[marq]

I have a cummins engine and when I am under a trailer I lose power going up hills and now it will only run 66 miles an hour. The speedometer is not working now when it once was. Any ideas what is wrong?

[Shnerdly]

My first thought would be a fuel filter or a fuel line collapsing inside.

[marq]

you got any thoughts about the speedometer not working

[Shnerdly]

It could be any number of things.

Bad Ground on the Speedo head or a bad speedo head.

Broken wire.

Bad sender at the Transmission.

[Kranky]

If the Cummins is an electronically controlled engine, then I would suspect a problem with the VSS (vehicle speed sensor) or it's associated wiring.

A problem with the VSS will cause the ECM to go into "limp mode" (HP derate) and will also cause the speedo to quit working, as there is no vehicle speed data being sent to the ECM.

The VSS is located at the transmission tailshaft, right where the old type speedometer cable would have been if it was a mechanical type setup.

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