The Stovebolt Site Forums The Shop Area The Electrical Bay How to test the ammeter?

I have been getting my 3800 back on the road. I have run new wires, rebuilt the generator and replaced the v regulator. I am having a strange situation occur where the meter shows charging when the vehicle has started in the same manner as my other three trucks. After 15-20 minutes of running/driving the meter drops to just into the discharge side. I have pulled the generator and v regulator out and tested them at my local service shop(the guy hooked it up and let me witness the testing, with both hooked up according to the manual, and all for free.) I have also traced every wire in the truck and found two loose screws, but other than knowing I have a few screws loose I found nothing out of the ordinary.

The armature shop guy suggested that the ammeter could be going bad to where after it has run it heats up and causes the issue. He mentioned the word "shunt" and a few other terms. He was not certain of any way to test because of it being an intermittent problem.

I have three original ammeters, two from jimmies(wrong color and marking) and one old chev. I would like to reuse as much of the original equipment in the spirit of being jack Benny esque! Is there a way to test these units? Or is it time for a replacement?

Thanks,
Sam

my ammeter shows a very slight discharge once it gets the battery back up and it always has .... if it's "just into the discharge side", it's probably where a normal neutral current flow condition registers, nothing bad about that, these are not precision instruments - if it bugs you, bend the needle



Bill

One minor detail, it does drain the battery over time. I did the run it till it won't crank anymore. I thought maybe the generator needed polarizing, it was good, maybe something else.

Good idea though. And I would definately bend the needle or grease pencil in normal.

No need to bend or mark. Just make note in your mind that is where normal is and watch for variations. My GMC is the same way.

What do you mean by "I did the run it till it won't crank anymore"? Do you mean you ran the starter until the battery was dead or you drove the truck and the battery went dead?

It looks like your ammeter is telling the truth. With the everything off, the meter should be centered. If you turn on the headlights, you should see it discharge. If you reversed the connections you would see a charge indication with the headlights on.

I understand the lights on showing discharge at idle, but not while at elevated rpm. It does work normally for 15-20 minutes, then it shows discharge to about a 1/3 of the way from 0 to D with no change regardless of what is turned on and off, and the battery eventually is low enough that the engine will run when hand started, but not electric started because there isn't any juice left. Also, when you run with the lights on and meter is showing a charge, normal bright lights, when the meter sticks to discharge only, all lights dim similar to match when the lights are on with engine not running.

Tomorrow I am going to try the other three meters and see what happens.

1/3 of the way down is not "just into the discharge side", it's discharging .... if it's not charging after a period of working properly, it's not the gauges fault, it's something in the gen/reg overheating although they work the first 15 minutes, that kind of problem won't show up on a normal bench test ... change the gauge and prove it to yourself, then try another generator and/or regulator - or have your guy run the current ones on his bench tester under load for 15-20 minutes .... also, running only the spark plugs shouldn't be drawing enough current to show 1/3 discharge, once it starts showing discharge, shut it off and starting at the battery with a meter, see if there's a current draw someplace that only happens when it's hot

and how do you "hand start" the truck, use a crank?

Bill

Yep hand crank. Works every time. PITA when alone or inclement weather. I have tried multiple voltage regs, and we did run it for a fair amount of time, not sure of the exact. Besides trying gauges I plan on another once over on the electrical system with the multimeter. I will let you know what I find.

I changed out the questionable ammeter for one of the spares. Everything seems to be working fine. I ran the truck almost all day, driving, idling with varying loads on the electrical system. Currently it reads as my other three trucks do. I plan on keeping my eye on it for the time being, but I think it is resolved. The old ammeter had some black gunk on the back when I removed it from the cluster. Any ideas?

Your meter could have been shorting out...making contact with ground. That would cause discharge. After reading your diagnosis, knowing your regulator is OK, I'm leaning towards your guage shorting.
See if there is continuity between each post on the ammeter and the housing. If there is, there's your answer.