Ignition coil and resistor, 69 280sl

[Lee Roessler]

I just replaced my coil and resistor (other wise known as a ballast).  I find that the resistor is getting hot/warm.  Is this normal?  Help somebody.

Lee in Lake Tahoe

[waqas]

Are you sure you have the correctly rated resistor and ballast setup? We have more information in the Technical Manual.

Another reason might be an imminently-failing interference suppressor condenser. Do you have an interference suppression condenser by the coil? It looks a bit bigger that the distributor condenser, and is mounted to one of the coil mount bolts. I believe only cars with radios installed from the factory had it.

If you have one, then make sure the resistance across it is zero (appears disconnected to an ohm-meter, or multimeter). This condenser is connected to the coil side of the ballast resistor. In the extreme case (see below), a short to ground on this side of the resistor will drop all 12 volts across the resistor, causing it to get very hot indeed, and in fact smoking! (again, see below). In this case, the car will not start, as the input end of the coil would be shorted.

In the more likely case of only a slight breakdown in the condenser (instead of a direct short), the condenser will look more like a resistor, causing a higher voltage drop at this point, and excess current flow though the resistor-- again making it warm or hot (but not necessarily smoke).

In my case, on the morning I was to drive to Blacklick for PUB'09, my interference suppressor completely shorted, and caused my ballast resistor to start smoking when I tried to start the car! (all 12 volts were being dropped over the resistor). I was too impatient to try and debug the problem, so I left the car behind. On my return from Ohio, I tracked the problem down to the shorted condenser. I just removed the condenser, and voila! Problem solved. Even the previously smoking resistor began to work normally. I replaced it anyway due to reliability concerns, for an overheated resistor may no longer have the same resistance or heat-range behaviour.

Hope this helps

[Lee Roessler]

Thank you for the reply Waqas.  I do have a condenser on the coil side.  The dealership did not have a condenser that had the connecting tab to I spiced the old one back to gether and reused it.  Question...Is a condenser necessary?  What does it do?

[mdsalemi]

I suspect that the ballast resistor does indeed get warm in operation.  If not, it would not be made so heavy-duty and not be made in a block of ceramic.  The ballast limits the current in the system, and in doing so draws a bit of it itself; hence the [heat-dissipating] ceramic.  Compare it to the tiny carbon resistors you see on circuit boards.

The condenser stores a charge and its function is to reduce the spark and arcing between the contact breaker points.

Of course with most electronic ignition schemes (Pertronix, Crane, or the 123 distributor) the ballast and or condenser are not required, depending on setup.  My Pertronix requires neither but both are left in place.

[waqas]

Indeed Michael is correct, but he is referring to the condenser on the distributor.  The interference suppression condenser referred to earlier is the larger one attached to the coil.

A condenser is simply a capacitor (also found on electronic circuit boards). This one is of course rated for automotive voltages and currents. In this case, the ballast resitor and capacitor form a simple RC filter at the input side of the coil. This RC filter serves to dampen the electrical spikes that are induced here due to the ignition process. These spikes end up finding their way throughout the on-board electrical network, including the radio (you end up hearing it on the speakers as a 'crackling').

I presume that MB installed this capacitor on cars that came with pre-installed radios.

[Dave Gallon]

Yes, the capacitor was fitted to cars with pre-installed radios. Also, there was an option (as supplied on my 280SL) for radio interference suppression for subsequent installation of a radio. MBNA was ordering cars that way for a time and then the dealer often installed a Blaupunkt Frankfurt radio. At this moment I have a 1967 250SL (not mine) and a 1968 280SL sitting in my garage which were both ordered by MBNA that way.

[wwheeler]

On my '68 111, I have an original condenser on the coil eventhough my data card shows the car was not equipped with a radio from the factory. A Blaupunkt radio is installed and I suspect it was from the dealer as Dave suggests.