be done with about 18" of fusible link also.

 

[73tlcv8]

used silver solder to connect the ss wire to copper butt splices then soldered the wires.

 

[AlaskanWheeler]

The issue I see is controlling your circuit at such a low resistance.

 

[AlaskanWheeler]

@73tlcv8 / @Rainman, curious if there has been any progress on this type of solution for our amp meters.

Thanks

 

[Rainman]

I'm electrically ignorant. Sorry. I'm serious too!

 

[73tlcv8]

I believe this shunt will work reliably but I need to get some time to install and test. I currently have the old style cluster so I have to switch that out and run the wires for the ammeter to the shunt. I have the black fuse holder and the green connectors, I just have to get time to make them up and install. It's been a tough spring at the power plant so haven't had much time to work on it. I will update when I get more progress.

 

[73tlcv8]

I made up the above pictured shunt and installed it between the charge wire and its fusible link and temp wired it to the new cluster sitting on the fender. Started the engine and the ammeter stayed at 0. Measured charge current at about 1 amp. Turned off engine ran lights, radio, fan for a few minutes (10-15) then restarted. Ammeter was just slightly above 0, measured charge current at 2.5-3 amps briefly then lowered back to around 1-1.5 amps. Since this ammeter only measures the charge/discharge of the battery and most of the loads are supplied before this point by the alternator, the current is always going to be low. I don't know why Toyota used a 50 amp ammeter, it will always be around 0 even with an accurate shunt. Probably why they didn't bother putting in a proper shunt originally.

 

[kellervernon]

read this thread.... Lost marbles and Pinhead break it down-
ammeter and fusible links

 

[73tlcv8]

Yes thanks. Since I am installing the 50 amp ammeter, I needed to create a shunt anyway so thought I would try to make it as accurate as I could.

 

[73tlcv8]

I was not happy with the results of the 50A ammeter, even with the correct shunt. So I decided to go a different route for my new cluster. This solution should work for those with a truck wired for the 50 amp ammeter since the ammeter plug has the battery voltage and a ground present.

I found a new oil pressure gauge and installed it in place of the 50 amp ammeter in the cluster. It is a simple one for one replacement. The oil pressure gauge is 0-6v for 0-full scale indication. I wanted to have the meter indicate over a range of 10-18v, a common range for after market gauges. If you look at the relationship between these two ranges, the 10-18v input has to be multiplied by 0.75 and subtract 7.5v (y=mx+b for those that remember) . ((6/8) = 0.75, and 10 x 0.75 = 7.5 so if we want 10 to be 0 on the meter we need to subtract 7.5v). I dug through my junk box and found some components that are close and made this circuit, the 2sc2579 is overkill but I had one so I used it. It could have been done with op amps or some programmable thing but I like simplicity for my daily driver:

breadboard the circuit:

Meter with 14V input to my circuit:

Now all I have to do is solder it up and install in the truck.

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