95 Mazda B2300 SE 2.3L just got this truck on craigslist, I'm under the impression it's very similar to rangers from the same era so this seems the right place for my question.

The previous owner was having electrical issues and had replaced the battery. When I bought the car, it was not charging (voltage from positive to negative battery terminals when car was running was 11.3V). I could only get the car started by jumping it. I replaced the alternator but that didn't seem to change anything. When I put the voltmeter probes on the case of the alternator and on the positive terminal on the back of the alternator, I get a voltage of 13.6V. The voltmeter indicates that there is electrical continuity from the alternator case to the negative battery terminal, and also from the positive post on the back of the alternator to the positive battery terminal. The research I have done so far made me think I have a grounding issue, so I've cleaned the grounds I can find. I found one from the battery to the frame and one from the back of the block to the firewall.

Why would I still not be charging if the alternator itself is generating a voltage of 13.6V and there is electrical continuity going back to the battery?

Thanks

 

I found this info helpful from another electrical post
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Battery's larger ground(negative) cable should go to a starter motor bolt, maybe bell housing bolt, on most engines, because starter motor needs 50-75 AMPs all at once to crank engine
So BOTH larger battery cables should run to starter motor, positive and negative.

On some it can bolt to the side of the Block, on engine side motor mount bolt

Other grounds you need
Smaller ground wire from battery to inner fender and Rad Support, for relays and headlights/horn
Ground strap from rear of engine/head to firewall, main ground for ALL cab electrics <<<< very important and often left off after engine work
Ground strap from engine to Frame, for tail lights and fuel pump and gauge<<<< engine and trans sit on rubber mounts, so frame needs a ground strap
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I haven't found the first and third grounds referenced here, and I don't think the negative battery cable makes it all the way to the starter motor, so I haven't had that negative battery cable off and cleaned it yet.
Thanks

 

The largest negative battery cable should run to the block, or starter motor bolt, this is the ground for alternator and starter, so starter wouldn't work if this wasn't OK

So with engine running your battery shows 11.3v?
But B+(terminal on back of alternator) shows 13.6volts

If this is the case then the Fusible Link on the B+ wire is burned out

A battery positive cable should run to the Starter Relay(solenoid) on the inner finder
On that same post will be several other wires, this is the 12volt Power Distribution Point for the vehicle

And B+ is hooked up there, via a Fusible link
A fusible link is a 6-8" wire that can handle higher amps without "blowing"
On a 1995 it should be a Grey wire

Starter relay POST---------(grey fusible link)---(black/orange wire)-----------------B+ on alternator

This wire transfers ALL alternator voltage to the vehicle, so without it you would see what you see now

You may need to uncover wiring harness to get to the fusible link wire, but it IS on that starter relay post with Battery cable

 

Thanks. I did more testing this afternoon, but still haven't figured it out.

Located the grey fuseable link connected to the black/orange wire that goes to alternator. The resistance in this cable was around .5 Ohms, so I'm inclined to believe the fuseable link is still OK. What I did notice is that the ring terminal that goes to the B+ post on the alternator is quite corroded, and the wire leading into the ring terminal has 30% of the strands broken off. Looks like a copper wire but in this broken section it is green. I will try to attach a photo.

I think I made a mistake before, when I measured voltage from the B+ terminal to the case of the alternator. I previously said it was 13.6 volts, but at that point I was still connected to the other vehicle I had used to jump-start my truck. Bit of a blunder... When I measured voltage again from the B+ terminal to the case of the alternator, I get a reading of 11.3 volts, which seems low for a brand new alternator (remanufactured). Engine idle speed seems okay, belt tension feels alright, and I don't hear the pully slipping.

To test if I was facing a ground issue, I devised a test: Use a jumper cable to carry the ground directly from the case of the alternator to the negative battery terminal. This did not seem to have any effect, and the battery voltage with the engine running and the ground being carried directly to the negative battery terminal was still less than 12 volts.

 

Then do the Basic Alternator wiring test

Key OFF
Measure battery voltage, remember it
test B+ terminal. should be "battery voltage", if not Fusible link is bad(1995)

Unplug the 2 or 3 wire connector on alternator
Test Yellow wire, should be battery voltage, if not fuse 17, 15amp, is blown(engine fuse box, 1995)

Test light green wire, should be 0 volts
Turn key on
Retest light green wire, should be battery voltage, if not fuse 15, 15amp is blown(CAB Fuse box, 1995)

If all wires test OK then truck wiring is fine, hook wires back up

Start engine
Test battery voltage, if its not above 14volts then alternator is bad, for sure, 100%, replace it

 

Okay will do. And to be clear, all of these voltages are measured compared to the negative battery terminal?

 

Or the alternators case, should be the same

 

When I test the voltage between the light green wire (on the 3 wire connector at the alternator) I get 0 volts regardless of whether or not the key is on. I checked the fuse box in the cab, but fuse 15 tested fine (.7 ohms) and also looks fine. I replaced it with a brand new fuse but the result of the test is the same?

I have started to try and follow the green wire from the alternator to the fuse box, but have only gotten as far as the firewall. From there it's hard to tell where the wire goes. I have the instrument cluster out right now to repair the odometer, so I was able to find one wire with continuity to the light green wire from where it disappears into the firewall. Going to replace the instruments in case that makes a difference.

 

Great, you found the problem

The light green wire goes to the Instrument cluster, its the "ground" for the Battery Light Bulb, so I assume the battery light no longer comes on with Key ON?

The fuse powers(12v) the battery light bulb socket

Diagrams below, C216 is on the cluster


How it works
When key is on Battery light gets 12v from fuse 15
And when engine is not running the Alternator's green wire connector is 0volts so "like a Ground"
So 12v and ground = Battery light comes on

Now an alternator can NOT generate any volts on its own, its needs Start Up voltage, and thats what the light green wire provides
The 12v from fuse 15 travels THRU the light bulbs filament to the alternator on the light green wire

Fuse 15------------------bulb-----------------------(12v)alternator(ground) = Battery Light ON

And thats the start up voltage

When engine is started and alternator is WORKING that light green wire terminal on alternator will be 12v, well 13.5v, and fuse 15 will also be 13.5v
So fuse 15(13.5v)------------bulb--------------------13.5v no more "ground" so Battery Light OFF

Clear as mud, lol

Anyway light green wire should have 12v key ON

 

Yeah, I also think we found the problem. Put the instrument cluster back in after I swapped in some new bulbs for the ones that looked burnt and now the battery light comes on with the key ON. With the truck running, the battery voltage fluctuates between 14 volts and some lower voltages. I think it's about 50/50 between steady 14 and jumping around lower. It is charging for sure though.

Thank you for all of the help. What was the source for those wiring diagrams? Would definitely be helpful for other projects.

 

Good Work

Voltage shouldn't be jumping around, should be steady, it will drop down to 13.5-13.8v after engine has been running for say 10min, but no "jumping"

Set Volt meter to read AC Volts, then test battery with engine running, should see under .5vAC, so under 1/2 volt AC, if higher then a diode is bad in the alternator, exchange it


Public Library's have Online services now, well they have had it for a long while, you need to get a library card and set up a PIN, its FREE, well if you pay taxes, it part of the service, lol
Then log in from home and look for Auto Repair section, lots of info including wiring diagrams