2000 Mazda B2500 - New alternator but battery still not charging

Honest John Back Room » Technical matters
What is life like with your car? Let us know and win £500 in John Lewis vouchers | No thanks
View Threaded
Sat 8 Apr 2023 16:20
2000 Mazda B2500 - New alternator but battery still not charging - brucewayne

Can anyone help me with this problem on my 2000 Mazda B2500 please. Got in it the other morning battery was too flat to turn the engine to start it. Charged the battery up and got it running but the battery light was on all the time. Put a meter on the battery with engine runnning and it stayed at about 11.9 volts even when I revved it up hard. Alternator gone, yes?
Fitted a new alternator, started her up, battery light no longer on, happy days! Used the truck ok, but a few days later, went to start her up in the morning and the battery was flat again. Charged the battery, started the engine, put the meter on the battery and got a steady voltage no matter what the rpm.New alternator not charging. I've checked every fuse I can find and there all all good. The battery is only a few months old and is a decent Yuasa brand.
Any ideas on what to do next anyone?

Reply | Report as offensive | Link
Sat 8 Apr 2023 17:05
2000 Mazda B2500 - New alternator but battery still not charging - bathtub tom

It's a twenty-year-old motor. Check all the battery, earth and starter connections for corrosion.

Edited by bathtub tom on 08/04/2023 at 17:05

Reply | Report as offensive | Link
Sat 8 Apr 2023 18:53
2000 Mazda B2500 - New alternator but battery still not charging - elekie&a/c doctor
At the back of the alternator is a large fat wire , which is the main power lead . Check for battery voltage at this point .
Reply | Report as offensive | Link
Sun 9 Apr 2023 00:36
2000 Mazda B2500 - New alternator but battery still not charging - brucewayne

Yep, did that, reads the same as at the battery.

Reply | Report as offensive | Link
Sun 9 Apr 2023 01:04
2000 Mazda B2500 - New alternator but battery still not charging - edlithgow
Check v belt tension.
Though it could easily be your new alternator is junk, especially if reman.
Reply | Report as offensive | Link
Sun 9 Apr 2023 09:37
2000 Mazda B2500 - New alternator but battery still not charging - brucewayne

Yeh, Have thought that. Although it came from a Alternator/Starter specialist who have been trading for many years. And it was £170 sodding quid!

Reply | Report as offensive | Link
Sun 9 Apr 2023 10:19
2000 Mazda B2500 - New alternator but battery still not charging - elekie&a/c doctor
How many wires on the back of the alternator?
Reply | Report as offensive | Link
Sun 9 Apr 2023 11:02
2000 Mazda B2500 - New alternator but battery still not charging - brucewayne

There's the main power take off. a single spade/socket connection and another socket with 3 pins which can only go in one way. I've seen testing alternator videos on youtube which say that the alternator needs 'telling' to produce power, and thats something to do with the battery warning light. But as the light goes out straight away I would have thought that side of the wiring is ok.

Reply | Report as offensive | Link
Sun 9 Apr 2023 10:59
2000 Mazda B2500 - New alternator but battery still not charging - Railroad.

Check to make sure you have a good battery feed at Ter B+, the main cable on the alternator. Carry out a volt drop test between battery positive and alternator positive which should not exceed 0.5v under load. Do the same on the earth side. The two smaller wires are for the warning lamp and a phase tap. The latter will be for the tachometer or anything else that requires a speed signal. This will not affect charging if it doesn't work. The other is the exciter wire which will be supplied battery voltage when the alternator starts to charge. Equal voltage on both sides of the warning lamp will extinguish it. Earth this wire which should put the warning lamp on. If it does you know the lamp circuit is good.

Reply | Report as offensive | Link
Sun 9 Apr 2023 11:42
2000 Mazda B2500 - New alternator but battery still not charging - brucewayne

Ok great. Thanks for the advice, will give that all a go.

Seeing as the warning lamp goes out as it should and the tacho works ok, would your gut feeling be that I've bought a dud alternator?

Reply | Report as offensive | Link
Sun 9 Apr 2023 13:09
2000 Mazda B2500 - New alternator but battery still not charging - elekie&a/c doctor
I know it’s a fairly old vehicle, but I would think the tachometer works from the engine control unit , rather than the alternator. Looks like it may be a dodgy unit .
Reply | Report as offensive | Link
Sun 9 Apr 2023 18:44
2000 Mazda B2500 - New alternator but battery still not charging - brucewayne
I know it’s a fairly old vehicle, but I would think the tachometer works from the engine control unit , rather than the alternator. Looks like it may be a dodgy unit .

Many thanks.

Reply | Report as offensive | Link
Sun 9 Apr 2023 14:12
2000 Mazda B2500 - New alternator but battery still not charging - Railroad.

Seeing as the warning lamp goes out as it should and the tacho works ok, would your gut feeling be that I've bought a dud alternator?

I would definitely check the main alternator cable to battery positive. Some go directly to the battery, and some go via the starter motor. A bad connection there could be your problem. A volt drop test will confirm that or otherwise. Alternatively temporarily connect a heavy duty jump lead from the alternator positive to battery positive and see if that cures your problem. If it does you have a voltage supply fault. If it makes no difference then you probably have a duff alternator.

Reply | Report as offensive | Link
Sun 9 Apr 2023 18:48
2000 Mazda B2500 - New alternator but battery still not charging - brucewayne

Seeing as the warning lamp goes out as it should and the tacho works ok, would your gut feeling be that I've bought a dud alternator?

I would definitely check the main alternator cable to battery positive. Some go directly to the battery, and some go via the starter motor. A bad connection there could be your problem. A volt drop test will confirm that or otherwise. Alternatively temporarily connect a heavy duty jump lead from the alternator positive to battery positive and see if that cures your problem. If it does you have a voltage supply fault. If it makes no difference then you probably have a duff alternator.

Ok, l'll try that and hopefully get nearer to sorting it out. Many thanks.

Reply | Report as offensive | Link

Looking for a garage you can trust? Honest John's Good Garage Guide has thousands of reader recommended garages, plus how-to guides and tips on servicing.

The Good Garage Guide

From which small car to buy at which price point to the best UK-built cars, our Top 10s have the lot. Click now for your shortlist.

Unsure of what to buy? Our Top 10s are here to help you....