Help needed to understand an alternator load problem

[nickboaz]

Hi all,

I am looking for some expert advice and knowledge to help get my head around an alternator load problem.

This has been on-going for about 18 months...

Vehicle summary;

1993 Mazda MX5 import.
Originally a 1.6 but I have put a 1.8 engine in from an early MK2 MX5.
I also fitted a UK spec battery at the same time.
The alternator is a used early mk1 1.8.
Engine harness and chassis harness essentially all original.
From all the MX5 knowledge I can harness, this should "work", and is certainly not an uncommon swap.

The symptoms;
Squealing alternator belt, and the alternator pulley gets very hot very quickly. If driven, the belt snaps in about 4-5 miles and displays signs of having been at high temperature.
If I disconnect the high tension cable from the alternator the problem goes away.
If I only disconnect the exciter wire and the charge light wire, the problem persists.

What I've done;
Started with belt tension - nothing made any difference.

Fitted brand new alternator. Still the same problem. Fitted another new alternator, and still the same.

I fitted another new battery. Initially this seemed to fix it, however quite quickly the symptoms returned.

I have checked the basics on the wiring - high resistance and shorts, continuity, and no obvious faults. I haven't touched any of the loom that might make me think there's an issue.

At this point, I don't have a good understanding of the loads and charge currents etc., hence coming on here.

The alternator is rated at 65A, and the battery is 425CCA and 46Ah.

My thinking is that too much is being asked of the alternator, and thus it is being retarded by excessive load, causing the belt to slip and get hot, and snap.

Please can anyone help me understand the load and rating relationship between the alternator and the battery? I feel convinced that there is a mis-match between the two parts. Or perhaps not?!

Thanks in advance.

Nick.

 

[static zap]

A picture of actual alternator terminals may help .
(Does Ignition lamp behave as expected .. coming back on when belt shreds)
...Some fancy alternators have a sense wire .. That if its not seeing the correct voltage will cause alternator to over work it's self.

 

[Wilko]

Hi - it sounds like the alternator is working into too much load. Can you confirm the battery voltages (ignition off, on, cranking, running, idle) and charging currents? In the olden days the regulator would fail and the battery would end up being cooked or not charged. I haven't seen an alternator break a fanbelt break in this way.

 

[LankyWill]

You dont have to worry about the CCA as this wont effect the alternator its just a reflection on the capacity, the higher the CCA the longer it takes to charge. Your Alternator has a bigger output than your battery 65A-46A, it could be the voltage regulator thats knackered, or it could be mechanical. If the bearing is loose it can cause overheating which in turn damages the alternator, have you tested the running load?

 

[PEG]

Hi,if you have changed alternators,and the fault persists,it would seem your vehicle has an excessive load on the charging circuit. This may or may not,be immediate,and may cycle in after initial start up.

You need to measure the current to and from,your battery and accessories. A DC clamp meter would be useful,for this.

It is not uncommon for the belt breaking/heating,under such circumstances,although other more serious damage could occur,if not rectified.

Although a bit agricultural,you could start with seeing how much load is being drawn,at ignition position of key. You could do this with no meter,by holding a blade type fuse,between the disconnected positive clamp,and the battery post. Get someone to turn the key to ignition on (not start) and see what size fuse will hold...if a 30A pops...you may have found a clue

 

[telectrix]

Hi,if you have changed alternators,and the fault persists,it would seem your vehicle has an excessive load on the charging circuit. This may or may not,be immediate,and may cycle in after initial start up.

You need to measure the current to and from,your battery and accessories. A DC clamp meter would be useful,for this.

It is not uncommon for the belt breaking/heating,under such circumstances,although other more serious damage could occur,if not rectified.

Although a bit agricultural,you could start with seeing how much load is being drawn,at ignition position of key. You could do this with no meter,by holding a blade type fuse,between the disconnected positive clamp,and the battery post. Get someone to turn the key to ignition on (not start) and see what size fuse will hold...if a 30A pops...you may have found a clue

a new twist on the bang test , then?

 

[PEG]

a new twist on the bang test , then?

Yep,but more of a "click" than a "bang"

A blowing,blade type fuse,on a vehicle,should make a sound,not dissimilar to a dry lolly stick,snapping. It is a sound all DIY car radio fitters,should be familiar with...

 

[Deleted member 26818]

Is the alternator pulley in line with the other pulleys?
Ave you tried pouring water over the belt while the engine is running and the belt is squealing?

 

[davesparks]

You dont have to worry about the CCA as this wont effect the alternator its just a reflection on the capacity, the higher the CCA the longer it takes to charge. Your Alternator has a bigger output than your battery 65A-46A,

CCA is cold cranking amps and reflects the amount of current the battery can deliver for the short period of time it takes to start the engine, not the capacity of the battery.
The 46Ah quoted is effectively the capacity of the battery not its output current, it means the battery can supply 46A for 1 hour (or 23A for 2hours etc) so that figure being less than the alternator output current doesn’t tell you much.

 

[ruston]

As @spinlondon has alluded to, are the pulleys all free, are there any idlers in the line that are worn ? Is the water pump on the same pulley system as the the alternator belt?
You need to make sure everything is running free as well as checking the suggestions already given.

 

[nickboaz]

A picture of actual alternator terminals may help .
(Does Ignition lamp behave as expected .. coming back on when belt shreds)
...Some fancy alternators have a sense wire .. That if its not seeing the correct voltage will cause alternator to over work it's self.

The ignition light works okay.
It does have a sense wire - ie a wire straight back to the battery. I have checked continuity and resistance, all okay. I ran an alternate wire too and it was the same.

 

[nickboaz]

CCA is cold cranking amps and reflects the amount of current the battery can deliver for the short period of time it takes to start the engine, not the capacity of the battery.
The 46Ah quoted is effectively the capacity of the battery not its output current, it means the battery can supply 46A for 1 hour (or 23A for 2hours etc) so that figure being less than the alternator output current doesn’t tell you much.

Thanks. Good to know.

 

[nickboaz]

Is the alternator pulley in line with the other pulleys?
Ave you tried pouring water over the belt while the engine is running and the belt is squealing?

Checked all that. It's so bad that silicone spray or even wd40 doesn't quieten it down.

 

[nickboaz]

As @spinlondon has alluded to, are the pulleys all free, are there any idlers in the line that are worn ? Is the water pump on the same pulley system as the the alternator belt?
You need to make sure everything is running free as well as checking the suggestions already given.

Yep, been through all this, and all good.

 

[nickboaz]

Hi,if you have changed alternators,and the fault persists,it would seem your vehicle has an excessive load on the charging circuit. This may or may not,be immediate,and may cycle in after initial start up.

You need to measure the current to and from,your battery and accessories. A DC clamp meter would be useful,for this.

It is not uncommon for the belt breaking/heating,under such circumstances,although other more serious damage could occur,if not rectified.

Although a bit agricultural,you could start with seeing how much load is being drawn,at ignition position of key. You could do this with no meter,by holding a blade type fuse,between the disconnected positive clamp,and the battery post. Get someone to turn the key to ignition on (not start) and see what size fuse will hold...if a 30A pops...you may have found a clue

I think this is getting down to business!
I'm not sure what current I should expect on the charging circuit under normal conditions, ie at idle with a good alternator and battery. Any thoughts?

I can test current no problem with the engine and ignition off. In fact I have, and there is 0A as expected.

So, if I put my ammeter (10A, no fuse... she's a beaut!) onto the battery positive side to measure the current with the engine running, I shouldn't expect to see much more than with the engine off but the ignition on? Correct? If I see a huge draw, then something is definitely amiss.

 

[nickboaz]

Thanks everyone for your comments and suggestions!

 

[Wilko]

... So, if I put my ammeter (10A, no fuse... she's a beaut!) onto the battery positive side to measure the current with the engine running, I shouldn't expect to see much more than with the engine off but the ignition on? Correct? If I see a huge draw, then something is definitely amiss.

Hi - Don't mean to teach you how to suck eggs, but if you put a 10A meter in series with the battery it will likely be fried when you start the motor. You need the clip on sort that can measure DC amps. They look like this -

 

[static zap]

Resting and charging voltages may well be clues .
resting 12.2 .. 12.8 ..even 13 if recently charged ..
Should be charging at 14.0 - 14.4 V typical ..
(if 16 -18 then that is cause attempting to over charge --and probably harming battery with lots of extra current )
.... Most car currents (ignition coils / Jammed-electric windows ) will be too much for your 10A range ... (especially charging current !)

 

[PEG]

Yep,there are methods of using that Tandy meter,even with it's 10A safety valvebut,too involved for explanation,here.

You really need a clamp or clip type,which will give you current magnitude and direction,and are available fairly cheaply. I have an ancient Durite clip-on,which was my go-to jobby,as a kid...

You need to know size,and timing of any load,which may not be present,until a period after starting,but maybe not at "ignition on"position.

I have very recently had such an issue,on a large,modern horsebox,where the belt was being flung off,soon after starting,but not on every start-up.

This was due to a PITA split-charge relay,for the leisure batteries charge circuit,and the fact that the huge,18 tonne wagon's factory fitted alternator,was only 45A maximum output.

I feel a load heavy enough to tax your decent sized alternator,would not be hard to find

 

[nickboaz]

Hi - Don't mean to teach you how to suck eggs, but if you put a 10A meter in series with the battery it will likely be fried when you start the motor. You need the clip on sort that can measure DC amps. They look like this -

View attachment 43377

Thanks. Yeah, I wouldn't try to start it on a 10A meter! In the past I have put croc clips on the terminal and lead, then slid the lead off the terminal.