What should ammeter read with headlights on?
Instructor
Joined: Aug 2017
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two things to keep in mind....your amp meter uses a shunt resistor, which over time can become faulty and give not so accurate readings...and second, try each headlight individually...sometimes bulbs get to a point where they have burnt there internal connections but bumped and 'rejoined' themselves but they tend to pull more current. I dont think that is the case here. I would suspect your amp meter is just out of sync and needs a calibration. Im not sure if thats even possible on these. Never tore into mine. There is however an old saying, if it aint broke dont fix it!!! But if your like most of us you cant rest until you have a straight answer. I have a 78 and my meter is just before the orange section...the cjarge light is on dimly, and at idle in gear my needle bounces with every thump of the motor......its charging now, wasnt when i bought it, lots of bubba wiring, had to replace regulator in alternator, did main bearing and diode as well, i suspect the triode is still bad....but on a good note i went to scrap yard today and picked up an original alt off of a 84 firebird!!! Im gonna slap her on tomorrow and see if it works!! This will also let me polish the original to a brilliant shine!!
Instructor
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I checked the resistance between the ground and hot leads of the front headlight connectors, as well as one of the front parking lights. As with the taillights, there was only about 1-2 ohms resistance. Again it seems like there should be infinite resistance between these points. Does this mean there is a short? Could this explain why my ammeter usually shows discharge with the headlights on? If there is a short, any suggestions on how to track down where it occurs? Seems like a daunting task as it could be anywhere. Again, all the lights work fine, but I fear I will either destroy the alternator or run the battery down if I run with the lights on for an extended time.
Thanks
Thanks
Last edited by san; Sep 10, 2017 at 10:37 PM.
Race Director
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From: South Western Ontario
Last post I meant you should not worry about your resistance tests. The tail light bulbs, front running light bulbs, dash bulbs and console bulbs are all connected in parallel. You can't check the wiring for a short at one of the light sockets you listed unless you remove every bulb that lights up with the headlight switch in the parking light position.
The alternator diodes should measure a high resistance, not cause a short measurement. Otherwise, the alternator would drain the battery when the engine is off.
You wont get an iffinate resistance with the alternator hooked up. It should read so many ohms because i think your getting readings from alternator diodes. Try disconnecting alternator and do your measurements again at light sockets. Also....keep in mind anything that is hooked to power lines also hooks to ground, so you testing one power line can inadvertently be testing the internal resistance of another device ---light/relay/resistor/coil/heater in back window if you habe a shark....or a light bulb on the other side....also....pull the fuses of each section your testing...this will isolate them from other power mains.
The alternator diodes should measure a high resistance, not cause a short measurement. Otherwise, the alternator would drain the battery when the engine is off.
Instructor
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Last post I meant you should not worry about your resistance tests. The tail light bulbs, front running light bulbs, dash bulbs and console bulbs are all connected in parallel. You can't check the wiring for a short at one of the light sockets you listed unless you remove every bulb that lights up with the headlight switch in the parking light position.
The alternator diodes should measure a high resistance, not cause a short measurement. Otherwise, the alternator would drain the battery when the engine is off.
The alternator diodes should measure a high resistance, not cause a short measurement. Otherwise, the alternator would drain the battery when the engine is off.
Last edited by san; Sep 11, 2017 at 09:37 AM.
Race Director
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From: South Western Ontario
Obviously, you have never paid any attention to the resistance of a COLD light bulb and it appears you also don't understand ohms law for parallel resistances.
A single tail light bulb filament is < 25 ohms hot. But, a light bulb filament is a much lower resistance when cold. It can easily be 10X less resistance when cold. So, a single tail light bulb will only be a few ohms cold. Put some of them in parallel as well as putting all the other dash and marker bulbs in parallel and the cold resistance of the circuit quickly measures a low enough resistance you can't tell if there is a short or not with a typical ohm meter.
Besides, the lights all work just fine so this should make it quite obvious that there is no short to ground in the light circuit.
Why'd you say diodes if you don't mean the rectifier diodes??
Last edited by lionelhutz; Sep 11, 2017 at 10:29 AM.
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Obviously, you have never paid any attention to the resistance of a COLD light bulb and it appears you also don't understand ohms law for parallel resistances.
A single tail light bulb filament is < 25 ohms hot. But, a light bulb filament is a much lower resistance when cold. It can easily be 10X less resistance when cold. So, a single tail light bulb will only be a few ohms cold. Put some of them in parallel as well as putting all the other dash and marker bulbs in parallel and the cold resistance of the circuit quickly measures a low enough resistance you can't tell if there is a short or not with a typical ohm meter.
Besides, the lights all work just fine so this should make it quite obvious that there is no short to ground in the light circuit.
Why'd you say diodes if you don't mean the rectifier diodes??
A single tail light bulb filament is < 25 ohms hot. But, a light bulb filament is a much lower resistance when cold. It can easily be 10X less resistance when cold. So, a single tail light bulb will only be a few ohms cold. Put some of them in parallel as well as putting all the other dash and marker bulbs in parallel and the cold resistance of the circuit quickly measures a low enough resistance you can't tell if there is a short or not with a typical ohm meter.
Besides, the lights all work just fine so this should make it quite obvious that there is no short to ground in the light circuit.
Why'd you say diodes if you don't mean the rectifier diodes??
Unless im wrong a short measures 0 ohms or real close too it....so testing a circuit and getting ANYTHING higher than decimal points in ohms means there aint one....so obviously you are saying that 1-2 ohms means there could be a short??? Is that what your saying??? Hahaha
Race Director
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From: South Western Ontario
You have to read <0.100 on the meter to be a short?
I guess you've never used a meter that measures 0.6 Ohms with the leads touching each other. Or one that bounces around between 0.4 and 0.8 ohms depending on where the leads are held?
The typical hand-held multi-meter IS SIMPLY NOT meant to do precision measurements on low resistance circuits. Nothing below 1 ohm for most meters and even possibly below a few ohms or more depending on the meter.
If you remove 1 running light bulb and try to measure the resistance the other light bulbs all in parallel WILL be WELL under 1 ohm of resistance. I bet closer to about 0.2 to 0.4 ohms. When the meter measures 0.6 ohms with the leads shorted together how exactly should you interpret 0.9 ohms when you measure the circuit. I didn't post that anything about measuring 1-2 ohms as a short. YOU made that up all by yourself...
And the ONLY external connection to rectifier diodes on an alternator is at the MAIN large output post. NOT the connector. It's not my fault you post wrong information...
I guess you've never used a meter that measures 0.6 Ohms with the leads touching each other. Or one that bounces around between 0.4 and 0.8 ohms depending on where the leads are held?
The typical hand-held multi-meter IS SIMPLY NOT meant to do precision measurements on low resistance circuits. Nothing below 1 ohm for most meters and even possibly below a few ohms or more depending on the meter.
If you remove 1 running light bulb and try to measure the resistance the other light bulbs all in parallel WILL be WELL under 1 ohm of resistance. I bet closer to about 0.2 to 0.4 ohms. When the meter measures 0.6 ohms with the leads shorted together how exactly should you interpret 0.9 ohms when you measure the circuit. I didn't post that anything about measuring 1-2 ohms as a short. YOU made that up all by yourself...
And the ONLY external connection to rectifier diodes on an alternator is at the MAIN large output post. NOT the connector. It's not my fault you post wrong information...
Last edited by lionelhutz; Sep 11, 2017 at 06:05 PM.
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Instructor
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You have to read exactly 0.000 on the meter to be a short?
I guess you've never used a meter that measures 0.6 Ohms with the leads touching each other.
If you remove 1 running light bulb and try to measure the resistance the other light bulbs all in parallel WILL be WELL under 1 ohm of resistance. I bet closer to about 0.2 to 0.4 ohms. When the meter measures 0.6 ohms with the leads shorted together how exactly should you interpret 0.9 ohms when you measure the circuit.
And the ONLY external connection to rectifier diodes on an alternator is at the MAIN large output post. NOT the connector. It's not my fault you post wrong information...
And I didn't post that anything about measuring 1-2 ohms. YOU made that up all by yourself....
I guess you've never used a meter that measures 0.6 Ohms with the leads touching each other.
If you remove 1 running light bulb and try to measure the resistance the other light bulbs all in parallel WILL be WELL under 1 ohm of resistance. I bet closer to about 0.2 to 0.4 ohms. When the meter measures 0.6 ohms with the leads shorted together how exactly should you interpret 0.9 ohms when you measure the circuit.
And the ONLY external connection to rectifier diodes on an alternator is at the MAIN large output post. NOT the connector. It's not my fault you post wrong information...
And I didn't post that anything about measuring 1-2 ohms. YOU made that up all by yourself....
Usually they should read .1 ohms and if you leave them touching long enough they will jump to 0 ohms. If your meter is measureing .6 ohms then its only about 40 percent accurate....thats how i interpret it. In any case a short will measure pretty close to the same as touching leads together, and like i stated originally, with light bulbs all in an acounted for, still wont read .6 ohms if there is no short. It will read just like his measurements that he got....greater that his leads touching. I clearly said you have you take into account that other bulbs are on the same circuit, i may have said diodes instead of triodes....i wasnt referring to RECTIFIER...which is whats its TECHNICALLY called, i was referring to the tripple diode on the AMP meter circuit.
Race Director
Joined: Apr 2007
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From: South Western Ontario
I use FLUKE meters all the time and they NEVER read 0.000 ohms with the leads shorted. FLUKE is one of the most respected hand-held meter brands available so your claims just holds no water whatsoever.
You also just proved you don't understand at all how meter accuracy is specified.
A tripple diode on an ammeter circuit???
SAY WHAT?
There is no ammeter circuit in the alternator. NOTHING in the alternator measures current. There is a set of 3 diodes in a module called a diode trio. But, they are connected to terminal #1 and reverse biased in series with the main rectifier diodes before reaching a ground connection, so they certainly won't cause any kind of low resistance measurement.
FYI, a triode is a type of vacuum tube.
You also just proved you don't understand at all how meter accuracy is specified.
A tripple diode on an ammeter circuit???
SAY WHAT?
There is no ammeter circuit in the alternator. NOTHING in the alternator measures current. There is a set of 3 diodes in a module called a diode trio. But, they are connected to terminal #1 and reverse biased in series with the main rectifier diodes before reaching a ground connection, so they certainly won't cause any kind of low resistance measurement.
FYI, a triode is a type of vacuum tube.
Last edited by lionelhutz; Sep 11, 2017 at 07:19 PM.
Instructor
Joined: Feb 2014
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From: Prague
My reading was normal, with small movements when turning things on. After I hit high beam my ammeter goes crazy, shows a big draw, needle hitting the left edge violently with pretty much everything I turn on - high beam, brakes, indicators. What could it be? I am ready to disassemble the interior and any help is greatly appreciated. Thanks!
Le Mans Master
Joined: Jan 2001
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Is this when you select HI BEAM? Or "pretty much everything I turn on"?
I was writing some guidance for checking HI BEAM circuitry when I realized that I did not really understand your statement.
I was writing some guidance for checking HI BEAM circuitry when I realized that I did not really understand your statement.
My reading was normal, with small movements when turning things on. After I hit high beam my ammeter goes crazy, shows a big draw, needle hitting the left edge violently with pretty much everything I turn on - high beam, brakes, indicators. What could it be? I am ready to disassemble the interior and any help is greatly appreciated. Thanks!
Team Owner
Joined: Jan 2006
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From: Crossville TN
I'm suspecting a 'mechanical' short in the hi-beam switch. Something is amiss in that switch and is going to a significant short-circuit when energized. Check it out with your ohmmeter with battery disconnected to diagnose it.
Instructor
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I use FLUKE meters all the time and they NEVER read 0.000 ohms with the leads shorted. FLUKE is one of the most respected hand-held meter brands available so your claims just holds no water whatsoever.
You also just proved you don't understand at all how meter accuracy is specified.
A tripple diode on an ammeter circuit???
SAY WHAT?
There is no ammeter circuit in the alternator. NOTHING in the alternator measures current. There is a set of 3 diodes in a module called a diode trio. But, they are connected to terminal #1 and reverse biased in series with the main rectifier diodes before reaching a ground connection, so they certainly won't cause any kind of low resistance measurement.
FYI, a triode is a type of vacuum tube.
You also just proved you don't understand at all how meter accuracy is specified.
A tripple diode on an ammeter circuit???
SAY WHAT?
There is no ammeter circuit in the alternator. NOTHING in the alternator measures current. There is a set of 3 diodes in a module called a diode trio. But, they are connected to terminal #1 and reverse biased in series with the main rectifier diodes before reaching a ground connection, so they certainly won't cause any kind of low resistance measurement.
FYI, a triode is a type of vacuum tube.
You are real good at twisting around what people say eh?? Reversed biased in series?? Really??? Pretty sure they diode trio is not in series considering there is three tabs to one. But hey im sure you can teist that around too..also...I didnt say the amp meter circuit was inside the alternator, it is on the circuit. Maybe instead of picking apart someone's advice you should pick apart your comprehension skills.
Last edited by san; Mar 30, 2018 at 12:12 AM.
Instructor
Joined: Aug 2017
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My reading was normal, with small movements when turning things on. After I hit high beam my ammeter goes crazy, shows a big draw, needle hitting the left edge violently with pretty much everything I turn on - high beam, brakes, indicators. What could it be? I am ready to disassemble the interior and any help is greatly appreciated. Thanks!
Instructor
Joined: Feb 2014
Posts: 161
Likes: 26
From: Prague
The short seems permanent now. After hitting the switch, all seems to be shorted. Will run diagnostics as you suggested, thanks!
Melting Slicks
Joined: Oct 2012
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From: Dunedin NZ.
An answer to the original posters question (in case anyone is researching the same problem). My ammeter was showing a approx 5 Amp discharge when travelling with lights on, turned out that the connection from the alternator to horn relay needed cleaning and tightening properly, problem solved.
Le Mans Master
Joined: Jan 2001
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Griff2002- I'm no longer sure where we are with your concern....... but anyway, the ammeter should not be displaying negative values with the engine running and the alternator working properly. Well, that's assuming everything works well and you don't have too many loads.
With the original alternator on my 1968 it was just barely able to maintain a positive value with all loads energized and the engine at idle.
Some of the bests modifications I made was to upgrade my alternator. By the way, long term operation with that "negative" indication is drainingyour battery.
Anyway - just FYI, but you need to address this before you take off on a long drive and find yourself with a discharged battery.
With the original alternator on my 1968 it was just barely able to maintain a positive value with all loads energized and the engine at idle.
Some of the bests modifications I made was to upgrade my alternator. By the way, long term operation with that "negative" indication is drainingyour battery.
Anyway - just FYI, but you need to address this before you take off on a long drive and find yourself with a discharged battery.
My ammeter has always been at around -10 (to left about halfway to the first mark) with the headlights on. It's been like that since I've owned it - maybe even a little worse before I replaced the alternator. With headlights off the needle is vertical, or a bit to the right (i.e. charging) if I haven't driven it for a while. It hasn't caused me any problems, but I don't drive much at night. Parasitic drain is minimal.
Is this normal. If not, where / how should I look for a problem?
Thanks
Is this normal. If not, where / how should I look for a problem?
Thanks
Last edited by carriljc; Mar 31, 2018 at 10:44 AM.
Le Mans Master
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Not sure that headlamps have fuses though....Depending on where you are in your troubleshooting this would be my recommendation:
1. Unplug both HI BEAM connectors from each associated headlamp carefully (so as to not disrupt the lead routing - part of our future troubleshooting). Then tape over the ends to avoid inadvertent shorting/grounding.
A. Turn on the High Beams again and see the problem is still there.
A.1. IF the problem is no longer there, then the problem is in either the connector(s) or the lamp(s). To further isolate, then perform the following:
A.1.a. Gently (careful to not disrupt lead routing) plug one HI BEAM lamp at a time and check your readings again.
A.1.b. If this locates your fault, then repair/replace that lamps and/or connector.
If the problem STILL EXISTS after unplugging per Step 1. above, then the ground/fault/short may be in the wiring (leads) that supply the HI BEAM headlamps. Select a side and inspect and gently ensure that the leads are not damaged, grounded, nor shorted (or otherwise damaged). Repair issues as found (one side at a time) and then run your tests again.
Just how I would go about it if the only thing causing the issue for me was the HI BEAM use. Hope this helps.
Last edited by carriljc; Mar 31, 2018 at 11:06 AM.