ALTERNATOR TESTING

ow

current /amperage/ and voltage are different.
turn on headlights and if amp gauge goes negative, something is wrong. Any mechanic can tell what is wrong with a good vat 40 tester.

good luck

 

To be clear, the amp meter swings negative when the lamps are turned on AND the engine is running at greater than 1500 RPM ?

If this is the case, that is, discharging with sufficient RPM, then either the regulator or alternator has failed.

First, using your DC voltmeter, check the voltage at the battery. Should read approximately 12 volts with key off. With engine at 1,000 RPM should read about the same. When revving to 2,000 RPM should read approximately 13+ (up to 13.8 at 3,000).

Second, use your voltmeter at the engine/passenger file wall fuse box. The large red wire, alternator output, should read the same as the battery. The small white wire leading to the back of the alternator is the field wire. This should vary in an inverse relationship to the battery voltage. As the battery voltage drops, the field voltage should increase. Measure the field voltage with lights off, then with lights on. The regulator should sense the drop in system voltage and send a higher voltage to the alternator field in an attempt to increase alternator output. In addition, as a helper increases engine speed, the field voltage should go down.

If the field voltage behaves correctly, that is, its output voltage goes up with load (lights), then the culprit is a bad alternator (most likely brushes soaked with oil). If the field voltage does not vary with load then the regulator is guilty.

Jim S.

 

If, indeed, oil is the culprit, it is rather easy to rebuild and clean the alternator. Should your testing reveal a bad alternator, removing it is 90% of the work. Dismantling and cleaning the internals of the alternator is the fun part. Let me know and I can walk you through the dismantling and reassembly of the alternator.

Jim S.

 

Unless I misunderstand what you write here, you seem to indicate that you have different readings on your multimeter on the input and output side of the large 60A fuse. If that is the case, you simply have a defective fuse or oxidized connections in the fuse holder. Such problems are not unheard of in a Dino. Make sure you remove the fuse and clean the fuse holder clips. Put a new fuse in, if you have one. You should then read the same voltage (or within a tenth of a volt or so) on both sides of the fuse. (Hope I am not insulting your intelligence here). Perhaps this is already the case and I just misread your post. If so, just follow jselevan´s instructions for the alternator and regulator, and you will be OK!

Lars

 

It could still be the fuse/fuseholder. At least, that is the first thing to check. What do you measure on the other side (output) of the big fuse? It should read simply the battery voltage. On the input side of the fuse (= thick wire from the alternator) you should read the higher voltage, 13.8 V or so if the alternator is charging. Perhaps you have the voltage there (it sounds like it), but your digital voltmeter is going crazy because the alternator output is rectified, unfiltered DC and the engine environment is very noisy. If you have an old analog meter laying around, use that one instead.
Yes, it could still be the alternator or regulator, but check the simple things first. Just my opinion.

Lars

 

Lars - thanks for clarifying things a bit.

With respect to all voltage readings, the black, negative lead should be held to chassis ground. The red lead is the one that you will probe with. The large fuse (60 amp), as all fuses, should read the same on both ends as it is simply a continuous wire. The large fuse (either side) should read the output of the alternator, which is fed directly to the battery. Thus, it is battery voltage. Lars is absolutely correct about the fuse holders being clean. However, the impact of an oxidized copper fuse holder is seen primarily on the field current (the small 16 or 8 amp fuse). Dinos, unlike most cars, have a long run from the regulator under the dash (or in the front bonnet in early cars) to the alternator, with several connectors/fuses. Hence, as the resistance across this distance increases the field current diminishes. Cleaning all connectors (with sand paper) from the regulator to the firewall to the alternator is always a good idea.

Lars is also correct with respect to the jumping of your digital voltmeter. However, you should be able to read a reasonably steady voltage at the alternator output. If you cannot, this may suggest that a diode has gone bad with large A/C component making its way out of the alternator.

Make sure that your black lead is grounded and that you are firmly pressed against the field fuse (either side assuming clean contacts). You should see the field voltage go down as the engine revs up. Better yet, you should see the field voltage go up when you turn on the lights. You have already found that the battery voltage does not go up with revs, hence the regulator, if functioning properly, will not sense a rising voltage and will not, therefore, lower the field voltage. However, if you artificially lower the the system voltage by turning on the lights, the regulator should sense this drop in voltage and consequently raise the field voltage.

In simple but technical terms that may make things clearer, the regulator is a feed back control system. When the system voltage drops (lights, other loads, or idle RPM) the regulator should increase the field voltage in an attempt to flog more current (voltage) out of the alternator. In contrast, when the system voltage gets too high (13.8+ owing to high revs) the regulator drops the field voltage to calm the alternator and reduce current output (for fear of destroying the battery).

It is the field voltage (really current) that creates a magnetic field in the alternator. The more current the greater the magnetic field and the greater the output. The lower the field current, the less magnetic flux and less alternator output.

Jim S.

 

I can see that I did not express myself very well in my second post above. Fortunately, Jim S. explained clearly what I wanted to say! And, of course, Jim is right that if one of the diodes in the alternator is shorted, you would get violent fluctuations of the voltmeter readings. A shorted diode may well be johnvwatts´ problem in this case - or it could be the usual oil on the slip rings, or just time for a rebuilt alternator. Statistical probability alone would point to the alternator having to come out of the car for one reason or the other.
I only got the impression from the initial question that johnvwatts measured two different voltages (the battery voltage and the higher alternator voltage) in the system - at the same time. That is of course impossible unless there is an open circuit somewhere between the alternator and the rest of the system. Hence my question about the fuses and connections.

I would suggest to anybody who likes to work on, and diagnose their own cars, to get your voltmeters out and take the measurements which Jim suggests, now, when the car is working normally. Keep those notes handy for future references. That way, you are familiar with the measuring points, you know what your voltmeter is expected to read and you know what the system was putting out when it was working properly. Diagnosing a charging system may seem intimidating the first time you do it, but it is really not that difficult. Having those reference measurements will make everything that much easier. And I can not think of a better guide for doing it, or a better explanation of how the system works, that that given by Jim S. above.

Lars

 

John,

I notice you give your location as England. If you're anywhere near London (specifically near the Hangar Lane roundabout) then I strongly recommend a trip to Sahib's Auto Electrical. The shop is old-fashioned, pretty untidy and the guys there can be a bit grumpy.

You will not find a greater breadth or depth of knowledge about car electrics (specifically old car electrics) anywhere. They are pretty reasonable too.

Might be worth a trip.

Regards,

Andrew

 

John,
I just noticed that you reported some time ago in the thread "Signs of a short?" that you also have had problems with the voltage regulator putting out too much voltage and burning out two relays. You then replaced the regulator with a newer type which seemed to solve the problem. All that sounded highly unusual, but when I put it together with the symptoms you describe now, things start to make sense: I believe you have a shorted diode in your alternator. Alternators are three-phase devices, so an alternator with one shorted diode will put out two thirds DC and one third AC. The voltage regulator will average the output and read it as the voltage being too low and try to push it higher. The new regulator may be a bit better in dealing with this condition but does not solve the problem. Similarly, your digital voltmeter is getting confused by the presence of negative pulses and displays wildly fluctuating values. If you switch the meter to AC you should read a fairly substantial AC voltage if you have a shorted diode. On a normally functioning alternator, the meter would only read the ripple voltage on the AC range, probably less than a couple of volts.
If it turns out that it is a shorted diode in the alternator which has been causing your problems, you certainly do not need to feel bad for having been stumped by it for a while. It is a tricky problem to diagnose. Best of luck, and I hope you let us know how the problem was resolved.

Lars