Classic/Antique Car Repair: mercedes injectors, stereo amp, ohm meter

Question
QUESTION: 0n my number 4 and 8 injectors the plug that plugs into the injectorhas no fire to it, the other 6 are fine what could this be?. it is a 1973 mercedes 450sl. also there is a box on the passenger side that looks about like a stereo amp it is made by bosche has tons of wires running to it and has a adjustment knob on it,what the heck is this? your answers to my questions yesteday was very helpful i left you great feedback and i thank you again,Jason

ANSWER: How are you checking the injectors for "fire"?

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QUESTION: i have got one of the testers with a long poited end and a ground on the other end and putting the pointed end in the plug that hooks to the injector while it is still plug to the injector. on the other 6 it will light up and flash as it was lighting as the motor hit on that injector, do you know what that box on the inside is, thanks again, jason

Answer
That appears to be the control module for the injection system. It sound like the transistor inside that box that controls the injector has fried. But check the injectors with an ohm meter and make sure that the two injectors in question are not shorted and drawing more current than they should. If they are they will destroy as many new boxes as you replace. All the injectors should have the same resistance.