Control box malfunction

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Control box malfunction

Postby Asolepius » Tue Mar 31, 2009 11:28 am

This is very strange. When I start the engine the battery charges absolutely fine, and all is well for about 20 mins. Then the ammeter suddenly flicks over to discharge, as much as 15 amps. It does this for variable periods of seconds to perhaps a few minutes, and then comes back to charge again. It seems to get worse the longer I drive. With the lights on it discharges a majority of the time, especially with headlights. I have adjusted it repeatedly with a voltmeter as per the workshop manual, although I only have a digital one and it may need a moving coil one (are these available anywhere these days?). I am guessing that this is a temperature effect. As the engine bay gets hotter maybe the cut-out point changes, and of course the temperature varies with speed etc. I have an electric rad fan and maybe this is heating up the control box when it cuts in. The fan draws 15 amps so when idling I am on 30 amps discharge! But the battery always starts the engine so I am somehow winning. Without the ammeter I would not know this was going on! Do I need a new control box?
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Re: Control box malfunction

Postby hoggie » Tue Mar 31, 2009 11:10 pm

After the car is warmed up and you see the lack of charge on the ammeter, I would be curious what the standing voltage is reading at the battery terminals. You would expect the discharge indication would see a voltage less than 12.6v. Might be another clue to the puzzle.

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Re: Control box malfunction

Postby Bugzy59 » Wed Apr 01, 2009 1:45 am

I wonder if you have a short somewhere. Try unhooking the front harness when this starts to happen. If it is heat, the Ammeter should show the same discharge. You can try the same with the rear harness.
I question whether heat on the control box could cause such a great deviation at the ammeter.
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Re: Control box malfunction

Postby Charlie Tolman » Thu Apr 02, 2009 12:26 am

The operation of the regulator box might improve after cleaning the contacts. The best tool is a small metal file. If emery paper or sand-paper is used, a residue will remain on the points that might affect the electrical continuity. Adjust the regulator afterwards.

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Re: Control box malfunction

Postby Asolepius » Thu Apr 02, 2009 3:14 am

Thanks guys. I already cleaned the contacts, and adjusted afterwards. I did the mechanical setting first exactly as per the manual. I think a short is unlikely given the delay before the problem arises. I will check the battery voltage but not for a while as I fly to Australia for 2 weeks tomorrow. I should mention that I tried a new dynamo and that made no difference. Frankly as a new control box is only GBP 26 that might be more cost effective that wasting all this time on it.
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Re: Control box malfunction

Postby vagt6 » Thu Apr 02, 2009 9:30 am

I had a similar problem, and after much hand-wringing I found two hot wires were bare and touching, causing all kinds of strange electrical problems.

Check your wiring, especially hot ones, for shorts and integrity. Maybe wires are pinched somewhere?

Let us know . . .
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Re: Control box malfunction

Postby Bugzy59 » Fri Apr 03, 2009 7:17 pm

I was sure I did not have any shorts. I later found out miswired the license plate lamp. When it got wet then the short happened. Humidity can also affect a short. The beehive lights are easy to miswire.
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Re: Control box malfunction

Postby Asolepius » Sat Apr 18, 2009 10:17 am

Hi guys - just got back from Australia and did the test that occurred to me while I was away. I started up from cold and confirmed that charging was OK. Then I took off the box cover and warmed the unit gently with the heat gun. Only up to about 40ºC I guess (ambient is 12 here at present). Started up again and lo and behold charging was minimal. Waited for it to cool, and charging was back to normal. This technique is called in my line of business challenge and dechallenge. So it's definitely a temperature problem. Now I just need to find out why it's happening, or just buy a new box and be done with it.
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Re: Control box malfunction

Postby Charlie Tolman » Sun Apr 19, 2009 12:22 am

With everything cool, remove the cover from the regulator (control box), start the engine, and note the charging rate.

Use the heat gun to warm-up the regulator, and observe which one of the relays opens or closes when the charging rate drops. This might help in the diagnosis.

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Re: Control box malfunction

Postby hoggie » Sun Apr 19, 2009 1:41 pm

Check out section N.7 of this pdf on the control box.
http://www.spridget-tech.com/service_ma ... tion-N.pdf

Here are the open circuit regulator voltages measured between the D terminal and ground with the A and A1 cables pulled off the control box and the cables connected together.

40C 15.3 > 15.9
30C 15.6 > 16.2
20C 15.8 > 16.4
10C 16.1 > 16.7

Also you might check out page 26 of this PDF on Lucas Control Box troubleshooting.
http://www.vitessesteve.co.uk/PDF/Lucas ... Manual.pdf

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Re: Control box malfunction

Postby Asolepius » Sun Apr 19, 2009 2:04 pm

Thanks Mike - I already have the BMC manual but the Lucas one looks much more useful.
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Re: Control box malfunction

Postby Asolepius » Fri Jul 03, 2009 6:03 am

Here is the final outcome to the saga. I gave up trying to adjust the control box. No matter what I did, I could not stop it from suddenly flicking over to discharge, and there was no reliable charging with the headlights on. So I ordered a new box from Moss, which was broken when I unpacked it so there was a wait for a new one. This is on the car now. Yesterday on a 100 mile trip all seemed absolutely fine, so I started to relax - until the dynamo failed. I was 40 miles from home and got back on the battery - fortunately in daylight. As it happened I had a new one in stock so that went on. It has a damaged tacho drive connection which is why I hadn't used it yet. I now have charging off the scale at 30+ amps, and 25 amps with the headlights on. The old dynamo was too hot to touch and smelling of insulation, so I think the windings have had it.

I suppose the new control box made more demand on the dynamo - last straw and all that. Sadly I can't swap the end bearing housings as the new and old ones are different, so will have to work out how to repair the tacho drive sleeve.
Last edited by Asolepius on Fri Jul 03, 2009 6:25 am, edited 1 time in total.
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978
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Re: Control box malfunction

Postby Asolepius » Fri Jul 03, 2009 6:05 am

Sorry - double post.
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978
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Posts: 489
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Re: Control box malfunction

Postby Charlie Tolman » Sat Jul 04, 2009 12:03 am

Hmmm. I would be suspicious about the new control box (regulator).

Thirty amps at 13 volts (approx.) is nearly 400 watts. What could possibly be requiring that level of power draw in a Sprite ?? Perhaps the battery, and that would become very warm after a few minutes. Headlights are probably 40 watts each, or less, and they would draw about 6 amps, total.

I doubt that the wires in the generator are large enough to continuously supply 30 amps, and the windings will become very hot, possibly fusing, especially the inner wires in a bundle. My calculations show that a #8 gauge wire is necessary, with #6 being better.

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Re: Control box malfunction

Postby Asolepius » Sat Jul 04, 2009 2:00 pm

Once the battery was up to charge the rate fell back to trickle. All seems well now. Out of curiosity I dismantled the old dynamo and found that the live brush holder had got very hot and melted its insulated mountings. The carbon brush was worn away to almost nothing, but the earth brush was OK. I'll keep a close eye on this!
Les Rose, Salisbury, UK
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