75 450 correct coil replacement

[joedee]

Hi, Replaced the coil in my 75 450sl. The new Bosh coil had the same markings as the old one B U T the engine would not start after a couple cranks; then a BIG explosion that shook windows and scared the neighbors. Put the old coil back and all is ok. What did I miss??? There are ceramic blocks that I guess reduce the voltage mounted on the firewall and I was told that maybe the new coil had internal resistors.

Need some help to identify what the situation is.

Tks,
Joe R.
Maryland

[GaryF]

Sounds to me like the coil had no discharge circuit on the HT side and eventually made its own which also made a big bang. Did you definately have the HT lead to the distributor pushed all the way into the central coil connector.
Alternatively, the wiring around the coil changed through '73, '74 and '75, have you definately got the right coil for your year ?

[Engr693]

I know that I read somewhere that these cars require the "blue" Bosch coil. I don't remember the reasoning behind it, just that other coils would not work correctly.

[Tomguy]

Measure resistance on the new vs the old coil, you may need to bypass resistors.

The "Blue" coil is NOT the right coil for these. The outside of the NOS ones are blue, the newly manufactured ones are I believe "Yellow". Blue coils have about 2 ohms*, the proper coil is under 0.8 ohms*.

* - If memory serves correct. I can check my "Blue" coil with my "NOS" coil

[Engr693]

Originally Posted by Tomguy

...The "Blue" coil is NOT the right coil for these. ...

I have a 1973 450SL and I do have the blue coil. My 72 parts car also has the blue coil...

If there is a break on which coil for which year car...that should be the decider...

[joedee]

Thanks for the info. The newer coil is blue like the origional and the ohm readings are the same for both coils.

Will check ohms again and try it one more time.

Thanks again,
Joe R.