Derby Bentley electronic regulator

[Nicholas Simons]

This is a call to any members who have a Derby Bentley fitted with a modern electronic regulator.

I have a 1935 3 1/2 litre Derby with a two wire dynamo. When I bought it, it had been well restored and the old mechanical regulator had been swapped for one of the new electronic ones sold by the BDC and Fiennes. Unfortunately, the chap who exchanged the regulator didn't retain the old one so I cannot restore this and refit it. The problem is that I think the charging is faulty. After starting the car I get a short burst of a few amps charge, followed by almost nothing for most of the journey. I would expect to see a continuous low charge of one or two amps as long as I was driving. What I get is the ammeter needle sitting just below zero with regular clicks further negative when the fuel pump operates. The battery is in good condition but after a few long runs the battery is obviously becoming discharged as the car gets sluggish to start. The regulator appears to be doing something as the overall discharge rate is not as high as if the dynamo were not working at all, but is not giving sufficient charge to keep the battery healthy.

Can any members with such a device fitted to his/her car tell me what characteristic they get during running? Any thoughts please from the other regulars?

[John Robins]

Nobody seems to be responding here, and I ain’t no expert, but it seems certain that something isn’t right, and if the dynamo is producing the goods at the beginning, but then the flow turns to discharge, maybe it’s the regulator or maybe it’s not!

Any cars I have been involved with that had the luxury of an ammeter on the dashboard that showed a discharge when driving definitely had a fault. The type of regulator fitted is irrelevant if the system doesn’t perform and I think your expectations are correct, so, starting from first principles, check the dynamo isn’t failing to charge when it gets hot, check your battery is holding a charge, and then find an auto electrics man with knowledge enough to sort things out.

All the best, and I’m sorry to have just stated the b—-g obvious, but it does seem to be a good start point.

Cheers
John

[Nicholas Simons]

Thanks, John.
At present I am pretty sure the dynamo is giving a good output as when I switch on the lights the ammeter doesn't show a discharge equal to the draw of the lights, it stays around the zero mark. The problem appears to be that the regulator is not controlling to a voltage suitably above battery nominal, in order to create a charge current, but to somewhere around battery nominal, thus failing to create a charge.
It is too early to go to an 'auto electrics man' whoever that would be for a 1935 car. I would have hoped that this forum would have yielded suitable technical advice, especially as the suspected part is a special sold by the BDC Derby Technical Scheme. I have tried to phone the BDC's Derby expert but the number given in The Advertiser is wrong and I cannot get a reply from the appropriate staff member at Wroxton.

[John Robins]

Stitch a suitable voltmeter into the circuit and go for a drive with a companion watching the dial. I still wonder if the dynamo is becoming lethargic when it heats up. If my third form physics still works, the voltage should read that of the resting battery, or more, once the engine has started, and if it isn’t equal at all points of the system then a wiring fault is present, but one assumes that you would have noticed that.

What about earths? If the chassis has received a coat of paint, then the earthing straps have been replaced, are they still making good contact?

Perhaps another note to head office is needed, but try and keep the greasy finger marks off the parchment.

[John Murch]

I'm one with almost zero technical abilities; but I would agree that one of the first things to do is to check all the earths. Especially that of the battery to chassis, give the terminals a good seeing to with a wire brush.
It is interesting that it will power the headlights, but not charge the battery, it does sound like a battery earth problem...
John

[Nicholas Simons]

The battery took about 15 - 20 Ah to get to full charge so was clearly well down on charge after a few weeks of use. Next job is to fit a voltmeter to the switchplate socket and go for a drive. I would expect to see a charge voltage of 13 - 14v. I still suspect the electronic regulator is u/s.

[John Robins]

I think that the minimum dc volts needed to charge a nominal 12v lead acid battery fully is 14.2, so look for about 14.5 on your voltmeter, but I work from memory. Certainly 13 to 14 is a bit low.

I’m not arguing with your view of the electronic regulator being on the blink, but just suggesting that other faults are available.

Having put about 4 thirty shilling scrapyard dynamoes on my £50 Vauxhall many years back before someone said was I sure the cutout was ok, which it wasn’t, it’s definitely necessary to explore all the angles.

[John Robins]

I’ve just been into the motor house, where there rest a couple of large, good quality lead acid batteries which are in good condition and which I keep charged for various 12V dc needs.

At rest, one read 13.2 volts, the other 13.1 volts.

One is now on charge, and using an automatic charger with a voltmeter across the terminals. The charger started by delivering about 3 amps at 14.18 volts. After about ten minutes the reading is less than one amp and 14.43 volts.

Usually, after an hour or so the charger sits down at a low current, and will trickle charge forever.

We’ll leave it going for a couple of hours and check the voltage necessary to overcome the more fully charged battery.

Cheers

[Nicholas Simons]

Update. My memory failed me when I said that the headlights were supplied by the dynamo. Wrong! Big discharge, so it's either a duff dynamo or the cut-out is not cutting in correctly. When I ran the engine at reasonable revs the red warning light went out, so I thought that the cut-out was cutting in correctly, but on much closer examination I see that the red light is still on but very dim. I suspect, therefore that the cut-out is not cutting in when the engine speeds up. Next job is to check the cut-out and its coils.
Examination of the cct diagram shows that when the cut-out is made there can be no potential difference across the red light.
It's probably not the electronic regulator.
Thanks for your responses.

[John Robins]

Just to finish it off, the charger is now giving about half an amp and the voltage seems to have settled at 14.41.

The problem now is whether the regulator has damaged the dynamo or vice-versa. Good luck with your research.

Cheers
John

[Christopher Carnley]

I have had some awful times with Derby charging circuits,everything including the ammeter has been suspect, and as the Green Book is of little use in this case I resorted to fitting a modern Durite type, with great success.

[Nicholas Simons]

For those of you interested I have now solved the problem and what first looked like a fault with the dynamo or regulator is nothing of the sort.
This car has been rewired in recent times and it all looks neat with the correct colour and gauge of wires.
I checked the continuity of all wires and the resistance of the dynamo armature and field. All ok.
I ran the engine and noticed that the cut-out was cutting in correctly as the engine raised above idle.
Dynamo field was giving 5 - 11V as it speeded up.
Dynamo output was giving around 14V.
I then checked the voltages across the fuse box. Fuse 2, the dynamo output was giving around 14V, whereas fuse 6 ( main feed through ammeter from battery) was still showing 12V. These two fuses are connected through the switch box with the rh lever. This connection showed a open cct connection when switched on.
A jump lead between fuses 2 and 6 gives the correct charge through the ammeter and the dynamo is now capable of running the headlights and still charge the battery. Unfortunately, the top connection to fuse 2 is perfectly good, so the fault must be at the other end of the wire.
The correct solution to this problem will be to remove the switch box and investigate. Not a nice job!

[John Robins]

At least it seems that it will be a cheaper fix than a dynamo rebuild!

Cheers
John

[Nicholas Simons]

John.
Only if I do the job myself. I hate to think what the bill would be to remove and rebuild the switch box if I took it to a vintage specialist. Anyway, they would probably have rewound the dynamo and replaced the regulator already during their fault finding.

[John Robins]

Yes, that’s true. Would an interim fix with a lockable switch hidden somewhere be worth considering or do you not wish to go in for that sort of thing?

Best of luck removing the switches.

Cheers
John

[Nicholas Simons]

John.
I've been thinking about a short term fix. The switchbox rh switch feeds two contacts, No 2 goes to the top of the dynamo feed fuse 2, and this is the one that is u/s. The other contact on the switch blade is No 3 and goes to the top of fuse 3, which feeds the fuel gauge, fuel pumps and ignition coil. This wire is 40/36 and I calculate that it is safe for up to around 11 Amps. As long as the charge rate doesn't exceed around 8 Amps for a long period this wire should be ok if I connect the terminals at the top of fuses 2 and 3 together. I will do the job properly over the winter.

[John Robins]

A look at the "Service Handbook", (not the driver's handbook, but a more comprehensive publication) for post war cars says the dynamo output can be as much as 24 amps at 13.0 volts.

While fully appreciating that this is not a Derby car, I wonder if your assumption of 8 amps may not be a bit short of the possible output from the dynamo, and suggest caution.

[Nicholas Simons]

John
The ammeter shows the current going back to the battery and there will be an additional current going down the single wire under discussion. This would be to any side and head lights operating. I accept that side and head lights will take up to 8 Amps, but only if they are switched on. I never drive my car at night and will ensure never to do so until I have done the job properly so I reckon the wire will not be over-stressed in the short term. Thank you for your concern.

[John Robins]

👍

[John Murch]

With apologies for a bit of thread drift - I hate driving at night, and on my car (3lt WO) the maximum charge rate is 8amps which can be increased but at the risk of overheating the dynamo. I do like to drive with my lights on during the day (and today, filthy weather on the M6/M42 made that essential) so some years ago I put led bulbs in side and headlights, which are now brighter and use a fraction of the current. This is very useful as in city driving the electric fan is often in use which uses a surprising amount of current.
The point of all this is to recommend led bulbs!
John.

[Nicholas Simons]

Since buying my 1924 3 litre WO I've noticed that WO was not as good an electrical engineer as Henry Royce, but we already all know that as Henry previously ran a very successful electrical engineering company in Manchester. The charging system on the WO cars is not a patch on Henry's design. WO did have a good idea in using the dynamo as a camshaft and drive gear brake.

[Nicholas Simons]

Update, for those interested. After a few hours on my back with my head underneath the dash, I managed to remove the four 3BA nuts that hold the switch box in place. The top two nuts needed a pair of nested box spanners to get the reach. One had totally blind access. Dropping the box I was able to slip back the cover and remove the screws holding the wires to the terminals. On the bench, the switch box was easy to repair. I was pleased that I didn't have to remove any wires at the fuse box end or remove any wires from the conduit, it was all done from within the cabin.
The dynamo now charges as expected.

[John Robins]

Well done to you.

No doubt you are feeling the benefit of a fully charged battery, and also a fully charged wallet!

Concerning your comment about vertical dynamos used as camshaft drives, maybe the owners of thirties ohc Wolseleys may not agree so readily that they were a good idea.

[John Murch]

Congratulations on sorting it, most rewarding.
Also thank you for telling us the outcome, so often threads remain with no resolution.
Regarding the dynamos on 4cyl WO cars, they are horizontal and driven from the end of the camshaft, thus acting as a bit of a damper.