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TR5/TR250 amp gage shows discharge when lights are on

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I was checking the wire diagram again, against what I have showing on my amp gauge. If the wires on my amp gauge are on the wrong post, wouldn't that throw off the ability of the alternator to charge. According to my wire diagram, the heavy brown wire from the battery-solenoid, should be on the right post of my gauge, I have it on the left. The brown-white wire which runs from my alternator to the amp gauge is supposed to be on the left post and its on the right.
 
Depends if you have positive or negative ground car. If you or someone else changed it from positive to negative ground you have to switch the amp wires to work. So yes you are correct.
 
The amp gauge is polarity sensitive, and will indicate the relative direction of current flow with relation to the way the wires are connected. The right side of the meter when looked at from the front is the same as the left side of the meter when looked at from the rear. The key is that when the alternator is charging the battery, the meter should show positive, and when it is not (and the battery is discharging) it will show negative.

So, it is very possible for the ammeter to show negative when the alternator is charging, if it is wired backwards. But I don't think that's your problem. Your problem is the alternator is not charging, and the ammeter is indicating the battery is discharging.

A fully charged battery, at rest, should measure around 12.8 volts. When the alternator is charging the same battery, the voltage across it should be around 14 volts. If the head lights are turned on, or off, the ammeter will show the direction of current thru it, and is normally wired to indicate the state of charge towards the battery. The ammeter plays no role in determining the amount of current put out by the alternator (the ammeter is just a gauge or indicator). The regulator enables to the alternator to put more or less current out to match the load including the state of the battery. The regulator on the alternator is a closed feed-back loop, attempting to keep the battery charged, regardless of the load -within the designed parameters.

If the ammeter is wired backwards, it will show plus when discharging, and minus when charging. With the key off, and the lights on, a properly wired ammeter will show discharge, while a reversed wired ammeter will show charge.

You should also note that the headlights are not protected by a fuse. If there were a short to ground, the battery would shove as much current thru the wire as the wire can take (until the heat generated would melt the wire or burn it open). This might cause the voltage to drop, as well as the ammeter to show negative. I doubt you have that issue, but try turning on only the parking lights (which are fused) with the key on and off and see if the ammeter shows plus or minus.
 
Reversing the wires won't affect how the battery charges or discharges. It just changes the indication on the meter--if positive meant charging and negative meant a discharge, it would become the opposite of that.

So, yes, if the engine is off and you turn on the lights, you should see a discharge. If the engine is running at a modest speed, the alternator should easily replace the load current and you'll see a small charge current or zero.
 
thanks for the reply, and I see your point. I have a new alternator and a new voltage regulator but the battery is not charging.
 
I see your point, and will have to do some more checking. I did notice that my not so old voltage regulator was fried when I took it apart to check it. I am going to check my new voltage regulator to see if the same thing happened to it. What would cause that to happen?? there's a transistor that connect the + to the F terminals on the v.r. and it was fried.
 
Don't know why I was thinking this was a TR3. Recently I was helping someone with a no charging TR3. So I assume before this all was well and the amp gauge was reading positive when the lights were on? Or did you just purchase it this way?
 
I have this car since 1975. We've come a long ways together. The gauge used to oscillate back and forth across the zero mark. I think that might have been do to a bad ground from the motor to the chassis. It started working erratically every since I had a mechanic replace my timing cover and do some work on the bottom front part of the engine, where I had an oil leak. they had to dismantle the front end to get to it. There are not a lot of wires down there, mainly the two horns....but I have had negative charging ever since then
 
So we know you have a good alternator now that is charging 14 volts? But we are not 14 volts at the battery. When you say your regulator was fried, was it noticeably burnt or just not working? I think you need to do your due diligents and check your wiring for any resistance or open circuits.
When I rebuilt my Tr250 I replaced the original alternator with a Bosch 55 amp (rebuilt) internal regulator. Moss has a diagram on how to convert to a internal regulator alternator. Mine lasted a little of 20k miles. It recently went bad and one nice thing with most Bosh alternators is the regulator is bolted to the back of the unit and is replaceable. I purchased a new regulator but it didn't fix it. There was something else wrong in the windings. This is what I used but got it from Rock Auto and not Ebay. Paid less the $90 for all and this one has a noise filter and I had to tape the exciter wire to the harness to eliminate it. Charging great now. Make sure you charge the battery before restarting after repairs or your alternator will overwork to charge the battery.

https://www.ebay.com/itm/Ford-Corti...lternator-Bosch-55-Amp-Generator/281785569362
 
IMG_0611.jpgIMG_0611.jpgIMG_0611.jpg
Hope you can see from the pictures what happened to my v.r. That black transistor broke in two pieces....
 
Do you have a good ground from the alternator to the frame? You mentioned the meter flicking back and forward. There is a ground strap located on the drivers side engine mount.
Any cranking issues?
I added a separate ground from the alternator ground stud to the frame.

David
 
Looks like the transistor's case simply fractured. I've never seen one do that without obvious signs of overheating. Unless there are signs of burning that are not visible in your picture, it's probably just a bad part, not blown by something that was wrong in the car's wiring. It's likely that your new one is OK and the problem is elsewhere.

I agree, extra ground straps are never a bad idea, even for parts like the alternator, which should be well grounded by their mounting. A little paint or rust in the mounting surfaces can cause trouble.
 
thanks I will check it out. I'm still messing with it. Took the dash off, again, and checked the amp gauge for continuity and cleaned all the terminals, etc, and checked all the wires again......not there yet.
 
Nose said:
I just replaced my alternator because my amp gage was showing discharge when I turned on the headlights. My new alternator is doing the same thing. It shows a discharge when the lights are engaged. I have a new battery and a somewhat new external voltage regulator. The alternator is only showing a little over 12 volts when running, and I would think it should show around 14 volts. When I turn the light on, the voltage drops to 11 volts. I could use some advise please. I have a 1968 TR-250. Nose wants to know what to try next.
Click to expand...
Well after several months of inactivity, I decided to try and get my amp gage to show a charge on my 68 TR-250 . I had the original seller send me a new alternator and a new voltage regulator and I checked all the wiring and still no charge. I tried everything that you folks so graciously volunteered to help me out. I decided to take both alternators and regulators to an auto electrical shop to test everything one more time. To my surprise, he told me that my alternator has a built in regulator. He showed me how to change the wiring so the alternator will charge the battery and now it works. Apparently the British car parts outfit that sold me one set, and gave me another set free, doesn't know what there selling. Anyway, I'm back on the road and loving it. Thank you all so much for your help...God Bless
 
Modern alternators all have built-in regulators. Back in the dark ages (like, 1970s or 80s), some alternator-equipped cars had separate regulators, and I suppose it's possible that replacement alternators for those cars might not have an internal regulator. But even then, I think it's likely that the replacements might have built-in regulators anyway, if only because the parts suppliers aren't bright enough to know this. Looks like you got caught in that dilemma.

Glad you got it sorted, though.
 
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