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Posted on Tuesday, 13 March, 2001 - 21:28:   Edit PostDelete PostView Post/Check IP

I have to put Bess, my Mark VI, on constant trickle charge when she's in her garage. No matter what type of driving I do - day or night, city or country - the dynamo just can't seem to keep the battery charged on its own.

At full clip on the highway during the day, with no accessories on, the ammeter never shows more than 5 or 6 amps of charge.

The battery is your standard truck battery and is less than three years old. Its fluid levels are fine but it may have gone to nearly empty once or twice, so I would not be surprised if it's lost its efficiency.

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Martin Cutler
Posted on Thursday, 15 March, 2001 - 09:41:   Edit PostDelete PostView Post/Check IP

Hi Chris,

Firstly, grab a multi meter and wave it at Bess, cars hate these things, and usually pull themselves into line when you pull out a multi meter. If this doesn't work, you might actually have to use the thing. Put it on volts DC, and wind up the 50 volt scale, and put the meter across the battery. It should be reading somewhere around 12 volts, hopefully more. Now, keeping an eye on the meter, start the car. The voltage should drop briefly, when the starter hits, then come back to the previous 12 ish volts. Now, gently rev the motor. As the revs rise, so should the output from the generator. The volts should rise, indicating it is charging. 13-14 volts would indicate everything is OK. More or less than this, and you have problems.

The generators on Bentleys are very good. They are Lucas Special Equipment, so beefed up bearings, etc. I would suspect the regulator before the generator. Also, the regulator is easier to check. The generator is a pig to pull out and replace.

Clean the points on the regulator, and see if this helps.

SAFETY TIP - Never close the points on the regulator whilst the motor is not running. This will cause the current from the battery to flow back to your generator, and could cause damage.

I used to be scared of this electrickery stuff, but the more you play with it, the better you get!

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Bill Vatter
Posted on Thursday, 15 March, 2001 - 12:34:   Edit PostDelete PostView Post/Check IP

There is a way to adjust your regulator to increase dynamo output. I wrote a long explanation on how to do that as well as some other things to check, but it was all lost when your spell checker crashed Netscape.

So here is my suggestion, as you requested:

Get rid of the spell checker. Sometimes less is more.

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Bill Vatter
Posted on Thursday, 15 March, 2001 - 14:13:   Edit PostDelete PostView Post/Check IP


Sorry to have expressed my frustration publicly in my last message. I need to control myself better.

My suggestions:

1. From what you say, your Dynamo and regulator seem to be working correctly. My Silver Wraith, WGC66 shows similar readings on its ammeter as yours, and my battery has not failed to start the car in the past three years.

2. You did not say if you have a disconnect switch and if you turn it off when the car is not operating. A disconnect switch should be installed in the ground (positive) lead between the battery and the frame. Alternatively, you can remove the ground wire from the positive battery terminal when you are not using the car, but this is quite inconvenient. More than one car has been destroyed by fire caused by an electrical problem that could not be readily disconnected. Get a disconnect switch, especially since you have electrical problems that could be caused by faulty wiring. If the problem goes away when you disconnect the battery when not using the car, there you have it. Something is causing a discharge when the car is turned off. May be difficult or easy to find.

3. Old batteries often don't hold a charge. Drying a battery out as you say might have been done is of course detrimental to battery life. Charge your battery, and leave it out of the car for the period of time you don't use the car. If the battery won't start the car after this time period, get a new battery.

4. If you haven't found the problem yet, it may well be in the charging system. Marty said the dynamo is very reliable. True, but the regulator is even more so. These "special" Lucas parts are very good quality; no "Prince of Darkness" here, as might be the case with other English cars. These regulators rarely fail, and we already know your dynamo is charging, although we don't know how much. However the regulator can be adjusted.

5. Check cutout relay operation. Take the cover off the regulator and you will see two adjusting screws on the top. The left one (left side of car) adjusts the cutout relay. This relay protects the dynamo, and prevents the battery from discharging through the dynamo when the engine is running slowly. When dynamo voltage drops below about 12.5 volts the relay "cuts out." Start the car and turn on the lights. At idle, you should see significant discharge on the ammeter. Slowly speed the engine up. You should see the ammeter suddenly jump up when the cutout relay closes. This should occur about 800 rpm. It is very unlikely that the cutout relay will need to be adjusted.

6. The right adjusting screw adjusts the vibrator contacts that control dynamo field current. You can increase dynamo output by turning the screw clockwise, but be easy with this. Factory specification is about 16.5 volts with no accessories on and the engine running fast enough for a steady ammeter reading. There is a procedure in the service manual for setting the vibrator contacts, but you can do as well by tweaking it up slightly until you get sufficient charge to keep the battery up. Run the engine at high enough speed to get a steady ammeter reading. Watch the ammeter while you turn the screw clockwise and you will see the charging current increase. Move it up only a little at a time until it is providing sufficient charging to the battery.

I hope you have success. Let me know what happens.
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Martin Cutler
Posted on Tuesday, 20 March, 2001 - 15:44:   Edit PostDelete PostView Post/Check IP

Hi Bill,

Excellent Post. I have a regulator in my Bentley that has failed. The extra coil for the temperature control burnt out. The standard (MG Magnette type) regulator I now have in the car is working fine. The regulators all have the date they where made stamped into the bakerlite. The one that failed was stamped 1967, indicating that it had been a replacement unit. I now have one in there stamped 1965. (I am assuming the original one would have been stamped 1951!)This would indicate that at least 2 regulators have failed in 106,000 miles. Not too bad really!
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Bill Vatter
Posted on Wednesday, 21 March, 2001 - 14:01:   Edit PostDelete PostView Post/Check IP


Thanks for this information. The regulator on my operating Silver Wraith is marked:

RB106-2037182L 12V 34 71 GHS

My spare regulator, which came from Silver Wraith WFC10 (organ-donor car that died from body rot) is marked:

RB106-2037182L 12V 53 69 GHS

The regulators appear to be identical in every respect except for the numbering difference noted. On these regulators, the numbers are engraved along the top surface (as mounted in the car) just next to the bakelite cover. There are no other markings, and nothing that appears to be a date.

On my MG-TD, (made in 1952) the regulator is quite similar in appearance but not exactly the same. This regulator has a number engraved along the left side, just adjacent to the cover, It is:

RB106 2 12V * 37182H 10-57

Perhaps 10-57 is a date, I don't know, and some of that number could indicate a manufacturing location. Anyway, this regulator works fine for the MG, and I'm sure it would also work on the Rolls-Royce, as both cars are positive ground, but it would probably need to be adjusted differently for the different dynamo characteristics.

In 1955, I spent 6 months in Australia, and there were many MG Magnette saloons running about Melbourne, but the MG saloon cars are quite rare in the US, and I haven't seen one since 1955. I suppose they have a similar electrical system to the TD, as they are of the same vintage, and I understand that MG cars were built from available stock parts and a minimum of original design. Perhaps the failed regulators you describe are not "Lucas Special Equipment" (made especially for Rolls-Royce) and therefore they should be expected to have lesser reliablity.

A correction to my previous message:

The Dynamo voltage of 16.5 volts I quoted is with the battery electrically removed from the circuit by removing and reconnecting wires according to the test and adjustment procedure of the service manual. If the battery were in the circuit, the dynamo voltage would be more like 13 volts as you described, held down by the battery voltage while the battery accepts the charge. If the dynamo were charging the battery at 16.5 volts, the charging rate would be extremely high, well over 100 amp, which is neither healthy nor possible with this equipment.
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Martin Cutler
Posted on Thursday, 22 March, 2001 - 10:37:   Edit PostDelete PostView Post/Check IP

Hi Bill, numbers are the dates. 34 71 is 1971, 53 69 is 1969. The only difference I can find between the standard regulator and the Rolls Royce "special Equipment" one, is the extra temperature control bit, which ups the charge rate for different climatic conditions. Apart from that, they look and operate the same.

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Posted on Saturday, 05 May, 2001 - 13:26:   Edit PostDelete PostView Post/Check IP

After returning from the Federal Rally in Mt Gambier, we discovered that the battery in our Mercedes 280SE was dead flat. This was immediately attributed to the electric clock - nothing else could have used power while it was sitting idle - and this particular clock is apparently notorious for being very thirsty.

On the same day I received my copy of London & Derby (the RROC(A) NSW Branch newsletter) in which the editorial told a tale of woe about an electric clock draining the battery of a Silver Shadow. I immediately disconnected Bess' clock and everything's been fine since - she starts every time now regardless of conditions, with gusto!

I've since been told that that particular clock needs to be reset *every* time the battery is reconnected, or the make-and-break(?) in the circuit stays closed, burns out and drains the battery. Someone in Melbourne supplies a replacement, solid-state circuit for these clocks, which eliminates the problem.

Who needs clocks in a car anyway? ;-)