Gilly,
A '74 SL would be D-Jetronic, not K-Jetronic. CIS wasnt used till about '76 or therabouts. OB Joe has enough to keep him concerned without confusing him further
Joe,
Bad connection would be my guess Joe - that big wide sucker back on to the ECU main board would be where I would start with some contact cleaner. If its like all three of my D-Jet cars there will be oxidisation all over the contacts. Theres no intermediate connections between injectors and ECU so if it isnt there then its within the ECU - best of luck.