Anyone fimilar with 9400i 870cm

[Mr Hagg]

Ok so depending on the alternator you have will depend on what the out put is. What I would do it at the alternator it self see what the voltage is coming out. Then check it in the cab. You will see a drop of like .2 or so thats ok.
The fault code you posted quick serve says its a clock battery.

SPN FMI Lamp Color Description
251 2 Maintenance Real Time Clock - Data Erratic, Intermittent, or Incorrect
Effect
None on performance. Data in the ECM will not have accurate time and date information.
Component Location
The real-time clock battery is secured to the engine harness near the ECM. Refer to Section E for a detailed component location view.

Under ordinary operating conditions, the real-time clock in the ECM is powered by the vehicle's batteries. If battery power to the ECM is lost, then the real-time clock will be powered by a battery in the engine harness (if equipped).

Ive ever saw what its speaking of but there is a jumper from the 4 pin connector to ecm, If you look upper left of ecm you will see the square 4 pin plug. follow wires back to battery source. About a few inches you should see a break out in it with 2 wires plug going back into ecm harness that will go to pins 35 and 47 on the engine harness side. Check volvage there.I have found in the past that where the wires split off will have a bad connection and drop voltage....