Stan, While all this seems neat, where does it lead? There are no schematics to isolate a component on the board as an issue. Wouldn't it be easier to pull all the harnesses except the 7 pin (power and tach) and the 5 pin (motor) and see how it runs. This eliminates those possible grounds in lighting, smoke and sound. If the engine runs fine, then install harnesses one at a time until problem comes back.
If still bad, replace the tach reader, though this should also cause sound issues. If still bad, and you are sure nothing mechanical, then replace the processor board. G
Hello G,
Well, hopefully all this leads to a smooth running SD70. The reason for my "neat" experiments is because I don't have access to his engine in which case I'd use other tools and methods. So I'm suggesting experiments which should be easy and relatively quick to perform. Based on the mechanical experiments others have suggested, I'm convinced there is an electrical problem.
If he were to perform no further experiments, harness removals, or whatever, I agree it's time to start swapping board(s). I don't mean to suggest trying to isolate to a specific component.
In John's first post, he suggests the problem is the tach reader. I haven't given up on that either. My last suggested experiment was to move the 3-wires to the tach reader. As you know, the tach reader is just an LED bouncing light off the stripes back to a phototransistor. With no schematic necessary, my logic is the current-transfer-ratio of photodetectors like this is less than 100% so the level of currents flowing in the tach wires must be just a few mA. If you run these wires next to high-current wires carrying pulsed currents 1000 times greater (a few Amps under heavy load), I'm speculating that extra tach pulses could be induced causing the engine to slow down under speed control. As shown below, in a diesel but not Premier SD70, the 3-wires (blue, org, grey) from the tach-reader go down the motor and co-mingle with the 2 motor wires (ylw, wht) and disappear under the boards. I'm suggesting he redress the 3-wires so they run by themselves, physically separated, direct to the tach reader - but I don't know if that's practical to do on his chassis.
