Temp gauge not accurate

[Fred Mc]

I replaced the stock temp guage with an aftermarket one and find it inaccurate as compared to the temp guage in the engine compartment. Its reads 35-40 deg lower than the mechanical one. Is this simply a function of old wiring in the bus(gmpd4106) or some other problem.
Thanks

Fred

[Iceni John]

What make of gauge and sender is it?   You did change the sender as well, yes?   Temperature gauge senders need good electrical contact with the engine block and ground, so you shouldn't use Teflon tape on their threads, just a thin smear of paste or grease.   Are all the connections and terminals clean, and all the grounds good?   Running a temporary new wire to bypass the original wire could tell you if the original wire is the problem.

John

[richard5933]

I'll ditto the comment about the sending unit. I ran into many problems trying to add modern gauges to my old bread truck, and ran into constant problems with weird/inaccurate readings.

The truck had started with a Ford V8 and was later converted to a Cummins. No one had ever bothered to change the sending units on the engine when they did the swap. Unfortunately, by the time I got the truck sending units were not easily found which would work with 30+ year old Grumman gauges. Ended up changing out the whole works.

Bottom line is that I've found it best to always replace the sending unit at the same time as the gauge, and always using a sending unit either recommended by the gauge manufacturer or made by them specifically to match the gauge.

Richard

[Fred Mc]

Thanks.