Ignition resistors and HEAT!! - PeachParts Mercedes-Benz Forum

PanzerSD · #1 07-16-2014, 09:25 PM

Just wondering. 79 450 SEL, with engine off but key on and ignition on are the resistors supposed to get really hot? I know they're resistors but like hot enough to burn paint?

Idle · #2 07-18-2014, 02:53 PM

If you are speaking of a ballast resistor, and I bet you are, their function is to reduce the amount of voltage going to the coil. You don't need the ballast resistor for your car to run, but you will burn through a set of contact points in just a few hundred miles if you don't have one.

Since they provide electrical resistance they get hot. On a basic level they are nothing more than an electric heater set to a very low power.

So if your's is getting that hot it sounds like it might be time to swap it out for a new one. If that does not work then you will just not be able to leave your key in the run position with the engine off since the ballast resistor is doing its' job which is to cut current to the ignition coil that is not needed.

And if your car is sending voltage through the system and it is not getting used it is going to turn to heat somewhere and the resistor is the logical place for this to happen.

Tomguy · #3 07-18-2014, 07:08 PM

I thought the 79s didn't have points? I am guessing yours does though. The key should never be left in "Run" with the engine off on these engines. It will kill the resistor and/or the ignition box.

Idle · #4 07-19-2014, 01:50 PM

Quote:

Originally Posted by Tomguy

I thought the 79s didn't have points? I am guessing yours does though. The key should never be left in "Run" with the engine off on these engines. It will kill the resistor and/or the ignition box.

The resistor is used on cars without points as well. They serve the same purpose: To cut voltage to the coil to around 9v since that is all that is needed to keep the engine running.

By the way.... You can drop to 7.2 volts and still run. Your engine will run poorly and at 7.1 it will quit. This will take place when one of these resistors starts to break down and choke off the volts to your coil. If the resistor gets too hot it will break down and 'melt' like a fuse, but unlike a fuse it will fail not because of too much current draw from a short in a component but from just being old and having too much current passed through it for too many years.

Also unlike a fuse you can see if a fuse is burned through. You just have to check a resistor with a volt meter to see if current is flowing through it since they are normally sealed in a ceramic housing.

On cars with Hall Effect or optical devices in place of ignition points nothing will burn out but the resistor will still get hot when the engine is in the run position if there is no draw on the current being presented to it.

PanzerSD · #5 07-19-2014, 07:27 PM

Quote:

Originally Posted by Idle

On cars with Hall Effect or optical devices in place of ignition points nothing will burn out but the resistor will still get hot when the engine is in the run position if there is no draw on the current being presented to it.

Well here's what's happening. Engine running or not, these resistors approach 300*F in no time. The insulation on the wires connected is dry and baked. The terminals are rusted anyway, so the vote is to replace. They function correctly but throw an awful amount of heat. I'll find out is new (maybe better designed) will run cooler.

Idle · #6 07-19-2014, 09:52 PM

I think new is the only option. It has broken down inside and is offering too much resistance, and this resistance to the electrical flow is being thrown off as heat.

But this part should be cheap as Mercedes parts go. Let us know how the replacement works out.