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For some time I have been troubled with a miss/loss of power when the headlights have been on for a few minutes. (Switching off the lights effects an immediate cure provided you don't run into something!)

It seems perhaps like volt starvation to the coil. I have replaced this without benefit.

Normal daytime running is fine, so I have ruled out distributor/condenser which has in any case been replaced.

The battery is in good condition, and the dynamo output is 8/10 volts at around 35mph.

I see 6.4 volts at the (disconnected) coil 'ign' lead with static engine, and 5.6 with the headlights on.

Any thoughts rude or otherwis would be appreciated.


Mike

Mike

Could you run a cable of appropriate size from ,say the starter to the coil so you are getting a straight power feed from the battery to the coil. a quick run up the road with the lights on would confirm that it is a voltage drop to the coil that is causing your problem? This won't cure it obviously but would allow you to work back through the wiring then to try and find the cause of the drop?

Steve

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Steve,

Good thinking Batman!

Will give that a try over the weekend. Got a run to the Bill Targett Memorial Rally on Sunday (in the light) so I can have a bit of a play around.

Mike

Hello Mike

Just cogitating further on your "fade out" problem, I think Steve's advice is on the right track.

There are two likely general "lines of enquiry" in my estimation.

One, that there is something radically wrong in the lamp department which is causing a really high current drain and bringing down the battery voltage - think that to be unlikely unless you have really gone overboard in terms of the wattage of the bulbs you are using and/or the missing effect is accompanied by drastic current discharge shown on the ammeter and probably smoke from somewhere,

The second, and probably more likely area is that you have a poor "joint" or connection somewhere back in the battery feed circuit which is common to both the ignition and lamp circuits, which probably means somewhere either on the battery side of the "A" terminal on the ignition/lamp switch or just maybe within the switch itself.

My hypothesis would be that with only the couple of amps which flow with just the ignition circuit in operation, the extra resistance presented by the poor/dirty connection would drop only a small and operationally insignificant amount of voltage so the ignition would operate more or less normally. But when the added current commences to flow with the lights switched on, the voltage drop created by the poort/dirty connection would increase proprtionally to the extent that the operation of the ignition is compromised.

I would hate to see damage done inside your switch, and I really don't know what type you have, Probably, I would slightly vary Steve's recommendation by making the following suggestion.

Instead of trying to run the jumper from the battery supply point (starter? or somewhere) to the coil, I would disconnect the headlamp wire feed at the switch and then make the "jumper wire" feed the headlamp circuit temporarily. If the car continues to run OK with that arrangement, then you have just about proved that the fault must lie back in the common feed from the battery. The likelihood of a crook switch should also be considered in that case. If you still have the problem with the lamps fed that way, then there would seem to be the more unlikely scenario of some drastic breakdown in the lamp circuit.

No doubt you will prove me wrong but that is the way it seems to me

Regards


Barry R

Mike
Dont forget the negative side of the battery which is common to the headlamps and the coil.
A bad connecting from the battery to the chassis could cause this.

Pat

Could be Pat, but would expect there to be also problems with starter motor performance if there was trouble in the earth return leg to the battery.

Regards
Barry R.

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Thanks chaps.

Progress seems to have been made, I am glad to say. For the time being at least the engine is running smoothly with the lights on.

Putting together all your suggestions, I came to the conclusion that the common factor was probably behind the control panel. This is an area which I had not previously attacked so after a deep breath, and a struggle with the two retaining screws (more anon) I pulled it out from the dash. Not a pretty sight!

Having marked every wire in sight I disconnected it and did a complete strip-down as this seemed the only way to get rid of all the crud.

Having re-assembled the unit I refitted to the car making careful connections - having to extend some leads through a block connector to save straining.

A test start up was performed without clouds of blue smoke, and I discovered that the summer/winter charge arrangements were now work for the first time in my ownership.

Coming home on Sunday I ran for the last twenty minutes with all lights on. There was no problem.

I am seeing about 7 amps discharge with just the ignition on (half or full charge positions) and feel that this is probably high. After re-setting the third brush I am getting a charge of 6/8 amps on half, and 14/16 on high at thirty mph in second.

With regard to the fixing screws which I mentioned above, I have completed a minor mod in that I have made a couple of captive nuts, and pop-riveted these behind the dash. Removing the panel in the future will be pit stop stuff. I recently did this for the coil also, as I do not have tripple-jointed four-foot arms.

Thanks again for all the input.


Mike

Hello Mike,

Glad to hear that you are making some progress.

I definitely agree with you that a figure of 7/8 amps is on the high side for just the ignition circuit.

While I would hardly put the original ammeter in the "laboratory precsion' instrument class, I would have thought that a modest deflection of about two amps would be what you should expect.

If you have or could borrow a reasonably heavy duty test meter it might pay to validate just what current is being drawn. Putting the ammeter in series with the lead from the coil to the distributor LT terminal with the ignition on and points closed would be one place to check. If that is closer to expectations then it would be nice to know where all your other valuable amps are going.

If you really are drawing say 8 amps on just ignition and then deflecting to 16 aqmps at 30 mph, then the poor dynamo is delivering about 32 amps. It would be an absolute understatement to make the observation that this is a fairly brutal overload.

Remember, Mr Lucas psecified 10 amps output maximum for the coil ignition dynamo which means 8 amps on the scale (2 amps taken to run the ignition)

I have forgotten or maybe have never know just what sort of a switch you have - pre 1932 combined ammeter and switch (SM5 panel) or the later stand alone PLC type. Would help to know.

Best of luck

Barry R.

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Barry,

Thanks for the quick response. No, fortunately, it is not quite as bad a 32 amps! The 16 amps charge was without the lights on, (sorry I did not make that very clear) which gives 20/22amps at 'peak' revs which is bad enough. I can knock this back on the third brush.

The coil does seem to require a high 4.8 amps, going direct from the battery with a slave lead, and using a good meter. Across the + and - terminals of the coil I see 1.8 ohms, and between + and HT 5.2k

Whilst the slave was in place I measured 6.7 volts direct, whilst there was at that point (coil+) 6.5 volts coming through the system. So not a hugh volt drop there. The car ammeter shows a couple of amps with the coil disconnected, and I suppose that this must be accounted for by the charging system and dynamo.

The panel is indeed an SM5, with ammeter, two switches, and ignition warning light.

Mike

Hello Mike,

Sorry Mike, the 32 amps was either a slip of the keyboard or my mathematical incompetence, maybe both.

What I was trying to say was that if you have an 8 amp discharge at idling ( before charge circuit is switched in by cut-out) and you then have say 16 amps shown on the ammeter at say 30mph then you are actually drawing 8 plus 16 amps out of the dynamo, which of course is somewhere about 24 amps.

That is a considerable amount more than the designed output of any Austin Seven dynamo.

It was your original indication that the "no charge" drain indicated on the ammeter was somewhere around 8 amps that alarmed me.

I think it is generally accepted that in a Lucas 6v system, the effective drain by the ignition with the engine running is of the order of 2 amps (maybe a little bit more "at rest with the points closed" because that is a constant current situation and not the "average interrupted" current that flows when the engine is running.

The primary resistance figure of 1.8 ohms seems to be around about what I would expect to measure, and I suspect that your actual ignition circuit is fine, but I do wonder where the rest of the current is being drawn from.

At speeds below cut-in voltage for the cut-out the dynamo should be out of circuit and the only other drain, with lights off, should be the load of the shunt coil of the cut-out. This is somewhere of the order of 30 ohms and therefore should only add about say 1/4 amp to the load on the battery.

Maybe, just maybe, your cutout is coming in too early and their is a drain back through the dynamo. That should be relatively easy to check and really does need fixing as it is not good for the dynamo

Maybe though your ammeter is a lot more sensitive than it should be and is tending to indicate higher currents than are really there, but one way or another, I would wnat to be sure if there really is excessive current being taken out of the system and, if so, eliminating the cause.

Your poor dynamo will thank you for it , particularly if you finish up with a reasonable total requirement and can end up adjusting the third brush to a total required output that is not excessively beyond the dynamo ratings.

regards

Barry R.