Ammeter behaviour and readings

[Tournesol]

Good evening All,

My car is positive earth and with Dynamo.

I've just replaced the regulator and dynamo as the ammeter was always showing in the negatives and battery was being discharged. All good now with showing strongly in the + with engine revving above 1500 which I trust is about right for a dynamo car.

Question I have is that running in town the ammeter goes from just in negative under iddle engine to almost up to 30 under revving with the needle moving around a lot. Is that normal behaviour?

Also am I right to assume that the battery is not getting a strong charge (if any at all) while driving in congested or slow traffic and that to achieve a good charge the car needs to be driven at a decent pace (not just reving the engine)?

As you gathered I am a total newbie..... but willing to learn... FAST!

Tournesol
PS: I've tried to search the archive for this but couldn't find anything, might need training on archive searching too!!

[billwill]

The control box won't close its contacts until the dynamo is generating more volts than the current voltage of the battery, so at low revs you will see only the drain on the battery from the ignition circuit and any other active circuits eg lights, trafficators, horn, electric fuel pump if you have one.

When the revs are high enough for the dynamo to cut in, I would expect to see it charging the battery to replenish the power used in starting the car, this could be as high as about 30 amps briefly, but when the battery is reaching the fully charged condition the charging current will diminish to a trickle charge of about 1 or 2 amps, even at highish revs.

If it still shows a high current your battery is probably getting too old or your dynamo regulator needs re tuning.

All in my opinion of course...

When the headlamps are on at high revs the current will be going straight from the dynamo to the lights without passing through the ammeter, so you won't 'see' that current.

[oldchieft]

The high curent could be a shorted cell in the battery and the erratic readings might suggest it is an intermitent fault.

I would borrow a battery off another car and try it with that.

It might be a bad conection or break in wire, so get someone to watch the meter while you poke and pull things.

Jon the Chief

[Craven]

Hi,
You say you have replaced the regulator, is this with a new unit or a used one? If a second hand unit then check out the condition of the contacts, they can be cleaned up with a slip of fine emery cloth or similar.
It?s a simple procedure to reset the cut in/out point and the charging voltage but be careful as it is a fairly delicate device, a volt meter is needed. Procedure in Workshop Manual.
Ron.

[Tournesol]

Hello again,

sorry forgot to mention both the regulator and dynamo are new, not refurbished. I'll take the car for a longer spin today to see if the ammeter stops charging the battery after a long drive. Last night the ammeter readings were eratic (under revving) in town but constant at about 15 when on dual carriagway with lights on. I am slowly growing confidence and getting further and further out of town.... (probably cursing my luck now!). I invested in a battery pack booster to be on safe(r) side.

Thank you for all the advices.

[denicholls2]

My assessment based on the details you've provided is that your battery is discharged and everything else is working just fine.

Dynamos (we call them Generators here across the pond) were replaced with alternators because the output of an alternator at low RPM is significantly better. If you are driving at low RPM with lights on, chances are very good indeed that your net status at the battery is discharge. This type of driving is common in urban environments and exacerbated by short days in wintertime. Even my early alternator Fiat 124 was incapable of staying ahead of the game in city traffic during winter absent driving like an Italian.

You might choose to up the idle a bit during this time of year just to get to a positive average charging state. It shouldn't take much. Or, you could start driving like an Italian.

A drawback of the ammeter is that it does not indicate the current state of your battery, only the current state of the charging circuit. (Although high output indications like you are seeing are arguably the exceptions to this.)

With a good battery and significant RPM to get the dynamo comfortably above about 13.8 volts, you should see only brief forays to the +30 sector of the world (mostly after starting, which presents the largest, but hopefully brief, current drain.)

[AHM]

My car is positive earth and with Dynamo.

Well done! Sounds normal - The regulator is a pair (or 3) of switches for limiting voltage and current so the needle will flick as they switch on/off

dnicholls2 +1