Author Topic: '98 T4 2.5 tdi alternator problem  (Read 1101 times)

Offline Ozz

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'98 T4 2.5 tdi alternator problem
« on: 30 May 2018, 21:58 »
Hi there,

I've been trying to sort out a charging problem on the van, but can't work it out. It's not a golf but I was hoping the wealth of knowledge here might be transferable!

2 weeks ago the battery stopped charging, battery light came on. I checked the battery voltage and it was at 11.9V. After jump starting, I saw that the voltage at the battery was at 12.9 at idle, but would go up above 16V when the engine was revved.

I thought the regulator must be broken, so last weekend I installed a new Bosch regulator and hoped that would fix the problem. However on a test drive the battery light was still on (didn't have time for voltage tests as it was late when I finished). Monday morning I couldn't even jump start the van, battery voltage was at 0.4V!

After trickle charging the battery the voltage came back, but at idle the voltage at the battery is now 16V going up to over 18 when revved.

Should I just replace the alternator? I would've thought a new regulator would sort the problem, as the alternator is clearly capable of putting out enough volts!

Any help much appreciated!
Ozz
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Offline itavaltalainen

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Re: '98 T4 2.5 tdi alternator problem
« Reply #1 on: 01 June 2018, 23:19 »
Which alternator is it?
Are the rings ok, did you look at them?

Could be a fault with diodes too....
Would also trace wire from dash to alternator to be sure that's ok.
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Offline Ozz

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Re: '98 T4 2.5 tdi alternator problem
« Reply #2 on: 02 June 2018, 06:55 »
I think it's a Bosch 90A alternator, though it was hard to see because of where it's placed on the t4.

Not sure what the rings are so no I didn't look st them!

Is there any way to check the diodes?

Thanks,
Ozz
Mk2 Gti 8v

Offline bgbazz

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Re: '98 T4 2.5 tdi alternator problem
« Reply #3 on: 02 June 2018, 08:18 »
I would be looking closely at the battery as a first step. Easy way to check it's condition is to totally disconnect it, charge it until you get around 13.8v (with the charger unhooked), then leave it overnight with nothing connected. A battery in good condition will read the same the next day. If shows more than about 1v drop, it is on the way out, or gone already.
I had a similar problem very recently and even though the battery (Bosch) was only 8 or 9 months old, it had failed. New battery and no problems since.
Mk4 Golf TDI Estate 4Motion, remapped 1.9. Sweet runner = keeper. She is the 66th vehicle I've owned and hopefully, the last.

Offline Ozz

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Re: '98 T4 2.5 tdi alternator problem
« Reply #4 on: 02 June 2018, 14:32 »
Well I think the battery might be dead, but I'm worried that it's the alternator which killed it, so I'm reluctant to replace the battery before checking the alternator.

Unless I'm mistaken, even with a dead battery the alternator shouldn't be sending 16-18V, the regulator should limit it to around 15.5 max?
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Offline bgbazz

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Re: '98 T4 2.5 tdi alternator problem
« Reply #5 on: 03 June 2018, 15:52 »
To my way of thinking, I would replace the suspect battery first and see how the alternator responds to that..a short drive will tell the story.

Depending on the brand alternator and it's individual capabilities, it is possible for it to go into overdrive and cook itself, trying to keep up.

Just to help clarify what I mean....my Nissan Patrol came equipped with two HD 12v batteries and an alternator to match. After a heavy day in the mud, winching every couple of hundred yards or so, these batteries would be way down on capacity...my fitted voltmeter would sit around the 18v mark for more than an hour on the drive home, then gradually creep back down to about 14v.
It became a topic of conversation during a club meet one day and we ended up comparing alternators fitted to similar model vehicles. From the outside, they all looked the same...but a check of the little specs plate attached to each one revealed a difference.
It may well be that your alternator is toasted because of trying to keep up with a dead, or dying battery, but I don't really think that the alternator would cause the battery to die. I would change the battery first...even if it's a known good one from a mates car...a short drive will do no harm and may just reveal the truth.

Worth a try, just to save you some money.



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Mk4 Golf TDI Estate 4Motion, remapped 1.9. Sweet runner = keeper. She is the 66th vehicle I've owned and hopefully, the last.

Offline Ozz

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Re: '98 T4 2.5 tdi alternator problem
« Reply #6 on: 03 June 2018, 18:25 »
Well I've charged the battery up and disconnected it this afternoon to see if it holds charge, and have an appointment at the garage for a free alternator diagnostic test, fingers crossed it's just the battery!

Thanks for the replies, will let you know the outcome! Pressure is on now as we'll need to sell the van soon to pay for our mk6 golf which we're picking up next weekend!
Mk2 Gti 8v