a puzzle for the elctronics guys

Birdseye

Birdseye

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I have been struggling for some time with a problem with the instrument panel on my volvo 2030. The problem is that the tacho flickers up and down by maybe 500 rpm and the alarm sounds intermittently with the voltage light flickering.

The alternator is putting out a steady 14.2 volts at the output terminals and is charging OK - I always thought that the voltage alarm compared alternator output volts with battery volts but the circuit is wired to compare volts at the output terminals with the output from 61 at the alternator back.

Anyway, to the wiring. Terminal 61 has two wires. One is a 12v positive feed via a 33ohm "excitation resistance " and the engine instrument panel on/off switch ( the ignition switch if you like) from the B+ terminal and the battery. This finishes life on one side of the potted indicator light / alarm unit. The other 61 wire runs directly to that potted indicator light / alarm unit. In which case how can the alarm unit be comparing the voltage at the B+ terminal and battery with the voltage at the 61 terminal since that resistance apart, they are connected together.

In any case, what fault can you get with an alternator that gives intermittent alarms but still charges as normal. Dont diodes either run or fail?

Lastly it seems to me that I have two problems. The tacho is fed off W and doesnt go near the diodes so a flickering tacho is unlikely to be an alternator issue and more likely to be wiring or the tacho itself.
 
D+/61 is the alternator field coil which needs to be excited from the battery to start generating.
In many systems the excitation feed is via the warning light, the other side of which goes to +. However if the bulb fails, the alternator won't (well it might but not reliably) start generating.
In the VP wiring, the excitation feed is via the resistor.
Because of the resistor in the excitation line which will have a voltage drop across it, you won't get 12V showing on D+ when the engine isn't running (or if you have a fault and the alternator isn't generating even though the engine is running), but something less. As the other side of the warning light is at battery voltage it will glow (but not at full brightness).
As the alternator starts to generate, the voltage at D+ will come up to system voltage and the light goes out.
There will then be no voltage across the excitation resistor either.
Does that answer the question?

I think the problem you have is most likely a bad contact somewhere.
 
Last edited:
birdseye said:
I have been struggling for some time with a problem with the instrument panel on my volvo 2030. The problem is that the tacho flickers up and down by maybe 500 rpm and the alarm sounds intermittently with the voltage light flickering.

The alternator is putting out a steady 14.2 volts at the output terminals and is charging OK - I always thought that the voltage alarm compared alternator output volts with battery volts but the circuit is wired to compare volts at the output terminals with the output from 61 at the alternator back.

Anyway, to the wiring. Terminal 61 has two wires. One is a 12v positive feed via a 33ohm "excitation resistance " and the engine instrument panel on/off switch ( the ignition switch if you like) from the B+ terminal and the battery. This finishes life on one side of the potted indicator light / alarm unit. The other 61 wire runs directly to that potted indicator light / alarm unit. In which case how can the alarm unit be comparing the voltage at the B+ terminal and battery with the voltage at the 61 terminal since that resistance apart, they are connected together.

In any case, what fault can you get with an alternator that gives intermittent alarms but still charges as normal. Dont diodes either run or fail?

Lastly it seems to me that I have two problems. The tacho is fed off W and doesnt go near the diodes so a flickering tacho is unlikely to be an alternator issue and more likely to be wiring or the tacho itself.
Click to expand...
All consistent with an intermittent or failed diode in the rectifier, or some other bad connection.
An alternator may well still give 14+ volts with one phase missing, just won't have much current available.
Check the obvious, e.g. brushes worn or stuck.
 
OK so far. As I see it, when I switch on the engine supply but dont yet start the engine, battery volts go directly up to one side of the alarm/ indicator lights system. The same volts are applied at one end of the excitation resistor but as current flows through the field coil, there is a voltage drop across the resistor and this is what fires the warning light and triggers the buzzer.

When the engine is running, why does the field coil voltage rise to alternator outrput voltage? Is there an internal connection?

If so, what would cause the voltage at the field coil D+ terminal to fluctuate ? I had a play with a voltmeter and I saw a voltage difference between 61 and B+ fluctuating between 0 and 3 or 4 volts. I am wondering if there is a voltage regulator issue since surely the regulator must adjust the field coil voltage to control the output voltage of the alternator? The alarm signals when they occurr seem to do so with some regulairty which suggests to me that it isnt a broken wire or bad joint. Whats more they are much more frequent at idle than higher up the rev range
 
lw395

I hate problems that I cant understand so thanks to you and others and some puzzling over the circuit diagram I have written this simple explanation. Let me know if it is right.

Inside an alternator there are two sets of coils, one fixed to the case called the stator and one rotating called either the field coils or the rotor. If you have a current flowing through the field coils, it generates a magnetic field within the alternator and when this field cuts across the wires of the stator, electricity is generated. In the case of an alternator, there often are three coils in the stator each one producing a voltage 120 degrees out of phase.

The output of these three coils goes via a set of diodes to the B+ and - terminals at the back of the alternator and hence to the battery. It isnt actually a constant 12 or 14 volts - there is a ripple in the voltage but its so fast that your meter wont detect it.

To start the alternator generating you need to have a current flowing through the field coils and so there is a wire to the coils from the battery, often via a resistor and the engine on/off switch. There also is an connection inside the alternator from the stator to the field coils but when starting up there is no voltage coming out of the stator so the feed from the battery is the one that matters. But as revs rise then so does the voltage from the stator to the point where this takes over from the battery feed.

The field coils are rotating and so there are carbon brushes to make a contact via a commutator. The voltage on these field coils controls the strength of the magnetic field produced, this in turn controls the amount of leccy put out by the stator and into the battery. You need to control this voltage output to avoid boiling the batteries so the brushes are part of a fancy bit of electronics called the voltage regulator. This looks at the output volts of the stator and controls the input volts of the field coils to keep output voltage where you want it to be. In the case of my alternator it does this not by lowering the positive voltage on the field coils but by raising the negative level above zero. After all, what matters is the voltage difference between the two ends of each field coil so 3v one end 12v the other is just the same as 0 volts one end 9 volts the other.

Your boat or car has a voltage alarm that glows red when the leccy is switched on but the engine hasnt started. The current to the field coils from the battery initially runs through the excitation resistor and this causes a voltage drop. Ohms law. It might be, for example, that when you switch on and the batteries are at 12v the voltage at the field coils is just 10 volts because of the resistor. Your alarm circuit looks at that voltage difference. When the engine is running and the alternator charging then the voltage at the field coils will have risen to the output voltage of the stator as I said earlier. So now there will be no voltage difference across the alarm system and therefore no alarm.

One final comment. The feed to the tacho from terminal W is just an alternating current feed from one of the stator windings and the tacho is simply counting the number of cycles per second

If my understanding is right then there are a number of possible causes for my problem, and I can assume that the tacho is likely a different issue.

Causes could be:

1/ voltage regulator intermittently failing / brushes etc.
2/ field diodes
3/ bad connection / broken wire between D+ and the alarms.
4/ bad internal connection between field diodes and 61 causing the alarm circuit to drop back onto the exciter resistance feed.

The one remaining puzzle is that it seems to me that there is a regularity in the alarm sounding which goies against bad connections making / breaking at random. Also, the alarm drops back to infrequent when revs are raised from idle to 1500 rpm.
Question is, what does this suggest to you about fault location?
 
I'd agree that description pretty much - just substitute "slip rings" for "commutator".

I would still look for a bad contact somewhere first. Beyond that I'm sorry I wouldn't care to localise. I've personally never encountered an intermittent diode as lw395 suggests but I'm not saying it can't happen. I would have thought the regulator more likely but can't see how that would affect the W output for the tacho.
 
I have had a tacho problem before - a sticky mechanism which was cured with a bit of silicone spray. So I am inclined to think that I have two unconnected problems. Got the panel at home today and will strip and lube the tacho. The plan then it to put it back on the boat and start the engine whilst monitoring the voltage output at the D/61 terminal. If that is fluctuating then its a remove the regulator job ( easy enough) to check brushes and if necessary replace the reg - its only ! ??? £ 30 or so. If the voltage isnt fluctating then I will run a seperate wire from the D/61 to the instrument panel bypassing the wiring loom, which should show if its a wiring issue

Make sense?
 
birdseye said:
It might be, for example, that when you switch on and the batteries are at 12v the voltage at the field coils is just 10 volts because of the resistor. Your alarm circuit looks at that voltage difference. When the engine is running and the alternator charging then the voltage at the field coils will have risen to the output voltage of the stator as I said earlier. So now there will be no voltage difference across the alarm system and therefore no alarm.
Click to expand...

I could well be wrong, but this part of your explanation sounds a bit over complicated to my ears.
In my understanding the "alarm system" is simply a lamp and a buzzer that are activated by current running (though I suppose your could call that comparing voltages).
Current will be running as long as the start switch (key switch) is on and the alternator is not putting out. When it does, the voltage at D+/61 will rise to battery level, current stops running, alarms deactivates.
I don't know why VP has a resistor inline, some other engines only use the lamp as a resistor, obviously there must be some kind of load or there would be a short. Perhaps VP uses a LED indicator on the panel?
 
The tacho, running from the W terminal, is fed raw ac from one of the three windings. If this is unstable, and alarms (which indicate a charging failure) occur at the same time, I would check the common earth path. This could be the chassis - clean the faces and tighten the bolts, or a nice fat cable if it's isolated.
Don't bother trying to analyse diodes and regulators, the problem looks to be before them - basic stator winding connections.
 
That makes sense Bill. Its the only common bit between the tacho and the voltage alarm, and thinking about it, the alarm does buzz at the same time as the tacho dips. However, the alarms seem to me to be regular intervals which doesnt tie in with a loose connection making and breaking. It also alarms much less as revs rise though the tacho flicker doesnt change , maybe even gets worse.
 
I think you're perhaps making it sound more like brushes now.
If a brush loses contact then (ignoring any minor effects of residual magnetism) you will lose all output, including from W for the alternator.
Brushes can stick in their holders.
 
That makes sense. At the engine end, only a short failure in the alternator could stop the tacho and trigger the voltage alarm at the same time and thats likely to be brushes or internal since an intermittent failure of the voltage regulator elkectronics wouldnt stop the self excitation or the initial excitation from the battery. But would such a problem with the brushes cause the tacho to drop back just by 400 or so rpm, which is what it seems to be doing ie it doesnt fall back to zero.

The other alternative is a break in the negative feed to the instrument panel. But then surely the tacho would again go to zero.
 
The rectifier is composed of 4 diodes. If one diode is shot, you will still get output from the alternator but only half the current, and the engine will need to run faster, to get to the correct output voltage.
 
sure tawhiri, but that doesnt account for the tacho movements.

The fact that its both tacho and alarm simultaneously says to me that its the alternator stopping charging - I have carefully checked out the onl;y common wire between engine and panel and that is OK. But what I hadnt realised until today was that a failure of thye voltage regulator could have the same effect. I had thought that the alternator being " self energising" meant that it would run without a regulator but of course it wont since if the regulator goes dead, there could be no surrent through the field coils.

I have checked the brushes and they are fine. So I am going to replace the regulator for £20 and if that doesnt work I will upgrade the alternator.
 
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