I seem to be having some kind of electrical problem.
For the last couple of days, every time i first start my car, my battery
“Charge” and CAT “Heat” light stays on. After a quick rev, the lights
go off.
Yesterday, on my way to work, the lights didn’t come on when i first
started, but did come on during the drive. The two lights came on
together, and were flickering/dimming.
The car still drove fine, and didnt miss a beat. Driving home it was
doing the same thing, but not as much. Once i got home i thought i would
break out the multi meter have have a look at the battery.
First up, the battery was boiling hot to touch ! I left it to cool for 15 min, and checked it.
Switched off, it read 12.5v ( no issues there)
Turned it on, it read 13v (again, nothing wrong with that)
Revved it, Still reading around 13v.
Because the two lights weren’t on, i thought, i would leave it until the morning.
This morning, Did the same proceedure, and the battery was still reading the normal voltage as before.
But this time i thought i would leave the multi meter on, and bring the
wires into the cabin, and take it for a drive. On the drive the “Charge”
light, and “Heat” light came back, and the meter was reading 19v and
over !!!
So now this leads me to think, this could be maybe an alternator fault? I
had a little poke around, but plug is ok, and belt looks fine, and that
ab out all i know on what to look out for on a alternator.
Does anyone know what could be the problem here, or if anyone has had this problem?
Does anyone know what i can try looking at next, or if they have a solution?
How much damage can i cause if i keep driving my car? I am going away on holiday from friday, but will need it until then.
Just got back from having another look, and noticed the battery is bulging. I removed it and had a closer look, and seen that it leaks when i tipped it to one side.
Hi, From the 19V reading and hot battery I’d say the battery is a casualty rather than the cause, it has simply overheated through being overcharged. A battery can be charged with (connected to) 19V but cannot be charged to, or supply that voltage, it will gas at 14.4V and start to lose water rapidly.
The only place that high voltage can come from is the alternator. This has diodes to rectify the AC to DC and puts out well over 12VDC. It then has an IC solid state regulator which controls the charging voltage depending on electrical load and that seems the likely culprit.
Fortunately(?) on the Mk1 all this is inside the alternator whereas in the Mk2 part is in the PCM/ECU.
I’d be looking for a good/exchange alternator and battery but by all means get a hands-on dignosis first by a sparky.
I woudn’t dissagree with Rich M because he may well be right. But it is possible that the problem is the battery. Nobody would leave an engine running and then disconnect the battery. The reason being is that chargers produce unstable and unpredictable outputs when not connected to a battery. If your old battery has gone faulty and gone more or less open circuit then this is the same as being disconnected. Your 19Volts may well be the result. As you have to buy a new battery I would do that first and with your excellent monitoring try it before tackling the alternator.
Hi RB, what you describe would be right for an unregulated supply (as in a basic battery charger for example), the load is lowered and the voltage rises due to resistance or supply impedance, ohms law. It wouldn’t happen with a working modern regulator though and that is exactly why they are there!
Hi Dave, the red terminal in the top pic is battery pos not earth.
I’m assuming your 12.5V and 13.3V readings were made with the meter connected across the battery terminals and so are correct.
I’m reading it that you then took off the meter probe from battery neg and put it on that red alternator terminal thinking it was earth and expecting the same 12.5V and 13.3V but you got 0.12V.
You then put it on engine earth after the light had come on and got the 17V reading.
If that’s right, the reason for that 0.12V reading is that both probes were connected to pos; one near the battery and the other near the alternator. The 0.12V meter reading is the voltage drop across the resistance in that pos loop and is about what we’d expect. When you transferred it to engine earth you got a proper reading and saw the engine light was on and it was at 17V.
To double-check that red terminal connection, leave one meter probe on battery neg and put the other to that red terminal and you should get battery voltage.
So our diagnosis is unchanged. Check again but it seems that the regulator works at first then breaks down and allows 17V - 19V instead of 13V.
It would appear that the regulator is breaking down. The fact that it’s reading in the right area and then bangs up to 17V and the charge light comes on suggests that this is the case. The heat light is possibly bleed over from the charge light bulb. The heat light comes on if thecat melts down.
The battery is screwed because it has been overheated ,
Replace the alternator and battery. While your aboiut it clean the earths to the PPF, Chassis and engine.
If you don’t need the car for a few days take the alternator to a good Auto electrician and get it rebuilt. It may be a cheaper alternative
Sorry I was on holiday, and then got caught up enjoying the car when i got back
So anyway, when i got back into the country, I went over to my parents place, and pick up a new alternator and battry that was delivered to them while i was away.
2 hours later, I had them both fitted, and with help of my multi meter, the car was happliy reading 14v again while driving.
Very happy with the results and the advise you guys have given me again !!
I left it to cool for 15 min, and checked it.
if I was guessing it would be the regulator in the alternator on a Mk1 but you don’t say which car.
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