CIL on Mercruiser with Mando Style Alternator

[pgbj]

Hi
I have a 1999 Mercruiser D-Tronic 4.2l diesel. It has a Mando style alternator with the output terminal, an earth and a sensing and excitation circuit. The sensing circuit goes to the battery; the excitation circuit goes to the ignition switch. The alternator works fine and passes all the tests in the Merc service manual.

However the charge indicator light does not work - always on - and I can't get my head around the wiring?

The light is connected to the purple Run wire in the instrumentation and then to a brown wire which is connected to the P terminal on the alternator. The P wire is described variously as PULSE or AC TAP. When running it shows, 7v DC and 7v AC - so it could be a pulsed positive waveform intended for a tach (not needed on my installation). So how is this supposed to control a Charge Indicator Light??

I have two wiring diagrams for early and late models and they both show this connection to the P terminal, so it was clearly intended.
As I have a separate Excite circuit, I dont need the light to provide current to the rotor to start the alternator.

The received wisdom on the internet is that the L2 terminal is the Light indicator on a Mando. However, the Merc service manual says this is a tie strap for testing the regulator output to the rotor and should be between 1.3 and 2.5v (which it is).

The light is a new bulb - I checked it was per the original Ansii 194 14v 027ma. So should be electrically similar -though perhaps lights at a lower voltage?

Anyone seen this wiring config on a Mando? Any insights?

(I have a voltmeter gauge so I can just disconnect it....)
Thanks

 

[Grubble]

Where does the other side of the alternator light go and have you checked the voltage on that side?

 

[pgbj]

As I said it goes to purple on the ignition switch (via a general reverse current protection diode)...

With key in RUN and engine not started, battery voltage less a diode loss, so about 11.8v, on the purple side; 0v nearly on brown side - hence light is on.
With key in RUN and engine running, alternator voltage less a diode loss, so about 14.2v on the purple side; 7.7v on brown side (with also 7.2v AC showing on my multimeter).

Presumably enough potential difference to light the bulb with current flowing back through the stator.
But how was this wiring ever supposed to work?

 

[Grubble]

If 7.2VAC is the correct output voltage from the P wire then the only explanation is as you suspect - you need a bulb that doesn't light when it only has about 7v across it.

I think I would also check that the diode is working correctly.

 

[pgbj]

My multimeter says 7v DC and 7v AC - it's a cheap multimeter so that won't be true RMS but it is AC coupled - but suggests it is a signal oscillating around 7v probably from 0v to 14v (would make sense for a tach output). So when it is 0v there is 14v across the bulb momentarilly,...

As I say, I cant see how in theory it was supposed to work.

(I have checked the diode in the instrumentation harness.)

Grubble, thanks for your thoughts. I might try putting a resistor in series to drop the voltage.

 

[paulbiom]

Apologies first of all for the thread resurrection but I've searched the internet high and low and you're the only person who seems to have found a similar problem to mine with the D-Tronic engine. I have the 2.8L version, year 2000. Older style harness.

Did you ever get your problem resolved or make any sense of the wiring for the charge indicator? I've got a mechanic doing the heavy work for me, not fit to get in the engine bay at the moment, but between us we're really struggling. Brand new alternator, all checks out OK, performs great. When brown charge indicator wire is connected to P terminal get approx 7.8 volts DC with engine running, 0 when off. Mechanic insists that this is not right (which would match your description of the warning light not going out above). So instead my brown has been wired to L2 which is what that terminal is designed for according to the alternator spec and the mechanic. Engine running, brown wire has 14 volts, 0 when off. Purple side has 13 volts give or take. With this setup the charging indicator works perfectly ,except you now can't switch the engine off. There is a 5v backflow from the brown wire, just enough to keep everything running, the only solution is to pull the brown wire or use the alarm test to drop the voltage.

I can't get my head around how 13-14v on the purple side - 7.8v on the brown side is supposed to turn off the charge indicator? Yet it seems that's how the diode has been setup, to stop a backflow of around 7.8v. To make matters worse the 2 of us can't find the diode in the instrumentation harness for love nor money? Do get a 1v drop on the purple side and only 5v backflow so assuming it has to be there.

Short of actually being able to find that diode and replacing it with something more beefy I'm thinking the best solution is to put a zener diode on the brown wire to get around 10v. Hoping this will be enough to have the charge indicator go out whilst at the same time be a low enough backflow that the engine stops. It still doesn't explain how the original wiring in the service manual from Mercruiser is supposed to work?

 

[pgbj]

In short never fully got to the bottom of it. I concluded that the OEM alternator P terminal was wired different to the 2x aftermarket Mando-style alternators I had tried.

On the aftermarket alternators, P was a pulsed or AC output of the alternator i.e. prior to the rectifier. So, depending on your multimeter, looks like 7v DC but is 0-14v AC. On the OEM alternator, in the Service Manual, P is referred to as "AC output" and is connected to the "charge indicator lead (Brown)". I confirmed that the wiring on my from-factory boat was exactly as per the manual. However it made no sense to me how it could work unless the OEM alternator had repurposed P to be say 14v or 0v signal. On the D-tronic 4.2l the brown lead is used for the charge light and as the positive side of the oil pressure buzzer - and in later models to suppress energising the starter motor when the engine is running.

L2 on both the OEM and aftermarket alternators was definitely the output of the voltage regulator i.e the excitation circuit output current for the rotor. The Service Manual gives a test procedure where L2 should be between 1.3-2.5v with ignition on but not running. This was the case on all the alternators I tried. So not a conventional "L" terminal either.

In the end I gave up on the charge light and rewired the oil pressure buzzer so that it got 12v direct rather than from P. This means the buzzer sounds when ignition is on but the engine is not running - like on many boats, whereas before it would only sound if the engine was running.

Wasted several days of a summer on this! I almost thought of sourcing the original Mercruiser alternator but it was way too expensive and not generally available compared to the aftermarket "equivalent" (or not so!). Just wanted to get on the water!

I did find the diode - sort of embedded in the instrument harness wiring. It tested fine. (Sold the boat now so cant be clearer than that)

 

[paulbiom]

That's really helpful thanks. All of a sudden the non standard wiring on mine (24v buzzers, diode on excite, all sorts of crazyness in the dash) starts to make a little more sense. The alternator I've just replaced was an aftermarket as well so I'm guessing whoever had it before discovered the same issue when they swapped out the OEM alternator and their solution was what I was left with. Also explains why the 2 alternators I've tested have performed exactly the same, never had access to an OEM. My plan and what prompted all this was to get everything back to factory so that should anything go wrong it would be easy to trace....guess not. Will look again for that diode and just double check all is right, but it sounds like it's just going to be a bodge job either way.

Thanks again for taking the time to reply, saved me more days playing when I should be out fishing