Alternator failing to produce power until revs rise

[Heathydee]

Hi people . My first post .
I have a 1989 turbo diesel Landcruiser with a 24 volt system . The alternator , for many years has needed to be spun up before it began to produce power and the dash lights extinguished . About 1250 engine rpm or three times that for the alternator . Recently it would not work at all so I pulled it off and gave it to the auto electrician . The regulator , brushes , bearings and the vacuum pump seal were replaced and I re-fitted the unit to the vehicle .
The alternator exhibited the same problem but now needs nearly 3000 engine rpm before it triggers and produces power .
Three wires to the alternator through the plug plus the main battery connection . The two 24v connections in the plug have power . The third is for the dash lights .
I pulled the unit off again and it bench tested fine . The auto elec. is a guy with a lot of experience and he is baffled . As am I .
Hoping someone can help

Heath

 

[ruston]

Has the auto electrician seen the vehicle and installation? If not
what kind of condition are the terminating cables in, and the earthing connections to the body? Are the batteries in good condition?
Is the pulley in good condition along with the belt , and not slipping .
Also are the battery connections to positive and negative in good condition to their respective terminations?
Assuming the alternator is ok as you have faith in the Auto Electrician, the problem seems to be one of delivery to the system or battery, unless this vehicle has external alternator controls that I am not aware of ( the auto spark would know that anyway). My memory is not great and I have not even seen one of those for years.
I think @PEG is an expert on these.
[automerge]1591438025[/automerge]

Has the auto electrician seen the vehicle and installation? If not
what kind of condition are the terminating cables in, and the earthing connections to the body? Are the batteries in good condition?
Is the pulley in good condition along with the belt , and not slipping .
Also are the battery connections to positive and negative in good condition to their respective terminations?
Assuming the alternator is ok as you have faith in the Auto Electrician, the problem seems to be one of delivery to the system or battery, unless this vehicle has external alternator controls that I am not aware of ( the auto spark would know that anyway). My memory is not great and I have not even seen one of those for years.
I think @PEG is an expert on these.

Ps . It may have a free-wheeling alternator pulley that is goosed.

 

[Heathydee]

The auto lec has seen the installation . Cables good . The batteries are good . The earth between the alternator and the vehicle shows no resistance .
I thought maybe that the current through the connecting plug may be passing through a dodgy relay so I wired direct from the batteries to test that idea . No change . Direct drive pulley .

 

[pc1966]

Did they check the bridge rectifier assembly? Was it new when you got it?

Typically the alternator relies on residual magnetism in the rotor (and/or the current from the indicator bulb) to get the stator voltage up when running in a sort of positive feedback arrangement as once the stator is generating power it feeds the rotor to fully magnetise it (at least, until the regulator pulls it back to keep the output voltage at an appropriate level).

It might seem like a dumb question, but is it a 12V alternator that has had a 24V regulator fitted?

 

[ruston]

@pc1966 this is what i started to type lol.
Mmm- Is the wire and lamp holder and the lamp in good condition and well connected?

 

[R-fur]

Is this an original installation? If it is not have the pulley ratios been checked. It sounds like the alternator is not spinning fast enough at tick over.

 

[EricMark]

Put in a larger warning lamp bulb. With some alternators there was a resistor in parallel with the bulb, may be that resistor is open circuit. It uses the current through warning light bulb to kick it all off.

 

[PEG]

Hi,i will do my best,but i have had a lot more Nissans,as the Toyota versions were much dearer and no better in the gloop

What version is this,a HJ60/61? from what i can remember,the alternators were fairly low powered,with the lowest power regulated externally. There was also an ignition main relay hidden away,which could be a source of fun.

Without knowing the exact system,it's all a bit guess-worthy,but if it has original gear,and has always needed a good rev the initiate charge,i would check the wattage/connections,of that charge lamp.

Your auto electrician would have had to do this,to check charge/no charge and output,to determine alternator was fine. A quick measure of crank and alternator pulley,would give you the alternator rpm V engine rpm,and therefore charge initiation speed.

These are pretty robust and straightforward machines,i would be surprised if it was a rocket science fault

 

[Heathydee]

Thanks for the replies people . Yes, the alternator is original installation .The pulley ratios look about 1 to 3 . Motor to alternator.
I will wire the alternator up directly using more resistance in the warning light circuit and get back to you .

HJ61 Peg. The vehicle was a grey import to Australia .I have had it since 2005 . The alternator is self contained with no external regulation 24 volt . 50 or 60 amps I think

 

[Heathydee]

Thanks very much people
I tried putting a resistor in series with the charge light wire with no result . The lights stayed on until the engine hit 3000 rpm .
I next wired up the alternator direct from the batteries with the plug out of the equation . An 8 ohm resistor used to simulate the charge lights . The alternator began to produce power on startup . So , I am thinking power is not reaching the alternator through the charge light wire . The charge lights must be a parallel circuit . Putting current to it extinguishes the charge lights .
Somewhere a relay , whose job it is to send power to the alternator through the charge light wire is not functioning . Finding that and finding a replacement is going to be fun so , in the interim , I will splice power to the wire through a momentary switch .

 

[pc1966]

I tried putting a resistor in series with the charge light wire with no result . The lights stayed on until the engine hit 3000 rpm .

You should be trying one in parallel with the light to increase current!

 

[PEG]

Yep,from what you have described,it sounds like the charge indicator lamp is not heavy enough. Try an independently powered lamp of 2 to 4 watts,see if this kicks it in to life.

The ignition relay is in one of the usual places,if this is faulty,and i would seek it out,before i bypassed anything.

I vaguely remember one or sometimes two fuses,on the charge circuit,not including the links on the heavier cables.

Find the problem,and it will not only make it more reliable,but greatly simplify any future diagnosis.

 

[ruston]

Yep,from what you have described,it sounds like the charge indicator lamp is not heavy enough. Try an independently powered lamp of 2 to 4 watts,see if this kicks it in to life.

The ignition relay is in one of the usual places,if this is faulty,and i would seek it out,before i bypassed anything.

I vaguely remember one or sometimes two fuses,on the charge circuit,not including the links on the heavier cables.

Find the problem,and it will not only make it more reliable,but greatly simplify any future diagnosis.

I agree with you , it is obviously a cherished vehicle and just getting it working, as to the satisfaction of getting it back to working, and in original condition.

 

[Heathydee]

I will do as suggested guys .
Now I have a logic trail to follow knowing that the fault has nothing to do with the alternator .

 

[ruston]

Good luck mate , and I hope you enjoy it for many years to come.
They are one of a kind.

 

[Heathydee]

I got hold of the relevant wiring diagram and have found the Charge Light Relay which appears to be working .

 

[pc1966]

I got hold of the relevant wiring diagram and have found the Charge Light Relay which appears to be working .

That is interesting and a bit more complicated than the older car alternators. It seems here the indicator lamp is simply that - it is not the source of rotor start-current, that goes vie the 'IG' line to the alternator.

Are you cable to measure the current on the IG line? Safest would be a low current DC clamp ammeter (most are AC only but an auto electrician might have one that goes down to under 1A).

 

[R-fur]

I agree with PC1966, another way if you cant get a DC clamp meter would be to put a 24v supply direct to the ign terminal, if this fixes the fault it means there is a resistance in the ign wire somewhere (a terminal or inside the relay) if it doesn't fix it, I would check the diode and resistor between the ign terminal and the rectifier inside the alternator.
Good luck

 

[PEG]

I got hold of the relevant wiring diagram and have found the Charge Light Relay which appears to be working .

OK,so the ignition relay and two fuses i remember,are on that diagram...identify and test both,and if the charge relay and wiring are ok,it should not be hard to find the culprit

 

[Heathydee]

After studying the circuit I think the problem lies after the IG connection enters the alternator . Either the diode or the resistor is open circuit . Current should move through the diode and resistor and the reduced voltage thereafter energises the rotor . This reduced voltage should be present and able to be measured in wire L which goes back to the ignition light relay and is blocked by the zener diodes . When the alternator begins to produce power the voltage rises in wire L , the zener diodes allow current through which activates the relay and cuts current to the warning lights .
There is no voltage at wire L when the ignition is on .
I have tapped into wire L using a momentary switch and the alternator produces power as soon as it it pressed .
I can see where there would be difficulty in identifying this problem up with a bench test .

 

[pc1966]

Can you identify the diode / resistor in the alternator, or is that all just a sealed block?

Your cheapest solution might be to put something like a 100R 10W resistor, typically metal clad and mounted on some sheet metal as a heatsink, and a 1-3A rectifier externally between the IG and L lines to essentially "parallel" the internal diode/resistor which presumably have an open fault.

This sort of idea:

Arcol, 100Ω 10W Wire Wound Chassis Mount Resistor HS10 100R J ±5% | RS

uk.rs-online.com

You could use a 5W 24V bulb instead, but then you ought to make the rectifier 3A to cope with switch-on surges. Cost difference is minor, something like this would do:

onsemi 200V 3A, Rectifier Diode, 2-Pin DO-201AD 1N5402G | RS

uk.rs-online.com

 

[Heathydee]

I will talk to the auto lec tomorrow about the internal configuration of the alternator . I didn't pull it to bits so have no idea of it's construction

 

[pc1966]

They might know, or at least could give to a price for replacing the module for a "proper" repair after verifying if that really is the underlying fault.

 

[Heathydee]

I spoke to the autolec this morning . He said the rectifier assembly is one solid state unit and unavailable . The fix will have to parallel the existing circuit as pc1966 suggested , which can be easily done externally . There is an electronics store in town with loose components available . I will take the details on components supplied by pc1966 to the electronics store and see if he has them in stock .

 

[PEG]

I spoke to the autolec this morning . He said the rectifier assembly is one solid state unit and unavailable . The fix will have to parallel the existing circuit as pc1966 suggested , which can be easily done externally . There is an electronics store in town with loose components available . I will take the details on components supplied by pc1966 to the electronics store and see if he has them in stock .

Hi,is this the same autolec who bench tested it and confirmed the alternator was fine?

I would advise caution,on him jury-rigging an additional,external rectifier,if a straight forward bench test of the unit,eluded him

Traditionally,a heart transplant is a healthy organ,fitted where the failed one has been.....not added with a velcro-sticky,to the ribcage,leaving the poorly ticker,squeaking along,in situ.....

 

[Heathydee]

I would advise caution,on him jury-rigging an additional,external rectifier,if a straight forward bench test of the unit,eluded him

Good advice there Peg . How many times does it happen you pay good money to a "tradesman" and end up doing it yourself ? I could tell a story about a wheel alignment professional who put the castor correction wedge under the back of the axle instead of the front and had the vehicle steering like a shopping trolley . I learnt all about wheel alignments after that little episode . I do that in the driveway using string lines and a steel tape measure .
As far as the alternator goes , I am looking for a replacement rectifier unit but in the interim I have ordered a diode and resistor for the external fix and will do the work myself.

 

[pc1966]

Let us know if that fixes things.

 

[Heathydee]

Sure thing . I had to order the parts off Ebay . The electronics store is gone .....maybe a Covid Closure , so it will be a few days until they arrive .
Interestingly , I applied 12v to wire "L" from the 12v sub-system I run in the truck , and it was enough to excite the rotor . The warning lights were not extinguished by this voltage .

 

[Lucien Nunes]

Be cautious about what you connect directly to the L terminal because this outputs full alternator voltage as soon as self-excitation is achieved. Drawing too much current from L (e.g. by allowing it to feed charging current back into the battery +ve) could burn-out the three field supply diodes in the rectifier. These are usually of much lower rating than the main +ve arm of the rectifier as the field only takes a couple of amps.

It's worth noting that an alternator with one or two field supply diodes open might generate OK but will require excess rpm to build up to self-excitation, since the average voltage supplied to the field is reduced. However, with this particular circuit using a voltage-sensing relay monitoring the L terminal, such a fault would probably also cause the charge warning light to stay on even while the main output terminal reaches normal voltage.

You mentioned that there was no voltage on the L wire (i.e. at the field winding positive end) when an ignition feed was present at IG, suggesting an open-circuit in the startup diode or resistor. If it was indeed zero volts that's fairly conclusive but terminal L might only be a couple of volts above ground when stationary, since the alternator requires a certain minimum field current to start up rather than a minimum voltage and much of the battery voltage is dropped across R. 100mA is often enough to trigger a 24V alt to build up at average revs, I note PC1966 has been a bit more generous with the 100 ohms to give it about 200mA (allowing for drop in the regulator pass transistor and diode and the battery having just started the engine).

 

[Heathydee]

Thanks for the advice guys .Keep it coming .I am learning as I go and having a bit of fun with it too. Electronics is a real gap in my knowledge . I am (was) a fitter-machinist/welder . With your help and Youtube I feel I'm improving my personal data base.
I understand now why flashing the "L" wire with 24v to excite the rotor is a bad idea

 

[Heathydee]

The parts arrived today and were installed . The warning lights extinguish and the alternator produces power at startup . There are 3.75 volts in wire "L" with the ignition on before the motor is started . The resisitor became warm to the the touch after a minute or two . I left it outside the wiring harness for better cooling . After startup there is 28v or so in wire "L" .
Thanks for the help people !

Heath

 

[pc1966]

The resisitor became warm to the the touch after a minute or two . I left it outside the wiring harness for better cooling .

If it is one of the aluminium types it should be screwed to a metal plate - any of the car bodywork will do (but grease any holes to minimise rust, not that landies are ever free from rust anyway...)

Otherwise it may get rather hot if ignition on and engine not running!

In fact any resistor doing that job will get hot, but the metal ones are easier to deal with and maintain below insulator-melting temperatures by fixing them to the chassis.

 

[Heathydee]

Thanks guys. All fixed with a fortnight's testing completed . The resister and diode were wired in externally with a heat sink attached . The ignition lights extinguish immediately on startup .
Good result