Jumping Gauges

[m139h2otruck]

Have installed a new (to us) gauge panel in the Mack tractor, and now the temp gauge "jumps" between rest and actual reading or full & actual when running. Have also had this problem with the water truck on the oil pressure, when running down the road, it will peg but if you let up on the gas, it reads right. Never have been too impressed with the electrical gauges used in these trucks, and have tested everything for ground and correct output on the senders. All check OK. Even when so far as to install grounds on the back of the water truck panel from each gauge mounting stud to the truck body itself. Used a new oil sender on the water truck as the gauge was upgraded to the 120 psi unit and the gassers came with 60 psi originally.

Also, now with a new fuel guage, it reads E or just above E with the tank over half full. This may be a gauge problem, as it might not be matched to the senders, but the old coil type gauge worked, just didn't look very good.

Any suggestions? Again, have checked grounds and sensors for correct ohm output.

 

[cranetruck]

........... Even when so far as to install grounds on the back of the water truck panel from each gauge mounting stud to the truck body itself.
Any suggestions? Again, have checked grounds and sensors for correct ohm output.

Have you checked the ground all the way to the batteries?

My xm757 8x8 has a lot of improvements over earlier and possibly later vehicles as well and one is in the grounding of the instrument panel. The panel itself is electrically isolated from the dash to prevent, so called, ground loops and has a dedicated ground wire going directly to the batteries negative terminal without having to rely on any frame or body fasteners.

The oil pressure gauge must have a solidly grounded sending unit (= engine must be grounded).
The fuel tank must be grounded for the fuel level sending unit to work. Again, the xm757 feature no less than 3 different fuel tank related grounds, one "static" discharge to the frame, one for the fuel pump and one separate ground wire for the fuel level sending unit.

Regarding the fuel sending unit, I have had one case of a float that was leaking and filled with fuel was always showing "empty" because it sank.

Edit: The point is, (m656/xm757/xm791 designed in the sixties) the importance of proper grounds were recognized and something needed to be done about using the vehicle frame for all ground returns.

 

[m139h2otruck]

Will check grounds again, however, on the water truck, the starter and ignition both work OK, so I think it may be the grounding of the sender itself. On the tractor, there is good continuity between the tanks and the frame so I think the sender(s) and the newer gauge are not compatible. The temp gauge on the tractor worked before I changed the gauge, so, again I guess I have to recheck the grounds.
Thanks

 

[cranetruck]

You need to check continuity all the way to the negative battery terminal, not just to the frame.....