Even after a plodding along in rush hour here in FL my temp gauge never gets above 1/3 and mostly sits at bottom range.
I put on a lower resistance sensor and got the needle to rise almost to the top just to verify the needle works.
Temp compensator board has been bypassed.
The engine fan runs as soon as the car is started. Normal?
Engine bay certainly feels hot.
Resistance checks out on new sensor. (Measured hot and ambient)
Both coolant hoses feel hottish, I will measure temp though

Looks like t-stat is stuck open, right?
Is the fan supposed to run constantly?

 

Your car has belt driven fan, and an auxiliary electric fan if memory serves. The belt one is ON all the time, the electric when A/C is on, and when the temperature reaches a certain point. Do you mean the electric fan is ON all the time?

Thermostat stuck open or not, the car should reach normal operating temp.
Measure reliably the actual temperature before anything else!

 

My '90 240 doesn't have a secondary electric fan, was that an option?

It tends to run real cold too and it isn't bypassed, only in 90+ degree weather with the AC in traffic on does it ever get warm.

fyi the skid-tray and a lower turbo air "scoupe" are in place, sucks in a ton of air when moving.

 

So resistance is where it should be at the sensor. The needle works I can do the wire to ground test to prove it.
I haven't had a chance to check for an electric fan but I dont think I have one.

When I pulled the sensor the first time about a 1/2 cup of antifreeze came out. The second time, nothing.
I work in HVAC so there is no shortage of accurate temp taking devices. What should the outside of the block read after being fully warmed up? Around 190?

 

Definitely only one fan (pulley). I have to see if there is a t-stat in there. Its my only hope!

 

Changed out the t-stat... No change. Ran all new wire to the first harness.... No change.

Is there a way to test the gauge itself?
Could I have a turbo gauge that uses different resistance measures?

 

I should mention that once you bypass the commentator its perfectly normal for your temperature to read low. Only rime it reads hot on my car is when its 90 degrees out in traffic with the A/C blowing.

 

I can't get the needle past 1/4 even in 90+ sitting in traffic. Also my dash lights seem really dim is it a sign of a bad ground? Where is the ground for the gauges anyway?

 

Behind the cluster there is a circle of wires for your dash lights, unplug and clean the prongs up. I dunno if its the "ground" but its what makes your lights work.

 

get or borrow an IR thermometer, and read the temperature on the side of the engine block under the intake manifold, also the thermostat housing. if its under 80C when the engine is fully warmed up, something is fubar.

 

Engine block gets up to 100C+. There are a bunch of different temp sensors with very different resistance ranges. Never did find the range specifically for an 86, but an educated guess looks like my sensor is reading the same. BTW the sensor is from IPD and the one I replaced says the same.
I will check the connections on the gauge, otherwise there has to be a different gauge or mine malfunctioned.

 

FWIW I think I got it licked. Looks like my comp board bypass wire wiggled loose.... All seems good now. Except this last time I went to start her I got my starter tooth grind sound....