Medium and Heavy Trucks
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Do you have the engine serial number
Could you get the serial number info from the tag on the valve cover. May be a bear to get to. Is the engine a 3126, 3126B, 3126E?
I have attached a 3126E schematic. As far as the truck side the easiest way to check the gauges is to see if you have voltage coming to the gauges from the engine ecm.
How about now
The sensor in the timing cover is the speed timing sensor. The sensor that drives the tach is in the flywheel housing(pin 36 at the ecm). The sensor for the speedo is in the tail shaft of the transmission(pin 38 at the ECM). track the wires from the ECM to the connector at the firewall. I have seen failures of the dash cluster that kept the speedo and tach from working. If you have signal to the cluster and it still doesn't work then the cluster needs replaced.
10-4 I would say you're onto the problem. With low voltage signals the least amount of resistance can cause a component to have problems. I look forward to know what you find.
Yes sir the info is the same. Pin 36 wont have power until the engine is running. Pin 38 will have power when the vehicle is moving.