strange S13 alternator test results

[metako42]

Was driving along recently and car just cut out. Was told by the roadside assist that alternator has packed it in and got a tow home. Decided to try to confirm the bad alternator and after charging battery up got 12.6 volts across battery with engine off and 11.7 volts with engine at 1500 rpm so looks like alternator isn't charging. Fan belt seems tight.

I tested the voltage between the battery neg post and the casing of the alternator - got a steady 0.01 volts at 1500 rpm so all good there.

Then I tested between the pos battery post and the pos terminal on the alternator - it went from 0.02 volts one second then all the way up to 8.1 volts and then back to 0.6 volts then 8.1 volts !!! What the hell is going on there? I tried to bypass the cable with another one but it didn't make any difference to the readings.

I tried to re-measure several more times (car was quite warm by now) and then suddenly it felt like something kicked in and I got a chirp from the fan belt and the battery started reading 13.9 volts instead of the previous 11.7. What suddenly kicked in - the regulator? Is the regulator faulty or maybe the alternator itself? Any ideas?

[jedi03]

sounds like bad alternator, however, I would also check relays and fuses

[metako42]

Thanks for your comment jedi03. I checked the fuses and fusible link already and even bypassed them both with a solid cable from the battery to the alternator and it made no difference to the voltage readings.

[Gearhead9992]

Just a bad alternator

[EnnEssEnnKAT]

Carefully disconnect the positive battery cable with the car running, that is the next easiest way to see.


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[anti tyler]

sounds like bad alternator, however, I would also check relays and fuses

Thanks for your comment jedi03. I checked the fuses and fusible link already and even bypassed them both with a solid cable from the battery to the alternator and it made no difference to the voltage readings.


The problem with that is, if the 75a fuse was blown you wouldn't be able to start the car.

Starter relay etc on a different track.


Just take the alternator off and have it tested. You may have a bad ground if not alternator.

[metako42]

So car was charging ok from the alternator for a couple of days but now its not charging again so sounds like an intermittant fault in the alternator. I'll take it off and get it checked and probably have it rebuilt. Thanks for the replies.

[jedi03]

I would suggest check the 75a fuse...had one blown but still made intermittent connection...pop the plastic cover and try prying the contact

[Agamemnon]

Also check the signal wire that goes from the alternator to the gauge cluster. The alternator won't charge without it.

[metako42]

Ok thanks - will check the fuse again but I did try bypassing fuse with a solid cable from + battery to alternator and it made no difference. Will also check wire to guage. If its working then alt light should light up when you turn ignition to ON correct?

[metako42]

I rechecked the + wire and I suspect either the fusible link or the ring terminal connection at the battery is faulty, but I can't get the fusible link out - it's stuck hard! I tried spraying it with CRC but didn't help. Tried pliers and even vise grips to pull it out. Anyone have any tricks to getting one of these out?

My second option is to bypass the old alternator cable and fusible link with a new cable with new inline fusible link wire. Btw the battery light comes on ok and goes out when running.

[metako42]

Well I tested the battery again today and the alternator wasn't charging at all again. So I made up a new alternator cable straight from the alternator to the battery positive and the alternator is now charging! So it looks like it wasn't a bad alternator at all but a bad cable/fusible link.

I still can't get the old fusible link out to check and clean it (I suspect some water got in to where the cables plug in underneath as it was a bit wet under there) but I'll go to a wreckers soon and look at a fusible link box and try to work out how it all comes apart and try to repair it.

The new cable doesn't have an inline fuse so I might get one of these battery post fusible links below until I can repair the old one. Thanks for all the suggestions.

https://www.jaycar.com.au/battery-terminal-fuse-100a/p/SF4182