Discuss Alternator failing to produce power until revs rise in the Auto Electrician Forum area at ElectriciansForums.net

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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:

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:
 
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.
 
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 .
 
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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.....
 
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.
 
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 .
 
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).
 
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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
 
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
 
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.
 
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