Alternator failing to produce power until revs rise

[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