What's this?

[garyni]

Apologies but I've recently found the gizmo (circled in the pic) in an old box of spares I got with my 230sl 14yrs back - It's located under the hood stay but can anyone let me know what it is and does pls?

[Bonnyboy]

That is your ballast resistor  - I understand that it is placed in the primary ignition circuit to lower voltage to the coil after the engine is started to reduce wear on ignition components.   

I have seen a couple pagodas with these cracked due to the hood stop springy thing popping out of the bracket and smacking the resistor when gas jockeys try to open the hood too much releasing the spring.    I made a little metal guard for mine to protect it. 

[garyni]

Now it makes sense...One of the first jobs I did to the car was to change over to a 123 electronic ignition system with a flamethrower coil - I'm wondering if the guys who fitted it removed this resistor and popped it in the trunk with some other spares? - As it's contactless I probably don't need it? - Thanks for the reply.

[ja17]

Yes, not needed with the 123 distributor. These are color coded for the resistance value. Different resistance is used depending on the type of ignition you may have. Early standard ignitions used the 0.9 ohm with a silver strap, You will also see the resistance value stamped in the strap itself. The later factory transistor ignitions used two blue ones 0.4 ohm and 0.6 ohm. The latest upgrade for the standard ignition used the red coil and a red 1.8 ohm ballast resistor. Be careful to use the correct coil with the correct ballast resistor or your points will not last or your spark may be weak. These ballast resistor housings are ceramic material because the resistance coil wire inside can become red hot and generate a lot of heat. Cracks in the ceramic material are usually a result of the heat. The ballast resistor will usually still function despite the cracks as long as the coil is still intact. However when cracked, the coil is exposed to moisture and may rust causing it to eventually break and fail.

[mdsalemi]

Yes, not needed with the 123 distributor...Cracks in the ceramic material are usually a result of the heat. The ballast resistor will usually still function despite the cracks as long as the coil is still intact. However when cracked, the coil is exposed to moisture and may rust causing it to eventually break and fail.

When I upgraded to the Pertronix system, one does not need the ballast resistor in that either. HOWEVER, rather than remove it, I use one end of the resistor as a tie-point for various wires in the ignition circuit.

Also, I had cracked resistors...I was able to purchase new ones (Beru brand) from a number of sources online, all very reasonable in price. (I believe my system initially had two different resistors unless my memory fails me). Also as I recall, the resistor coil was exposed from the back leaving the coil somewhat exposed anyway. It is quite possible that after decades of use, the potting material may have fallen out...

[wjsvb ✝︎]

Those of us who used to drive old Chrysler products in the 60's & 70's will remember carrying replacement ballast resistors in the trunk for when they (inevitably) failed on the road.  Still available and cheap, though I don't know if a Mopar one would work.

[Jordan]

When I upgraded to the Pertronix system, one does not need the ballast resistor in that either. HOWEVER, rather than remove it, I use one end of the resistor as a tie-point for various wires in the ignition circuit.

That's odd.  I too have the Pertronix set up but it is wired into the ballast resistor.  One wire going to the coil and one to the ballast resistor.  I'm pretty sure that is what was given in the instructions when I installed it a few years ago.

[mdsalemi]

That's odd.  I too have the Pertronix set up but it is wired into the ballast resistor. 

Don't know what to tell you. Installed mine in 2009 or so, been running like a champ ever since. Using the Flamethrower black coil. No ballast. Remember I have a different distributor and ignition than a 230SL.