Charging system

Thread Starter

Coucou80

Joined Apr 7, 2018
78
Hello,
I'm trying to diagnose a charging issue on my car. Using a DC hall effect clamp, measuring current at the wire that connects from the alternator to the battery positive terminal is about 40 amps (ack its somewhat high when no load is on the electrical system).

Now for the strange part. I'm using the DC hall effect clamp on the ground wire at the negative terminal of the battery and I'm measuring about 5 amp. Shouldn't I be measuring the same amount of current at the negative terminal then what was measured on the positive side ?

Best regards.
 

Thread Starter

Coucou80

Joined Apr 7, 2018
78
Yes, you should. Does the clamp meter show the same value (but opposite polarity) if you turn it round?
I haven't tested the clamp meter on the opposite side (i.e flipping it arround), but let say one of the batterie cell is is shorted... would that produce the reading I'm seeing now?
 

Ian0

Joined Aug 7, 2020
10,035
Are there any other wires on the battery positive terminal? My experience of car wiring is that loads of accessories just get attached to the convenient bolt on the terminal post.
 

Thread Starter

Coucou80

Joined Apr 7, 2018
78
Are there any other wires on the battery positive terminal? My experience of car wiring is that loads of accessories just get attached to the convenient bolt on the terminal post.
Correct there is two big wire attached to the positive terminal. One going to starter and one coming from alternator. That same last wire feeds in to a junction box of various fuses.
 

Ian0

Joined Aug 7, 2020
10,035
If 40A is coming from the Alternator, and 5A is going from battery negative to chassis, then 35A is going down one of those other wires.
Does the battery voltage look about right? (13.8V to 14.7V)
 

Thread Starter

Coucou80

Joined Apr 7, 2018
78
Just wanted to close the loop here...
I think load wise this is correct (40 amps). Its a 6 cylinder engine, with about 3 amps per coil pack right there this is 18 amp + daytime runing lights + fuel pump and cabin dashboard + ECU...
So I think this is correct.

I don't know why I've seen somewhere that if the charging system is correct (with a battery 100% charged) drawn load should be 5 amps...

To conclude, this tells me that the infamous alternator is working all the time.
 

Ian0

Joined Aug 7, 2020
10,035
Sounds like a lot to me. I would expect more like 5A. 3 A per coil seem a lot, an old-fashioned coil and points system would use less than that for the entire engine.
 

Thread Starter

Coucou80

Joined Apr 7, 2018
78
Sounds like a lot to me. I would expect more like 5A. 3 A per coil seem a lot, an old-fashioned coil and points system would use less than that for the entire engine.
I agree, but this is the best explanation I could came up with. This a a electronically controlled coil system, it also have a variable valve system (hydrolicaly controlled with two actuators (one for exhaust cam shaft and on for intake cam shaft). Each spark plug have its own coil, so there is no rotors and main coil distributor cap. I guess to better see the effect of the load induced by each coil I could use a hall effect clamp with an oscilloscope where we could see the current spike at each fireing event.
 

LowQCab

Joined Nov 6, 2012
4,209
The Current requirement to operate all Electronic-Engine-Components is most likely around ~35-Amps.
The Regulated-Alternator-Output-Voltage is high enough to continuously charge the Battery at ~5-Amps.
The Alternator is Grounded by way of being bolted to the Engine-Block.
All Electronic-Engine-Components are Grounded directly to the Engine-Block.

There is nothing wrong with the initially observed Current measurements.
.
.
.
 

Tonyr1084

Joined Sep 24, 2015
7,956
Couple of thoughts: Check the battery voltage in the morning after letting the car rest the whole night. The voltage should be 12.6V at rest. If the battery is old that resting voltage could be down around 12.0V. That's an indication of an old battery. If it's below 12V I'd be looking to replace the battery. The battery condition can affect how the alternator responds.

NEXT, start the car and watch the voltage. If it goes up to 13.8 volts then that suggests the battery is in good condition. An older battery can see 14 volts. A weak (near end of life) battery can see as much as 15 volts. 14.5 is not uncommon for older batteries.

OK, NOW: Check the condition of the ground connections at all points wherever you can. An older vehicle may be experiencing rusting at some of those grounds. Rust can cause high resistance which can draw some amperage. I had an older car with rust issues. Headlights would not light unless I stepped on the clutch pedal. One night no matter what I did I couldn't get the headlights to come on. Opened the hood in the dark and saw a spot of light right at one of the ground connections. Touched it. Stupid thing to do - it was white hot. AND it was the last good (if you can call it that) connection. The next day I went through all the ground connections and found all of them having rust issues. Once that was fixed there were no other problems electrically speaking.

WHAT NOW? WITH THE ENGINE OFF Check the belt. A slack belt will allow the alternator to slip under load. Had that issue too on another vehicle. Could start any time all day, no problems. At night, after driving with headlights on, shut it off and it would not start. The battery was discharged. Bought a new battery but that did not solve the problem. Exasperated I tugged on the belt and discovered it was quite loose. Tightened it and that was the end of the problem.

SO:
  1. What's the resting voltage of your battery?
  2. What does the voltage go up to after starting the engine?
  3. WITH THE ENGINE OFF can you turn the alternator without rotating the engine? If so - the belt is slack or slipping
IF:
  1. Resting voltage is above 12.2V the battery is old but still in fair condition. You may get another year out of it.
  2. Immediate running voltage should be above 13.6V and not exceeding 14.5V
  3. IF those conditions are not met WITH THE ENGINE OFF does the alternator slip on the belt when you turn the alternator? If so - get a new fan belt. Or tighten it if that's the case.
Report your results. We'll go from there.
 
Last edited:

Ian0

Joined Aug 7, 2020
10,035
The Current requirement to operate all Electronic-Engine-Components is most likely around ~35-Amps.
The Regulated-Alternator-Output-Voltage is high enough to continuously charge the Battery at ~5-Amps.
The Alternator is Grounded by way of being bolted to the Engine-Block.
All Electronic-Engine-Components are Grounded directly to the Engine-Block.

There is nothing wrong with the initially observed Current measurements.
.
.
.
Where's it all go? I've not owned a petrol engined car since everything became electronic - mine have all been diesels, and a lot of that is still rather mechanical. I know that there are electrical valves for the common rail fuel injectors, but the rest is done by a high-pressure pump driven by the engine. My petrol-engined classic car has an alternator that is only 34A. The ignition took about 1A and that was all there was.
 
Top