Very faint battery light - constant

[cn90]

Maybe the.mechanic forgot to tighten the red cable on the alternator?

[andrewwynn]

I forgot to tighten the nut once.

Made the connection incandescent!


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[akwoods]

OK, sorry for the very long gap between updates here. The main update being that the car has continued to show a dim dash charge light and the battery discharges if the car is not driven for 3 days or more. The car is our second car, so I regularly sits for a week without being driven. I haven’t had time to work on it, so I’m just jump starting it as I need to use it. I got some time over the weekend and checked all of the connections between the alternator and starter and the starter and the battery. All are tight. I thought there may have been an issue with the cable from the starter to the firewall (battery terminal) so replaced this. At the same time took the suggestion to make sure that (on the starter) this cable sat above the cable from the alternator, it was already. Replacing this lead did nothing. Replacing the battery didn’t help. Measuring the charge rate at the battery is 14.3. When the car is off, this drops to 12.8. I still haven’t measured amps from the alternator, but figured if I have 14.3 volts at the battery (in the boot) that this isn’t the problem. Please correct me if this is not right. Also, someone mentioned the ignition switch could be the problem. Could this cause the dim light and the battery draining issue? Because the new battery continues to drain after 3 days, despite charging at what appears to be full and constant charge during driving. It seems to be less about the amount of charge going into the battery and more about some sort of parasitic drain of the battery that also gives the symptom of a dim battery light when the car is running. Any further thoughts appreciated.

[wpoll]

I’d still be very suspicious of the work done to replace the starter motor, or even the starter motor itself... was it a cheap copy part?

It’s no coincidence that this issue began when the starter was replaced....

[andrewwynn]

If the diesel is wired like the sparked engines, disconnect the main cable at the jump point and measure across the new gap you create. You should not get a voltage. If you do either the alternator or the starter has a fault that’s drawing current always.

In fact if you get a voltage reading, then measure amps. If you get an amperage reading then disconnect the main cable at the alternator to measure if you can still measure voltage/current going to the starter.

Neither starter nor alternator should draw current when the engine is off but both are connected to an always live battery cable.

A draw at the starter while running should not feedback info the alternator circuit monitor so I am suspecting something in the alternator like a leaky diode allowing current to go the wrong way. Not enough to stop voltage production but throw something off.

Measuring AC on the alternator output (o-scope even better) can be very helpful. I think I'm used to seeing 400hz 0.2v ripple.

I don't think anything else is attached to that main cable so it will bea very quick test to confirm yes/no one of the two items are faulty.


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[akwoods]

Thanks everyone, @Andrewwynn thanks for making this easy to understand and easy to follow. When I got to run this test, the battery was flat, I measured 2.93 volts when testing between positive and negative terminals in the engine bay. I pulled the positive off and tested the gap and got 2.93v there too. I charged the battery, turned the car off and got 12.2v at the terminals, then 12.2v at the gap once the cable was disconnected. Hopefully I did the test right, measuring between the positive terminal to the battery and the disconnected cable to the starter?? If this is correct, then it would appear the new starter is constantly drawing voltage (or the alternator is knackered). I’ll take it back to the mechanic that changed the starter and get them to look at it. Thanks again.

[andrewwynn]

When the jump point is disconnected there shouldn’t be any current path and could be either the starter solenoid or the alternator but more likely the alternator.

Hopefully the diesel is similar to the gas model to isolate the alternator and confirm which is the problem.

When I’ve seen similar it’s been in the diode stack to the stator.

The alternator has a set of six diodes to convert the 3 phase AC into pulsed DC and the DC side is directly attached to the battery where current can only go out to the battery.

You may be able to read voltage over the megaohm gap on the diodes so your need to measure ohms and/or current to confirm if there’s actually a parasitic draw.

The starter should have a full open so ∞ Ω but the diodes reverse should have effectively ∞ ohm also but might be 10-20 M Ω so able to let enough current through to get a voltage reading but should be a very very tiny current like 5-20 μA. (micro not milli).

I would measure Ω across the gap and once you confirm the range is at least in the 100s of k Ω then take a current reading across the jump point.

If I had to guess I would expect to see 10-20 M Ω and 12/15000000 or 800 nA (0.8 μ A).

https://chatgpt.com/share/69bfc90c-f...7-291185920175


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