91 Ohms is a little high for an OEM meter. Now replacement temp meter resistance can be anything depending upon design. The reason for this is that the inside of the meter has a bi-metallic movement that moves when heated. The heat is supplied by a resistance wire wrapped around the bi-metallic bar that measures 21 ohms. As the wire heats up, the more the bi-metallic bar moves. A new meter may have a more sensitive movement and only need a 91 ohm heater.
This is why they usually recommend replacing both the fuel gauge and the temp at the same time by the same manufacturer. Does your fuel meter also measure 91 ohms across the terminals?
You should be able to read the resistance of the sending unit with an ohm meter when it's in the block.
It's probably the best way to check the sending unit.
Disconnect the ignition coil
Turn the ignition ON
Check to see if Temp Meter is pinned, if pinned, turn ignition off.
Take the wire off the sending unit.
Turn ignition ON if turned Off
Check to see if the meter is still pinned , if pinned, turn ignition off
IF it still pins you have a short some place.
______Try take the wire off the back of the temp meter and see if it still pins.
Still pinned then you hooked something up wrong or there is a problem with the meter
A common problem is to have the meter rotated a bit in the housing and short out one of the terminals.
To check Resistance of Temp Sending Unit
Ground one lead of the DVM to the block.
Touch the other lead to the terminal on top of the sending unit.
73 ohms Cold - Test when engine is slightly warm. A stone cold engine will read 400 ohms
36 ohms Beginning of Band
13 ohms End of Band
9 ohms Hot
Resistance across OEM meter terminals= 21 ohms
To make the temp meter peg you will need to have near 0 ohms.