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MG TD TF 1500 - Charge warning light woes

I have just had an interesting experience. In my TF I run a Race Mettle altermator, Lucas C-39 look-a-like. Lately I have noticed that the auxiliary voltmeter I have mounted showed no charging activity.
I sent a e-mail to Steve at Race Mettle, and he responded quickly and comprehensively as to possible causes, but asked me to check my warning light bulb first. I did, and found that the bulb holder had become loose from its retainer in the dashboard. I put it back in place, and once again I have voltmeter readings of 13,5 volts + at medium engine speed. Quick fix, and excellent info from Race Mettle. I love those five second repairs!

Just wanted you to know.


Jan Emil Kristoffersen

Charging systems with a warning light will not charge when the bulb is not connected or in working order.
Len Fanelli

Domestic cars with a charge light only have a resistor in parallel with the lamp to provide excitation if the lamp filament is open - around 100 ohm. Perhaps Race Mettle can suggest something similar.
JE Carroll

The charging light on my TF has become intermittent. I chalked it up to a loose bulb or fixture, since the ammeter shows a charge any time above 1500 RPM. Am I damaging anything by running it with the intermittent charge light?
Jack Long

In the TD/TF system the Ignition Warning Light plays no part in the charging process except to give the driver something to look at. Bud
Bud Krueger

Thanks, Bud, that was my understanding as well. I know the MGB is a different critter and that the red light is critical to the charging circuit. I wonder, given that there is a factory ammeter, why they even bothered with the light on the TF.
Jack Long

The charge indicator light is only an issue with an alternator, which is what I have in my TF


Jan Emil Kristoffersen

Here's a typical simplified alternator diagram with a charge light providing excitation when the ignition is on. It also shows the parallel resistor that's in many installations.

I had the same problem recently with a tractor I converted. I simply bought a rebuilt 10SI for an '80s GM car and paid the core charge. Rather than install a one wire regulator in it I just used a charge light to activate it. Since I didn't bother to add a resistor the alternator quit when the bulb burned out.

The machine has an amp meter I could have just hard wired it but I decided to wire a light.

JE Carroll

Jack, with the dynamo set up the gauge and the light are showing you two different things. The light gives you an indication as to whether the dynamo is putting out the specified voltage or not. Think of it as pressure in a garden hose. The amp meter is showing the amount of draw on the system and is an indication as to whether the dynamo is keeping up or not. Think of the battery as a water tank and the amps as the amount of water flowing in or out. Putting a draw on the battery is like draining the water tank. When you drain more than what is going in the amp gauge will show that...even though the dyno is running correctly. When the draw is consistatnly greater that the flow going in you would have to upgrade to a larger amp dynamo...think of it as using a larger diameter hose to get more volume..while still maintaining the same pressure (voltage).

With the alternator you only see voltage (think of it as a pressure gauge in a water line) and when the alternator doesn't keep up with the demand you see the voltage drop on a gauge. The light serves the same function as on the dynamo but it is incorporated into the charging circuit on the B.
MG LaVerne

This thread was discussed between 27/04/2014 and 01/05/2014

MG TD TF 1500 index

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