Dim battery light after start up

[jmc330i]

Started the car (M44 converted) to come home this evening and whilst it was at idle to clear the windscreen, I noticed the battery light was very dimly lit. It stayed on whilst idling and being revved a little and was the same after I switched off and restarted the engine.
The light went out when I turned the heater fan to full speed, maybe because of the sudden extra load?

Any ideas what would cause the battery light to be lit dimly?

Cheers

[jmc330i]

Sunday morning bump

[Speedtouch]

Check the alternator belt tension; a loose belt may be slipping, particularly in wet weather, leading to the alternator not turning, thus not charging the battery.

It should have approx. 0.25" of give when pushed down by a thumb in the mid-point of its longest length between pulleys.

[jmc330i]

Thanks Maurice In my tired state after a long day at work I did check the belt tension, only to remember after checking that the M44 uses one auxiliary belt with a tensioner. The belt and tensioner were replaced during the summer so they should be good.

What's confusing me is the light will be lit dimly under all engine speeds but will go out if I switch the heater fan to full speed.
Is this pointing towards the alternator?

[Brianmoooore]

If you can get at the accessories socket, behind the glove box, measure the voltage on pins B, C and Z with the battery light glowing and with it not glowing.

[jmc330i]

Thanks Brian, I'll check those and get back to you, if this rain ever stops.

[jmc330i]

Typically the battery light hasn't come on at all this week so I've not tested the voltages yet, but I had another issue this morning that could be related???

The last two days, when I turn the key to start the engine there was a 2-3 second delay before the starter would turn, but this morning there was the delay and then a clunk from the starter and no starting (it didn't sound like the solenoid clicking). After 10 or so attempts, the starter reluctantly turned slowly and the engine fired up.

I had the same issue with the starter last year, I cleaned all the earths as suggested and it's been fine until two days ago. I think it was Brianmoooore that suggested that the starter maybe on its way out.
I do have another nearly new starter now and another alternator from a spare engine I've just bought, so will get those swapped at the weekend.

Would the starter/alternator/battery light issues be connected?

[maxfield]

On my E36 M44, my battery light came on faintly for a short period and went off. Thought nothing of it at the time. Did think to myself at the time I should test voltage in case of an issue. But being the lazy mechanic I am and can't be bothered to do much on my own car ignored it.

A week later I broke down in the middle of the busiest junction in Mansfield at rush hour.

I would definitely check the alternator output before it's too late.

[jmc330i]

Thanks Jamie. After not starting this morning, it's been spot on all day and started every time
I'm leaving it in the workshop at work tonight so I'll get on with changing the starter in the warm tomorrow morning. I'll check the alternator as well and change it if needs be.

[jmc330i]

If you can get at the accessories socket, behind the glove box, measure the voltage on pins B, C and Z with the battery light glowing and with it not glowing.

With the light glowing (not fully on) I've measured C - 14.02v, B & Z vary but both are just over 10v.
With the light as dim as it will get, C - 14.02v, B & Z vary but both just over 11v.

I've not been able to get a reading with the light off as it's always lit very dimly now.

[jmc330i]

I've just left the car idling with the multimeter on the battery and the voltage is around 14.02v, drops to around 13.5v with some load (lights and fan). It does also randomly drop to around 12v and even as low as 10v while idling. I briefly saw 9.3v.

I will change the alternator at the weekend, but is there anything else like a poor earth connection that could cause the issue?

Cheers

[DanThe]

Easy enough to go round the main earth points, remove, clean the mating faces, grease and refit.

[Brianmoooore]

With the light glowing (not fully on) I've measured C - 14.02v, B & Z vary but both are just over 10v.
With the light as dim as it will get, C - 14.02v, B & Z vary but both just over 11v.

C is the output from the small terminal on the alternator (blue wire) which connects to one side of the warning lamp.
14.02 volts here is about what I'd expect.

Z is from a permanently live fuse in the fusebox and B is the 12 volt switched supply from the ignition switch, which is connected to the other side of the warning lamp.
The voltage between B and C is what is making the light glow, and with things working properly, should be zero.

C and Z are the same voltage, so voltage drop from a faulty ignition switch or its wiring can be eliminated as the cause.

The light is glowing because the battery voltage at the fusebox is low, so the next thing to check is that the voltage at the battery + terminal is the same as at Z, and if it is, the next question is why is battery voltage low?

The next thing is to charge the battery fully, and see if the warning light then stays off, and then to run the car to see if the battery voltage drops back to where it was.

[jmc330i]

Sorry for the delay, I've been working away so not had much time.
The car is now back to normal. The starter motor did finally die, so a replacement solved that.

With the light glowing (not fully on) I've measured C - 14.02v, B & Z vary but both are just over 10v.
With the light as dim as it will get, C - 14.02v, B & Z vary but both just over 11v.

C and Z are the same voltage, so voltage drop from a faulty ignition switch or its wiring can be eliminated as the cause.

Did you mean B and Z? C and Z are not the same.

The light is glowing because the battery voltage at the fusebox is low

I would say that's the answer. The car died on me early one morning, engine cut out, lights out, no electrics at all - not a great experience at 50mph in the dark on an unlit road!
As best I could by torch light, I checked all the fuses, power and earth connections and everything looked good, but a small (desperate attempt) tweak of the power cable going to the fusebox brought the car back to life.

Once at work, I opened the fusebox up and found the power cable to the fusebox was loose inside with some charring around the connection - a fairly close call!!
With it all cleaned and tightened up the battery light is no longer glowing.

[Brianmoooore]

the power cable to the fusebox was loose inside with some charring around the connection

That will be the problem. The voltage being developed across this resistive connection is the voltage that was making the light glow.