Ignition light blinking at some RPM

I would check all the connections are good and clean as a starting point

Cleaned all connections in the charging circuit. (cleaned the connector blocks under the fuses while I was in the area) Checked continuity of the wiring between the charging components as well. Voltage (and resistance) between the battery +ve and the engine is basically nil. I had cleaned the earth strap a couple months ago chasing some other problem so that's still good.



Connected an analog multimeter set to AC voltage to the wire to the regulator, and the needle twitched in time with the flashing light. This is with one lead on earth and the other just touching the wire without disconnecting it. It also jumps similarly on the brown-yellow input wire to the 3AW, thus the warning light's flashing.



So, I opened up the alternator. It was uncooperative (or I was impatient soaking the bolts) and I broke a rusty bolt off in the housing. So, I took both alternators apart to use the end cover from the spare. (Remember they both behaved the same except one being noisier) One of the brushes was indeed short so I put in a brush from the spare. The winding measured 4.0ohms which is about right in the manual. My digital multimeter has a diode setting. It gave very similar readings for all six, and only gave readings in one direction for each.


Reassembled and much cleaner, I put it back in the car but the problem is still there. Hmmm...

If I recall correctly, the AC voltage on the lead to the regulator when disconnected (the 3-pin connector at the 4TR unplugged) did not jump around, and the warning light was on solid. I can check again this evening and check with all the electrics on then too.

Hi Ben
From recent experence with several strange electrical issues with my car I would say recheck the earthing. The main points to go over carefully are the two earth points in the engine bay where the connections for the front lights are. On my car there is one near the battery (the main one) and one in a similar spot the other side. Both were cleaned carefully on my car and did not fix the issue. It turned out to be the connectors where the wire had virtually corroded off the loop lugs but was heald on to the lug with the crimp to the plastic insulation! Not easy to see. Looked ok until it was wiggled around a bit then it fell off! New lug and bingo all fixed! Another area where the same occured was the fire wall plugs. This is where the engine bay wiring connects to the inside wiring. Same deal looked ok and got cleaned but corroded strands of copper that no longer made good contact with the lug was the issue. Wire soldered back on to the lug fixed it.

Note that this is not from direct experience of your type of charging circuit but from thinking about what a grounding issue might do to a 3AW. I think looking at the 3AW wiring it is a little heater and a bi-metalic strip. What could be happening with a bad ground is the bulb is causing the current to flow in the 3AW heating the bi-metalic which switches off the bulb so it cools again and the bulb comes back on reheats switches back off etc etc. This would happen at a higher engine speed due to the resistance in the bulb where the normal grounding path is bad. Not sure if I have explaned the process well enough and I could stand corrected on how the 3AW should operate!
Good luck.
Nick

I agree with everyone's thoughts - electrical problems on cars can be very frustrating and appear illogical due to interactions between components, wiring, earths and connectors.

What's really confusing is that you have the same problem after swapping everything. That means either each have the same fault or it's in the wiring. As you have checked the wiring and connectors that means further checks on the components.

Having determined that the voltage doesn't jump around with the 4TR and 3AW disconnected and it does with the 4TR connected and 3AW disconnected that points to an issue with the 4TR (or wiring).

I suggest the following:

Double check all wires and connectors especially power and earth.
 
I think it would be helpful if you could check the voltage at the battery as well as from the voltmeter you fitted inside the car to get an accurate picture less affected by other part of the wiring. If the voltage fluctuates there at the same time the warning light comes on then check the voltage to the alternator field coil to see if the 4TR is causing the issue. If it is, disconnect the 3AW warning light module to see if the voltage still jumps around. If it does that points to the 4TR being at fault.

I've found this circuit for the 4TR diagram. There is nothing immediately obvious that would cause pulsing of the field coil at certain rpm ie a particular alternator output voltage. The most likely internal fault would probably be the wiper terminal of the pot (R2),like a volume control on a radio gone noisy. If you adjust the pot it will change the voltage and may improve the contact. Just turning it forward and back may be all that's required followed by a spray of contact cleaner.

The other possibly suspect item is the zener diode. As the voltage from the alternator increases to the desired output it switches off the current to the field coil for a short period and then the circuit switches it back on again to stabilise the voltage and does this continuously. In my experience zeners usually fail short circuit but who knows with one this vintage? The circuit design is quite primitive as zener diodes especially of this vintage often don't have a clean distinction between being on and off at the transition point, maybe an issue as it ages? As the unit appear to function other than a certain rpm that suggests the other components are probably OK.

The circuit itself is quite simple and it looks as though individual components are replaceable at quite a low cost. If you can get to the back of the pcb it's worth checking all the solder joints as some of the components look as though they could vibrate and cause fatigue. I think you could find all you need in Canada at RS components, Farnell (Newark, Element 14), Mouser etc if needed. You don't need to be an expert with a soldering iron with good old style components like these.

Not very impressed from the pic of the 4TR internals, the capacitors seem to be marked 12volts,not really high enough to cope with inductive pulses, I would suggest a minimum of 25V. In fact you may notice that the photo of the board and the circuit diagram do not agree (eg the circuit shows 2 transistors and the photo only one). It would probably mean reverse engineering what you have in your actual unit, but it is simple and a single sided board so it shouldn't be too hard).
 

David

David, thanks for the interesting document in your last post. The 'diagram' link in the earlier post doesn't bring me to a diagram though. Not really that confident in my individual electronic component repair abilities... I should also admit that as this problem has been so tricky to solve, I'm starting to lean toward converting to negative earth and using the known good 17ACR I have on hand.

I have one older 4TR regulator (which I drilled the rivits to have a look inside) with big components soldered on a simple board and a pot for adjusting. and I have another probably newer 4TR which looks the same from the front but with a blank metal cover and no pot on the back side. Unfortunately they both behave the same wrt my voltage fluctuations!

Work has taken me out of town for a few days so I won't be able to test anything until the end of the week.

I made the switch to negative earth with a more modern alternator and have steady 14.3V when charging. So that's sorted out my charging issue for now. I haven't made any permanent changes to the wiring, so if I ever get a good 4TR I can consider putting things back and rejoining the Positive Earth Society.



As you predicted, something will now need to be done about the blower fan direction, but I'll chronicle that in another thread. (and when I get around to it!)