Generator control unit fault, repair help sought please!

[Chris Wilson]

Hi all, I have  a question with regard to a commercial diesel generator control panel that was pre fitted to a 20Kw single phase gennie I bought as a none runner and rebuilt the engine on.

The control unit gave a few issues but it had been damp and re-seating the socketed MCU sorted all but one out.

It has a safety function in so far as if the (car type) charging alternator fails it can optionally shut down the engine. This means if the fan belt fails it doesn't overheat as the belt also drives the water pump. I'd like this fully functional but alas it seems faulty.

Basically I envisage this is what SHOULD happen. The Paris - Rhone alternator has two terminals. B+ and D+      B+ is the heavy lead direct to the 12 volt battery, that charges it. D+, sometimes call the WL, warning light terminal, is the terminal by which the alternator is initially excited via the dashboard warning light feed on older cars. I *THINK* the 47 Ohm 6 Watt resistor shown in the schematics below mimics this bulb.

Then some magic circuitry measures the voltage on this wire, and if it's at resting battery volts (about 12.3V) the charge warning LED should illuminate. Once the alternator is running the WL terminal sees about 13.8 or more volts and the LED should go out unless the alternator / belt fails. Then the control pane lhas the switchable option of either just showing the warning LED, or both showing the LED *AND* shutting the engine off.

The LED does not illuminate with the "ignition" on, it seems to take its feed from the 12V from the fuel pump relay via the 47 Ohm, 6 Watt resistor. If I measure the voltage on the WL wire at either the control panel connector or at the WL terminal on the alternator, to ground, with the fuel pump activated ("ignition" on), I see about 1.8 Volts.

With the WL wire to the alternator disconnected from the alternator terminal I see battery resting voltage on the wire, about 12.3 Volts. Sometimes connecting the WL wire to ground multiple times will bring the LED on. Thee wire itself is fine, as are the spade and eyelet terminals, I see the same random illumination grounding at the control panel terminal itself.


Does the highlighted BC557  TR1 circuitry form a voltage divider and changes the state of TR1 according to it's base voltage level? How might I test if this is working OK please?

After this it seems the microprocessor decides what happens,  in accordance with a dip switch that selects warning LED only, or LED plus engine shutdown, via a "Timer" output pin on the microprocessor. There's also a signal from the processor to a multiplexer chip...

I think I should start with the TR1 area... Can anyone be so kind as to verify if I vaguely  understand the TR1 stuff and how I might test it please? Any other places of interest to check?

Thanks and a very happy, healthy and prosperous 2024 to all members.

The files should be clickable / expandable thumbnails and a couple of .pdf's

[p.larner]

is this mechanical fuel injection or common rail?,ie has it an hp injection pump+ecu or just a pump?

[Chris Wilson]

It's a 3 cylinder N/A Perkins with a basic engine driven mechanical metering unit, no electronic engine control.

[Chris Wilson]

I should have mentioned that there is a diode inserted in the wire from the warning light terminal of the alternator to the control unit that is something the control unit makers added but is not documented save in the schematic of the meter and relay box below.

[indeterminate]

Is TR1 turning on/off
Take sum voltage readings around tr1
it could be a bad cap or diode / transistor
or one pin on the cpu that still has a bad contact.

[Chris Wilson]

Is TR1 turning on/off
Take sum voltage readings around tr1
it could be a bad cap or diode / transistor
or one pin on the cpu that still has a bad contact.

Firstly, the important things, happy New Year and I am very grateful for you responding at this busy time!

I will take some voltage measurements around TR1 as you suggest and see what changes when the WL terminal on the charging alternator is connected and disconnected.  Any idea while the seemingly afterthought diode external to the control unit module was added?

Very much appreciated, all the best to you.

[indeterminate]

To compensate for the voltage drop across D25 witch lives between bat and the 47 ohm resister.

[BrokenYugo]

All ground connections good? Chassis to batt, batt to block, alternator to block, controller grounds, etc. common cause of all sorts of weirdness.

Big resistor probably is there to mimic the bulb, they pretty much all work and wire up the same way. You might sometimes also have a second "sense" control wire to give the option of the internal regulator reading the voltage from a place besides the output post (useful for multi battery charging though diodes, long battery wires, etc.), but that's it.

[Chris Wilson]

Thanks again for the help.  The issue was the generator needed to be running and making power with the contactor energised. It is hard coded in the MCU and testing statically gave erroneous results. I rang the makers and they put me straight. Sorry to have wasted anyone's time, I assumed testing without it (the main 22kVA alternator) making power would be OK...

With it running pulling the WL control panel terminal low, (to ground), killed the fuel pump and flagged a charging fault as it should.  Duh....