How to test Faulty Alternator

[Nuggit]

Hello,

I had some issues with my year 2000 T4 1.9 TD. Whilst driving, the rev counter would drop to zero and the oil light would flash with 4 beeps. But would still drive fine. Then it started happening more often, this all happened within a few days, but when the revs drop, if i put clutch down, and gassed it hard, i would see some life on the rev, but not much at all.

Battery is still fairly new and when I was testing the engine running voltage, it was less that 12, in fact it dropped down to 6! Which i think indicates the alternator right ?

Now I can't turn it over even, and when I turn the key, half way, the glow plug light flickers on and off extremely fast and the 104 glow plug relay clicks in time with that (very fast!). Which I also think indicates alternator/not enough juice, as now battery hasnt been charged enough?

I just wanted some confirmation really as I cant drive it to the garage! But can walk to euro car parts and get new one to fit.

Right now tho the battery measures 11.5V which i thought should be enough to turn it over.. and last night when I jump started it, it ran for a couple minutes, then went to a horribly Lumpy idle. Couldnt rev very high/well and so I turned the lights on to see how well they shine in all this, and the second i switched them to low beam the engine died.

Any info useful!! Any way I can test the alternator without it running ? ?

Thanks in advance!!

[Crasher]

That sounds like a faulty crank speed sensor but you need a code read first.

[Nuggit]

Ah right okay, do you know if I can test with home tools/voltmeter -- and how ?

(just for interest) Heres a video of it running, after I jump started it. The battery was showing 11.5v but still didn't start, so jumped it. Testing volts at battery showed 6v while running, and as you'll see in the vid, its VERY lumpy :( [ FYI: it is responding to the revving, nowhere near as much as I was giving it tho, you just cant hear over the relay madness]


https://youtu.be/YV4vaSMxtUo?si=IbEM9KZJqkmnU-jp

[Crasher]

The crank speed sensor can be tested with an oscilloscope but it is easier to watch the RPM signal on a diagnostic system, often they won't come up as faulty even when they are and if they do come up as a code it is often wrong and another component, such as the EGR valve on the EA189 engine) is spiking the sensor.

[Nuggit]

Ok, thanks Crasher. You've helped me greatly in the past with my old Golf so again appreciate your help! I'll find where it is under the hood and see if its visible corroded or damaged, just for my sake. Probably get a mobile mech out to see if they can code it, but like you say it may throw off red herring results. I'll update the thread once I get it sorted :beerchug:

[Crasher]

The crank sensor is an evil git to do, buried behind the oil filter assembly and accessed at an angle though a hole in the block using a 4mm ball ended LONG hex key and if you round out the screw, box out, flywheel off, sump down.

[gareth23]

I think you need to sort out your voltages before looking at other stuff.

First of all are you sure your meter is fairly accurate?

After that you should be getting no less than 12v across the battery with everything turned off, any less and its flat and needs charged. With the engine running you should be getting around 14v which doesnt change much when lights etc are turned on.

You mention a battery voltage of 6v, draining it that much has possibly killed it, running it that low several times will definately kill it.

A decent electronic battery tester will tell you the capacity of the battery as well as the voltage to give you an idea how healthy it is. The last battery I replaced was reading ok voltage wise but the capacity was down to 18ah instead of its rated 85ah.

You can test an alternator without running it but it involves taking it apart, much easier to get the engine running and measure the voltage it delivers to a known good battery.

[Nuggit]

So I got my mobile mech out to code it, and have a look. He said alternator.
He jump started it with his van while the leads were still on it was running perfectly. So I think my lumpy idle was because fuel system was running directly off battery, so when I turned lights on, it killed it. But we tested volt and it was 6v and dropping after he jumped it. When I jumped it previously I used a battery jumper as no 2nd car!

Anyway long story short, as it ran well after he jumped it with more juice, I took an educated gamble on it being alternator bought and fitted today. Runs like a dream, reads 14.3v running now.

Thanks for the help :D

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