Hi all,
I am trying to figure out if I have a problem with the electrical system in my car or not...
Starting from some background:
For some time I noticed that, in idle, with the lights on, the lighbulb in the dash indicating a fault in the lights was getting on, and switching off again when I was increasing the RPM. I noticed that I was getting some rather low voltage, in the range of 11.4-11.6V in idle with load.
Battery is a big Bosch less than 6 months old, so that's fine.
I started investigating and the first obvious things I found was the fuses were *massively* oxidised (after 33 years...). Cleaned them up, changed where possible, cleaned all contacts, put back together and now the alarm doesn't go on any more.
I have gained some fraction of volt, but still the voltage is low. When I checked in idle, it was about 12.5V with no load.
I have changed the regulator, got up to 13.5-13.6V with no load, but when I put some load on it goes down again below 12V and keeps dropping.

So it was time to take some proper measurement.

- The alternator provides 50A in idle and almost 70A at higher RPM, so I would say that is working nominally.
Then I measured the battery drain with the car off:

- internal light: 1.4A
- turning the key (not starting the car): 6.4A
- fan on position 1 and 2: 8.4A / 10.4A
- lights in daylights / headlights / fog lights: 13A / 21.2A / 31.6A

Then I repeated the same measurement with the car in idle:
- idle: 16A (charging)
- fan on position 1 and 2: 12A / 10A (charging)
- daylight: 7A (charging)
- headlights: 0A
- fog lights: -11A (discharging)

Knowing how much the alternator is generating, I have calculated an extra 28.4-29.6A load present when the car is running.
This explains why the voltage goes down when I add some extra load (I kept the radio off, not that I have anything huge but it takes a few more amps, or the wiper, etc.).

I can't really figure out what in my car would be consuming that much when the engine is running.
As it's a good, old mechanical car, there are few things that run on the 12V. The only difference between the engine off and on that I could think of are the fuel pump, and the high voltage circuit to generate the sparks... but would those two consume 360W?
Does anyone has any idea if this is normal (I don't think so, because I never had this issue of the low voltage) or what I could check to try to understand where the issue is?

Thanks

 

What year production is your car?
The general rule of thumb is that the fuse ratings will be ~2.0x-2.5x of the normal consumption of a circuit. So you can estimate the expected current consumption from that angle too reviewing the fuse box.
The dual fuel pumps on KE-Jetronic emissions take 8A each, so that it 16A right there.
The EZL is probably around 5A (just a guess), ECU is probably similar. It all adds up to a lot of amps.
The auxilary fans also take up a lot of current, when they turn on my voltage drops 0.5V.

The electrical system in these cars were a bit marginal. The system can not take all appliances to be on at the same time.
All this said, I'm guessing your alternator maybe a bit weak and can not put out the needed amperage.

 

My car is from 1990.
Yes, the auxiliary fan indeed consumed a lot, I noticed when it started while I was taking the measurement.
Today I have checked the consumption of the fuel pump when you turn the ignition and the pump runs for a few seconds.
I get 2A discharge just before turning the key, 16-17A when the pump is running and 6A afterwards. So the pump should be drawing about 10-11A. I don't know if I have two pumps, I thought only one. Searching online I see a declared consumption of 6A for W124 bosch fuel pumps, so if I have two it's ok, if I have one it's too much.
The alternator (cold) gives me the correct current (50A idle, up to almost 70A). I know -and I have noticed it- that once it's hot it generates less power, but it seems to still be doing its job well enough.

I'll have a look at the fuses and keep investigating. Even considering 11A for the pump and the 5+5A that you suggest, I'm still about 10A short of what the car is consuming once the engine is running.

 

The ECU is probably a few amps, and your are also charging the battery, that is probably a few more amps, that probably makes up most of the 10Amps you are missing.
Even the HVAC fan is still running at very low speed (unless you turned the system off) is a few more Amps. Also there are electrical circuits in that instrument cluster, they will consume some energy.

I am still thinking something is not 100% with your charging system (alternator+regulator)

 

No, no, I have taken into account the current going to charge the battery, that was the calculation at the beginning. Also the current to power up the dash.
Between ignition on and engine running, there are 28-29A more of consumption. With 50A generation in idle, whatever is left goes to the battery. HVAC was off, as well as the internal fan (I checked the consumption of that one too).
Without any other load on, in idle the battery is being charged with 16A. That leaves 34A out of the 50A generated. With the ignition on the consumption is about 6A, this means that there are extra 28A going somewhere.
Actually, with ignition on the ECU should already be running, because I already get all the signals from the test connector, so that should be part of those 6A consumed when you turn the key.

 

If you are measuring the alternator output current you must have a current probe. This is my assumption. You can attempt to remove the fuses one at a time to measure the current dissipation of all the auxiliary circuits as well with an DMM as well. BTW the alternator in my car is rated at 80A, not sure at what rpm.
The expected behavior is ~14V upon start-up, drifting down to 13.5V once the battery is back to 100% charge, with no auxiliary circuits running.
Mine only goes below 13V (like 12.8V) if I have AC running, blower fan on and the auxiliary fans in front of the radiator are on high speed. If the aux fans shut off, it is back to 13.1-13.2 volts.

I'll be curious to find out if any of the other circuits powered by your fuse box use significant current. I doubt it though.

- Cheers!

 

I am using a probe, yes.

Would it be wise to remove fuses with the car running? I imagine that also the engine control unit is protected by the fuses...

Now you make me doubt of the 70A rating that I remember. I need to check again, the label has become almost completely erased and it was really hard to find the P/N.

 

I thought my W201 model is the lowest amperage alternator used by MB at 80A rating. I could be wrong. I have never had a reason to measure the current output. But I know they were equipped with 80A alternators. Larger models had larger alternators as far as I know.

What you can do is remove all fuses that you know are not involved in engine management prior to starting the car. The OVP has its own fuse and in most models they are two 10A fuses mounted on the OVP. Yours could be a single fuse. But this is all separate from the fuse box. And after starting the engine you can use your 10A capable DMM to measure the current thru the fuse terminals.

 

Perhaps check to make sure your rear defroster is not stuck in on position. That would take a lot of current >>10A. I assume it is on a relay circuit.

 

Very good idea! I´ll have a look at that later this afternoon!

 

So, I have checked the defroster and it was working properly.
Today I took the car to the garage for other things and they confirmed that the alternator is providing neither enough voltage or current and it has to be replaced.
So it was actually a problem with the alternator and not with the load. Well, it's not that surprising either, considering that it is already more than 33 years old and it has done its job.
Thanks for the support and the ideas!!

 

Great to hear it was diagnosed. My initial instinct was along the same lines. Happy motoring!