on a 98 s10 engine is there two temp sending unit
looks like i have one next to the thermestact and another on the driver side manifolds?
i have a 99 chevy blazer that i swap engine on it from a 98
but the connection to the temp unit are both the same!

 

The earlier engines had two sensors, one for the gauge and one for the PCM. The later engines only had one. You should use the sensor from the 99 in the driver side cylinder head and either plug the other hole or just leave the sensor from the 98 in the hole.

 

the engine that i have now has two sensors with two wire plugins
but my wiring harness only has one two wire plugin
i have plug it in and guage reads no higher 129
i did change my thermastat wondering if im doing something wrong

 

i have 2 temp sensors on mine.. first one coolant temperature sensor in the intake with (black connector end) and the other one is called a temperature switch in the driver side head has a (light grey end)

 

Reelbroke2,
could you please take a pic of them for me
i wounder what the temp switch is for?
what color wires are they?
does anyone have schematics on this?

 

Like I said, swap over the sensor from your old engine into the same location on the replacement engine. This should result in a properly reading gauge. The sensors may be different resistance ranges for the given engine temp range.