problem truck

I am looking for advise on a 12.7litre detriot truck year 2000 FLD, the mechanic who went out said the engine came up with the fault saying Missing SLR's and had 255 counts. the mechanic cleared the fault and the truck started, he then drove it back until he was about 80km from the depot the engine started to sound that it was miss firing and producing black smoke when it . he cleared the faults which again showed Missing SLR's. he then managed to get back to the depot. we then changed the SLR and the SRS sensors. checked all the wiring and ran the truck around the yard no problems, sent it out on a trip and 200km away started to misfire and black smoke cutout again showed missing SLR and refused to start. the truck has done this a third time, every time shows missing SLRs I have checked and changed connectors. my next solution is to open the front timing cover and check the pins to see if they are worn. I would be open to any suggestions before I go this route. My apologies for any wordings or discriptions that may be wrong and hope every one understands what i am meaning