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Discussion Forum

Amp Gauge wiring question????

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Author  [Modern View]

02-04-2012 13:46:38

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In the process of converting Ford 8XX to 12volt. Had to install a new amp gauge. I found a VDO at NAPA but the wiring diagram is a little different than the 800 12v conversion diagram on this site. I would like a little help on the correct way to wire this up so I don't fry anything. Here are the two wiring diagrams.

Here is the VDO wiring diagram:
third party image
Here is the 12v conversion for the 600/800
third party image
This post was edited by Greg1959 at 13:56:02 02/04/12.

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02-04-2012 15:08:03

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 Re: Amp Gauge wiring question???? in reply to Greg1959, 02-04-2012 13:46:38  
Thanks guys! I think I understand. :D

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John in La

02-04-2012 14:57:10

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 Re: Amp Gauge wiring question???? in reply to Greg1959, 02-04-2012 13:46:38  
Lets take the bottom diagram first. I drew it so I can talk about my thought process in how it works.

On the right side of the amp gauge is the load and alternator. This will be your L post on gauge.
On the left side is the battery. The starter relay is just a convenient place to hook this wire because it is a direct connect to the battery.
The starter is the only thing out of the loop and you really do not want to read starter draw with a amp gauge.

How it works.
When you start the tractor the battery feeds the lights and or coil threw the amp gauge. The gauge shows a discharge.
Once the alternator starts to charge it will supply these items and send any extra charge to the battery showing a positive charge.
Once the battery is full the alternator will still supply power to the coil and lights but none to the battery. In other words the volts on the battery side and the alternator side will be the same. This is when the needle will set in the middle (no charge or discharge)

So the bottom one is correct.

Now lets look at the top one.
The alternator and load (key switch) is hooked to one side of the gauge. (Same as my diagram)
The battery is hooked to other side of gauge using the starter relay as a hooking point. (same as my diagram)

But wait. They have the fuse box hooked direct to the battery and out of the amp gauge loop. This means any power going threw the fuse box will not be read by the gauge. So that is wrong. The fuse box should be on the key side of the gauge. To make it correct the wire coming from the fuse box to the battery needs to go from fuse box to L on amp gauge.

Rather than hooking all these wires to the gauge we use a terminal block on our tractors and since the lights is the only fused thing on your tractor you would use a inline fuse instead of a fuse box.

Sorry I got so long winded but rather than tell you how it should be I tell you how it works so you can understand the underlying principle.

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02-04-2012 15:01:22

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 Re: Amp Gauge wiring question???? in reply to John in La, 02-04-2012 14:57:10  
John, you are retired. You don't have to solve complex problems anymore. How bout my simplistic answer.



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02-04-2012 14:14:48

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 Re: Amp Gauge wiring question???? in reply to Greg1959, 02-04-2012 13:46:38  
install yer new amp gauge in the same place in the wireing as the old one.

basically.. genny / alternator and all loads on one side of ammeter.. battery by itself on the other.

wire it so charge is a positive deflection, and lamps on with engine off is a negative deflection.. and you are golden.. net charge / discharge preserved.


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02-04-2012 14:59:03

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 Re: Amp Gauge wiring question???? in reply to soundguy, 02-04-2012 14:14:48  
Agree and SG ought to know.

Your ammeter knows not voltage, per se. It responds to electron movement...current flow. Actually you need not change your ammeter with a change of battery voltage. the resistance of the meter is say 0.01 ohms. Passing 10 amperes through it, regardless of the voltage that made the current flow in the circuit in the first place produces a drop of 0.1 volt, nebulous in a 6 or 12v system.



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