3.8 coupe not charging

[abowie]

I'm perplexed by a charging problem I'm having with a fully restored 3.8 coupe.

This car had a standard Lucas dynamo wired through an RB340 control box with entirely standard new wiring from Autosparks.

Over the last few years I've had intermittent failure to charge the battery, with the generator/charge light coming on.

I've twice tested the generator following the service manual and it seems to be producing the correct output for rpm. I checked and repaired what might have been a problem with one of the rear connectors, and then it worked briefly, and then failed again.

The RB340 seemed to be ok, again tested following the service manual. To be sure I have changed it out twice, once with another original unit and once with a new one.

But I still had a failure to charge the battery.

Recently, in desperation, I decided to exchange the dynamo out for a Tudor Dynamator. I gutted the control box so that I could use it purely to connect wires together. See viewtopic.php?f=3&t=21933&p=176938&hili ... or#p176938

On doing this initially the car didn't charge but after I had driven the car a few minutes it started to charge OK.

This morning I took the car for a run, and the same thing happened. Initially no charge with battery reading 12.2v, then suddenly after say 5 minutes it started charging reading 13.8 to 14.0v and the charge light went off.

I can't work out what's going on. I've replaced every component in the charging circuit but still have the problem. Is there a component I'm missing?

[Tommd]

Andrew, you certainly have a frustrating situation.
Now that you have gotten rid of the original generator and voltage regulator, you really have a completely different system than before and possibly a different problem.
However, I do see two items that could be similar.
1. Generator/Dynamator ground. I assume both units were bolted to the engine block without any rubber bushings etc. if not already confirmed, maybe remove the bracket and be sure there is no insulating paint, dirt, rust etc. between all mating surfaces.
2. The charging wire (not the field wire) between the dynamator and what ever route it takes to get back to the battery. You say this is a new harness. Occasionally a wire end may be poorly crimped and cause a poor connection. Check this carefully.

Next step would be troubleshooting when it acts up. Does it fail on a regular basis for troubleshooting?
Tom

[abowie]

Good thoughts both. I did wonder about the engine earth.

I’ll go and have a look.

[Tommd]

You responded while I was still making additions, did you see my thoughts on the wire crimp?
Tom

[abowie]

I'm going to make up another wire to bypass the loom. And probably replace the engine earth cable under the car.

Realistically, it has to be the wires. There's nothing else.

I'll keep you posted.

[mgcjag]

Hi Andrew....just about to fit a Dynamator to my C type ..a couple of things....ignition lamp need so be 2.8w to excite the alternator so check for poor connections and looking at your previous post have you fitted the old dynamo larger pully....an alternator needs a smaller pully to spin faster....Steve

[abowie]

Thanks for that Steve.

I bought a pulley from Tudor. I'll change out the globe too.

Yesterday I noticed that when I pushed the starter button the voltage dropped from 12.8 down to 6 or 7 volts while the starter was going and the engine was turning over much slower than normal.

I'm not sure what this means. I'll have a look at my other 3.8 and see what it does, but I have all of my cars stored off site now so I'll have to wait til I get to them.

[paydase]

I think that the earth may well be the culprit: not only the cable but also the bolting itself.
Maybe unbolt the cable and sand the metal where it bolts

[Tommd]

From where to where did you read the 7 volts?
Tom

[abowie]

I have a voltmeter in the cigar lighter socket.

[Tommd]

That reading does indicate a problem but not where:
1. Bad battery. Place a voltmeter on the actual battery pos and neg posts and crank. If 7 volts, probably bad or discharged battery.
A 3.8, so assuming positive ground.
2. Place voltmeter leads between battery position post and engine block. Crank. If more than 1 v, trace through each ground connection, cleaning each up until the voltage loss is minimal.
3. Place volt meter leads between battery neg posts and battery cable on starter and crank. Again should be minimal, less than 0.5 volts. If not clean up connections, including solenoid issues.

At least I would start here.
Tom