Ammeter Question

[RJS]

Hi,

I recently left my headlights on for 45 mins parked in the garage with my engine and ignition off. For some reason, this overheated my ammeter (I know this because I saw the smoke from the dashboard). I pulled the gauge and found the connections extremely corroded and rubber connector covers melted.

I cleaned everything up and it all appears to function normally again.

So, a couple of questions:

1) is it possible the connections on the ammeter were slightly corroded to begin with and this caused the overheating? Or, should I suspect something more than the ammeter?

2) does it make sense this would have only happened when engine and ignition were off? Why wouldn't this have occurred before when driving with the headlights on?

Thanks

Bob

 

[RJS]

Actually, as I think about number 2), if the ammeter is in series with the battery, this makes sense. With the engine off, the headlights would be puling all the voltage from the battery, via the ammeter (ultimately stressing it pretty hard). But, with the engine/generator running, the headlights will be drawing voltage directly off the control box & generator (not via the ammeter).

Agree/ disagree?

Bob

 

[TR3driver]

1) Most likely, the terminals/connections were slightly corroded to begin with. That produces heat, which causes more corrosion (copper oxidizes much more rapidly when it gets hot) and even more heat. If there were too much current flowing (due to some other problem), you would see that on the ammeter.

2) Just so, with the engine running, the generator powers the lights (and everything else) so there is little or no current going through the ammeter.

 

[RJS]

Thanks Randall,

Appreciate your input. At this point, everything is working just fine. If I have time, I may pull the gauge again just to double check the resistance across the gauge with a DMM and make sure no issues there (any ideas what is acceptable resistance?). Also, will apply some dielectric grease to the terminals in hopes of preventing future issues.

I find if I remove the glove box I have pretty good access to the ammeter and it shouldn't be more than a couple hours work one evening.

Bob
PS: I clicked the headlights on last night with engine off and the ammeter read -7 amps. Trust that looks normal(?)

 

[TR3driver]

The ammeter's resistance should be very low, just a few milliohms (which is too low to measure accurately with an ordinary DMM). I once measured a TR3 ammeter at 1.2 milliohms (.0012 ohms) and a friend measured his TR6 at 2.7 milliohms. So basically, your DMM should read the same as it does with the leads shorted together.

Using pliers to pinch the Lucar connector closed just a bit more will also help prevent future problems. It should be a very snug fit onto the ammeter terminal. Although I've not tried it (yet), a product like Caig De-oxit should help too.

7 amps seems a bit low to me. The stock headlamps should draw about 3.5 amps each, but there are also 4 (or is it 6 on a 4A?) parking/tail filaments at about 1/2 amp each. At any rate, it certainly wouldn't hurt to poke around and see what voltage drops you can find going to the headlights. If everything is healthy, you should be able to get within about 0.5 volts of battery voltage, right at the pins of the headlight bulbs.

If the current is still on the low side, it might be time for new headlight bulbs. Their hot resistance goes up with age, which of course reduces both power and light output. Normally they burn out before this process goes very far, but sometimes they just fade away.

PS, I should have added that the ammeter is not the most accurate of devices. It may simply read 7 amps when the actual current is 9 or 10 amps.

 

[houleyh]

Is there an easy converion from ammeter to voltmeter on tr3A's?

Thanks

 

[TR3driver]

It's certainly easy enough. Just securely jumper the brown and brown/white wires together (which you can do either where they are disconnected from the ammeter or near the control box & starter solenoid). Remove the ammeter and install any standard 2" voltmeter, then wire it to the ground tie on the back of the panel, and the white wire from the ignition switch. (On the starter switch might be an easier place to put it.)

Personally, I prefer the ammeter, as it tells you what is happening now as opposed to what happened in the past. But it's personal preference.

 

[71MKIV]

I second what TR3 driver said. The dash mounted ammeter is not accurate enough to do any troubleshooting with other than a "go-nogo" type of thing. You figure that even a +/- 30 amp guage has to have a 60 amp range, it would be very hard to have it be accurate closer than 5 or 10 amps.

If it reads a - when the engine is not running, and zero when it is, then you know that the generator is working.

 

[RJS]

Cheers,

I had the same question about a voltmeter as well. But, in the end, I think I will stay with the ammeter, clean up the contacts again (apply some dielectric grease to them) and call it a day. I also won't rely on it as a precision instrument. More of just a general indicator of the current state of charge on the battery (i.e. charge or discharge).

Lastly, if I understand the functions of these two gauges correctly
Ammeter: measures state of charge/discharge on the battery in amps (condition of the battery)
Voltmeter: measures the specific output of the alternator/generator in volts (condition of the alternator/generator)
Correct?

Bob

 

[TR3driver]

Ammeter shows you current going into or out of the battery (charge or discharge) without really saying anything directly about the battery's condition at the moment. But if the generator/alternator is working properly, you can infer the battery's condition by how heavily it charges when the engine is turning faster, load is light, etc.

The ammeter will also tell you things like whether your brake lights are working; you left the headlights on; the generator is overcharging (which can kill the generator); alternator has popped a diode, etc.

Voltmeter gives you more of the battery's condition at the moment; but you have to infer from that whether the generator/alternator is working or not. IOW if it reads 12.5v, you know that the battery is somewhat discharged, but not whether it is being recharged or not. But if it reads 13-14 volts, then you can be confident it is being charged to at least some extent.

Biggest advantage of the voltmeter is that it doesn't require those heavy gauge, unfused, always hot wires running to the dashboard.

 

[MichaelG]

2) Just so, with the engine running, the generator powers the lights (and everything else) so there is little or no current going through the ammeter.

Randall,how does the ammeter provide charge/discharge information if the current flow is no longer going through the gauge? I am planning to install headlights relays, and assumed that if power to the headlights was wired directly from the battery that the ammeter would not show the headlight "discharge" when the headlights were turned on/off ...what should I expect?

Also I am planning an alternator conversion, and have seen the threads regarding the shunt installation, etc..which also led me to believe that all the power is always flowing through the ammeter, although it was hard to imagine 45+amps flowing through the TR ammeter harness configuration.

If the alternator has it's own internal regulator, I thought it could be installed using "pass through" connections in a gutted control box in order to keep original functionality, but if wired directly to the battery the ignition light would not work and and ammeter would not be accurate.

Can you shed some light on this?

Thanks

 

[TR3driver]

Mike,

Normally, the ammeter is only between the battery and the rest of the car, so it only reads charge/discharge to the battery. In the 'normal' case where the generator or alternator is supplying all of the loads, and the battery is not being charged, then the ammeter reads 0 regardless of how big the load is.

If you hook your headlight relay source directly to the battery, then the ammeter is going to show charge with the engine running and the headlights on; even if the battery is being discharged. This seems like a bad idea to me, but some folks do it that way. Connecting the alternator directly to the battery is even worse, as now the ammeter will show discharge all the time, even if the battery is being charged.

I ran for many years using a 60 amp Motorcraft (Ford) alternator charging through the original wiring harness, and the wires were never a problem for me. The stock TR3 wires are actually heavier gauge than those found in my 1980 Chevy with an 80 amp alternator, so I have to think they are capable. However, I did wire my alternator in a "remote sensing" configuration, so that it's output to the battery would not be limited by the voltage drop through the wires and ammeter. With this configuration, I would see the full 60 amp output going to the battery immediately after a cold start.

But again, current only flows through the ammeter when the battery is being charged or discharged. So in the 'normal' case where the alternator is only supplying current to the rest of the car, there is no current through the ammeter (or the wires to it). I used a new 8 AWG wire (mostly because I had some on hand) to run from the alternator output to a new tie point (really just a 1/4" bolt), which is where I tied in the power feeds to all my high-current items (headlight relays, stereo, radiator fan).

Initially, I used the original control box as a tie point to the original harness. But it eventually developed a high resistance at one of the terminals and so I wound up having to splice the NW and NU wires from the original harness together with my new 8 AWG wire from the new tie point. Next time around, I will simply use one of these to both join the original NW & NU and to power new high-current items.

The warning light can be made to work even with an internally regulated alternator; but you will have to connect the light to the terminal on the alternator that is intended for the purpose. I used an externally regulated model mostly because it was slightly smaller (and cheap at the time); but the light still had to be rewired to the appropriate terminal on the regulator. BTW, depending on the alternator you choose, you may need to add a resistor in parallel with the dash lamp. I found that a 35 ohm, 5 watt resistor would help the alternator start reliably without having to rev the engine up every time I started it.

Does that answer the question?

 

[MichaelG]

Thanks Randall, Very helpful, I can now weigh my options with a better sense of the pros/cons for different alternators and installation schemes.