Alternator problem?

[gotoPzero]

I have a machine (perkins diesel) and its been slow starting the last few days despite being ran for a couple of hours.

I tested the battery and it tested ok but was only showing 12v. I did a battery charge test and it was showing no input.

So I got to the alternator and tested the output direct and was only seeing about 12v.

Took the alternator off and had it tested by a local place and they said its duff so got a new alternator.

Put the new one on and started up but still seeing 12.1 ish V. If I run the charge test I see no input error again.

I have checked the belt - its fine. I have checked the fuses all fine.

Any suggestions??

The wires running from the alternator to the battery are fairly short so I cant see any issues with the wiring.

Should I add an extra earth maybe? I am confused!!!

[Hammer67]

I would start by not assuming the new alternator is functioning correctly.

Have it checked in the same way you did with the old one.

[Polly Grigora]

Alternator on your machine very likely relies on the battery warning light to excite its rotor which in turn gives a low revs cut-in speed

Is the battery warning light working correctly?

Without a battery warning light the alternator needs to be running at a high speed to cut-in

[gotoPzero]

Polly Grigora said:

Alternator on your machine very likely relies on the battery warning light to excite its rotor which in turn gives a low revs cut-in speed

Is the battery warning light working correctly?

Without a battery warning light the alternator needs to be running at a high speed to cut-in

Cheers for this the auto electrical place we got the alternator from said the same thing.
So we have just checked the bulbs and they are all ok.

However the battery warning light is not working so it looks like thats probably going to be the fault - on with it now.

I will report back!

[Polly Grigora]

Ok then

There should be an ignition switched supply to one side of the battery warning light (might be a green cable)

The other side of the warning light (might be brown/yellow cable) should go directly to the alternator and it's terminal labelled D+ or 61

The cable connecting to the alternator D+ or 61 terminal can be disconnected and held to an earth point like the engine block to check the circuit

With ignition on and the alternator end of the warning light cable connected to the engine block the warning light should be at full brightness

[gotoPzero]

Thanks trying this now.

The alternator bulb is ok and I have tested it and it lights up.

But with ignition on its not lighting up, nor is it lighting up with the engine running.

So something is wrong there I think.

[gotoPzero]

OK so D+ to ground and the battery lamp lights up on the dash.

So I assume the circuit from the battery > dash > alternator is ok?

However the battery light does not light up at any other time.

Is it possible its a relay? There are only 2 relays in the fuse box, one is defiantly for the glow plugs.

The other I dont know.

[Polly Grigora]

gotoPzero said:

OK so D+ to ground and the battery lamp lights up on the dash.

So I assume the circuit from the battery > dash > alternator is ok?

However the battery light does not light up at any other time.

Yes warning light circuit has now been proven to be good

Warning light not lighting up when connected to the alternator points to the alternator not being to earth (assuming new alternator is good)

Is the alternator rubber mounted and its earth lead from engine block to alternator body or mounting bolt not connected or faulty?

If the warning light cable is definitely connected to the correct terminal the warning light will illuminate if the alternator has a good earth

If the warning light doesn't illuminate even though the alternator earth is good and it's connected correctly the alternator is faulty

[gotoPzero]

Thanks no its hard mounted.

If I put my multimeter on the +ve terminal and then meter out all around I get battery voltage - so I assume the earth is ok?

I think next step is to get the new alternator off and get it tested.

[gotoPzero]

Thanks for your help today btw!

[Polly Grigora]

gotoPzero said:

Thanks no its hard mounted.

If I put my multimeter on the +ve terminal and then meter out all around I get battery voltage - so I assume the earth is ok?

I think next step is to get the new alternator off and get it tested.

Mmmm, little chance of a bad earth then

Better be safe than sorry

The only possible problem with using a meter between battery + and alternator body is that a bad alternator earth could still show voltage but fail when loaded with the 5 watt battery warning light bulb

To be 100% sure

Ignition on, warning light connected, connect any length of cable you have handy between battery negative and alternator body

By any chance have you been supplied an alternator with a warning light terminal in a different place on the back of the alternator

Have you seen a D+ or 61 marked on the back of the alternator?

gotoPzero said:

Thanks for your help today btw!

No problem, glad to possibly be of some help, it's got me away from the war in Ukraine

Hope you get sorted

[gotoPzero]

Whats weird is the battery light is not coming on only when directly connected like we tried.

I will check the earth again in the morning - will also post a pic of the current set up.

Cheers!

[gotoPzero]

Here is the pic cheers

[gotoPzero]

Why do I suddenly get a feeling that black wire should be going to one of those top bolts?....

[finlo]

gotoPzero said:

Why do I suddenly get a feeling that black wire should be going to one of those top bolts?....

Would make no difference, the bottom ones maybe?

[Polly Grigora]

Ignore everything I've posted

Wasn't even close

Have never come across a Perkins engine with that type of alternator, all the ones I've been involved with had Lucas alternators

That alternator does look like it has a built in regulator

Can't see any terminal letters

If alternator has a built in regulator, 1 of the thin cables should be for the warning light and the other thin cable should be an ignition supply (check for this supply)

The thick black cable if it's earth (does look like an earth) should be connected to the earth stud marked E because a screw won't tighten up as tight as a nut and could work loose

Main + should be marked

Don't know if the following is correct for your alternator, would be best to contact the supplier

Marked IG (ignition) and L (warning light)

[gotoPzero]

Yes I think the top pin is ignition and the bottom lamp

If I put the bottom wire to ground the lamp comes on.

The earth - that should be to one of the top bolts? i.e top left?

Cheers

[Polly Grigora]

Earth is ok where it is for testing, will work loose over time if left there

Cheers

[gotoPzero]

Sorry its an Isuzu not perkins..

The top of the 2 spade connectors is switched ign. So ign off = 0v. Turn ign on and its live.

The only weird thing is the voltage I am getting is about 11.5v at that spade when I turn the ign on. The battery voltage is 12.5v currently.

Not sure why there is a drop?

The bottom spade is battery light and the light lights up on the dash if you earth that spade.

The light is still not lighting up though.

I am going to ask for the wiring diagram I think thats all I can do now!

[Polly Grigora]

gotoPzero said:

Sorry its an Isuzu not perkins..

ok then


11.5 volts should still be good enough, even though there is a volt drop somewhere

You could connect battery + to the ignition terminal to possibly prove me wrong though

Everything points to it being wired correctly

Edited by Polly Grigora on Thursday 7th April 10:34