The 'E' Type Forum

when I start the engine the ammeter shows a discharge as expected but once the engine is running it still shows it discharging. When anything electrical is used ie indicators,brake lights etc,it shows even more discharge. This means that the car can only be driven for short journeys because the battery is constantly being run down. Its a new battery and the dynamo was checked and found to be working ok. Any ideas on what to check/replace next?

Hi there, was the dynamo polarised before use ?

regards Barrie

This sounds to be a Current-Voltage Control regulator fault. The regulator on a dynamo equipped car should be a Lucas RB 310 or RB 340 type, housed in a black box underneath the heater box and to the rear of the battery.For details have a look at pages 27 to 30 in section P of the E type manual, reproduced on this site at

http://dl.dropbox.com/u/8496016/Service ... uments.pdf


First thing to check is that all the wires are properly connected - they go into screw type connectors on a switch block at the base of the control . The control itself, under the pull off lid, consists of three spring loaded solenoids . As the engine speeds up one should "pull in" and engage the dynamo, and it should then drop out as engine speed falls to idle . If nothing like this is happening it is worth gently pressing the cut out solenoid ( at one end above the B terminal ) into contact with the engine running at say 1500 rpm to see if this produces a charge, but have an assistant ready to stop the engine and disconnect the battery if it sticks in when the engine idles , because if that happens the battery is trying to drive the dynamo

You should note that there are modern reproduction versions of the RB regulators around, many of which are quite useless, and if this is your trouble you are better off trying to find a genuine one off e bay

I didn,t specifically check for polarity on the dynamo when it was checked. Never gave it a thought to be honest. The black box under the heater box is the original that has always been on the car. Not having the slightest clue of how electrics work the box was fitted back on to the car and connected so I don,t know the condition of the insides. Can it be taken apart and cleaned or overhauled?

The dynamo need to be polarised to work. If you google it you will find the info required. The RB 340 has a cover fitted with expanding plastic rivets if you push with a small screwdrier in the middle off the rivet the centre will be driven out of the back and you can then remove the cover. With the battery disconnected , check the contacts are clean and not burnt clean up with fine emery. Worn brushes on the dynamo are another possible trouble spot - give the dynamo a tap with a brass punch and a hammer carefully around the rear plate area - this will have the effect of re-seating the brushes and they may make contact and the dynamo will operate for a while - new brushes will obviously be needed.

hope this helps regards Barrie

Can it be taken apart ? Yes by a specialist but no for you and me . Having said that, you could put it on the bench and with a continuity tester check the integrity of the coils etc , using the diagrams in the manual which I provided the link for . The spring tensions on the various parts tend to be critical and this is why you need to know what you are doing to re-assemble them . As I said, first check that all the wires are properly connected in the right places , and then do the test which I suggested .

tinworm wrote:The dynamo need to be polarised to work. If you google it you will find the info required.

http://www.jcna.com/library/tech/tech0023.html