Electrical power loss

[JSabah]

So the other week I found that the battery is the SP250 was too low to start the car. After charging fully, I went for an 80 mile trip. Half way there I stopped and restarting wasn't as easy as I expected (it usually fires right up and it turned slowly at first). Yesterday again it didn't have enough juice to get it going (charger said it was at 68%). The battery is only about a year or so old and the generator was supposedly rebuilt. I realize I can have the generator and battery tested (my charger didn't indicate any issues but it is not always accurate). My question is could there be something slightly on even when the key is off and if so how would I find it. In the past on other cars, I've found bad grounds that create electrical "leaks" usually thanks Tina bad ground ... But only when the ignition is on ... Also, how would one check the output of the generator?

Just for info: SP250. No radio or additional equipment. Generator and neg ground. Electronic ignition.

[Christopher Storey]

If you have any kind of ammeter ( e.g the current function on an electrical tester ) you can test for this by disconnecting the battery positive lead and putting the tester in series with it i.e. between the +ve terminal and the +ve lead. If there is any current flow with everything apparently switched off then you have a leakage, and ( depending on how may fuses you have ) you can find the affected circuit by pulling fuses in turn until the leak stops. Not so good on old cars with few fuses, but very easy on moderns with multiple fuses. If you have no ammeter, a parking lamp bulb with two wires soldered to it will do the same job - ir it lights you have a leak

[JohnM]

Hi Josh,
is this your first run since the generator was rebuilt?
If so, has the generator/control box been "flashed" to negative earth, as it would originally have been positive earth?
My personal experiences have been that the control box is a more likely culprit than the generator itself in cases like this.

Depending on your confidence checking these things, you (or your electrics guy) might find these links useful:
http://www.dynamoregulatorconversions.c ... dynamo.php
http://mgaguru.com/mgtech/books/pdf/Luc ... _Tests.pdf
http://mgaguru.com/mgtech/electric/et120.htm
http://mgaguru.com/mgtech/electric/et121.htm

Hope that might help - good luck, I know how frustrating this can be! (I now have an alternator )
Cheers,
John

[Chris S]

Hi, best way to 'simply' test a dynamo is to charge the battery for 24 hours, leave unconnected for a further 12 hours. Check the battery 'no load' voltage. It should be at least 12.5 volts. anything less - replace battery. If battery OK, reconnect, start engine, run at fast idle, (about 1000 RPM) for 3 minutes, and check the ammeter if fitted. It should initially charge but very quickly fall back towards zero charge, but not quite. Keep the revs at fast idle and check battery voltage. It should be about 14.5 volts. Switch on headlights and ammeter should still show a charge.
If all above is NOT as it should be, then the charging system is suspect.
Test dynamo by, with engine stopped & ign off, remove the 2 wires (one thick, one thin) from the back of dynamo, link the terminals together with wire. Move the wires so they are not touching anything. Connect voltmeter (at least 20 V DC. scale) between the link and earth. Start engine and note voltage as the engine is SLOWLY accelerated. The voltage should very quickly climb to 20 volts. Don't let the voltage go much above 20 Volts. If it takes beyond 1000 RPM to get to 20 volts, then the dynamo is suspect. If all OK then voltage control/cut out box is at fault.
Hope this helps.
Best rgds
Chris
V8 Saloon registrar.

[Ian Slade]

The SP is fitted with an Ammeter as standard, after starting the meter should show about 20 Amps this should drop over about 1-2 miles to zero, at 2000rpm with the headlights on the meter should read zero or centre. The SP as you say has a dynamo so will charge at about 13.8 volts, age of a battery is no indication of condition, a good battery fully charged will hold its charge of 12.9 volts for more than half an hour with a indicator lamp(21W) wired across the terminals. Batteries can die within weeks of purchasing though most last for 4-6 years, there is usually no difference in a battery with a 1 year warranty or 4 year warranty, you are just paying an insurance premium in the upfront cost for a replacement in year 4.

[Sydsmith]

This is question that come up from time to time and very often boils down to bad connection to the battery terminal, the charging system can be fine the battery condition fine but if there is resistance between the battery and it connection there will be trouble.

One quick check is when the car is cold start the engine, quickly feel the battery posts, if one or both are at all warm clean the connections.

The best way to clean connections is to dissolve two table spoons of washing soda in a pint of warm water, dip and leave the connectors in the solution for a few minutes, dry off connector and coat with a thin film of Vaseline.

You might also like to check the output voltage of your battery charger it has to be at least 14 volts, I have found several new ones which have less than 13 volts output.

[JSabah]

Thank you all for you assistance and guidance. I took this opportunity to by myself a multimeter as up until this point I had been using a circuit/continuity tester and/ or a dash light bulb and wire.

I 1st tested the battery to make sure it held a charge and that there was no "leakage". All good there - a few weeks after a change still had 12.6 V and disconnected the positive and but my meter in line - nothing being drawn.

I then re "flashed both the generator as well as my dyno. After that, I tested the dyno by connecting the two terminals together on the back of the generator and checked the output with my new toy ...at 500-700 rpm I only had about 2 volts (my memory may be fuzzy hear) but at 1000 rpm it was only at 11.5-12.5v .... So it seems my dyno was not rebuilt correctly. So while it may cost me a few $$ it is an easy fix and I won't have to hunt around for mysterious "leaks".

As an alternate (no pun intended) dose anybody have a source/name of the alternator that looks like a generator in the US ... And can it be covered with the aluminum Daimler cover or does it need ventilation?

Thanks again

[JSabah]

Also, I know that the SP250 came with an Amp meter originally, but the person (whom I have high regards for) highly recommended changing to a Voltmeter as a mater,of safety and other reasons), so I had a voltmeter made to look like the original.

[Ian Slade]

Before you rush off spending more cash Josh, check the output at 1500 and 2000 rpm, due to the high revving engine the dynamo is not good at low rpm, if it is giving an output of 12.5 volts at 1000rpm I suggest the dynamo is OK, if you don't achieve 13.5 volts at 2000rpm I suggest ypur problem is in the regulator and may need adjustment. I believe there is a section in the workshop manual in setting the regulator, best of luck.

[JSabah]

Again, thank you. I was not intending on spending a lot. I thought my 1st step would be to inspect and possibly change the brushes and possibly the springs, although it seems that it is not as easy to find the springs (new old stock only available from what I see on on line) and possibly polish/clean the stator (not sure that is the correct name).

From my understanding from the earlier posts and now reviewing the workshop manual, the Dynamo is supposed to be putting out 20 v with the car running at 1000 RPM and I suppose the job of the regulator is to limit what goes through the system. It is confusing though because the specs of the Dynamo mention 13.5 v while the Dynamo is at 2000+ RPM (and I don't know what that translates to for RPM of the car). I any case my output may get me by if I run the car at the higher rpm's but with city driving the average may be on the low side, hence my discovery of the issue after 400 miles of driving over not quite a year.

My preference is to get the charging system in order (now that I have determined it is not the battery and there is no drain), be it the Dynamo and/or the regulator. Unfortunately, both have been rebuilt during the restoration so I can't say 1 has been attended to and the other not ... But u have more faith in the person who rebuilt the regulator (he also did the fuel pump and wiper motor) rather than whoever went through the Dynamo as they were subcontracted by the guys who rebuilt my engine (and who I now have less faith in) - if it had actually even been done. So 1st step seems to be remove the Dynamo and check the brushes.... And possibly the springs, then reinstall and recheck output.