MGB Temp sender issues

[Stewart]

Blew a head gasket on my 75 and had the head checked over by the machine shop which tanked the head magnafluxed it, skimmed it and fixed a few valves that were leaking. Head went back on runs great but the temperature gauge reads low 1/2 way between the C and the N. Previously ran dead nuts in the center. After 200 miles it dies completely and verified dead by my ohm meter.

Replace with new red topped b710 one from moss gauge works again however when the engine is at room temp the needle now goes to 1/3 of the way to N. With the engine running it goes 4/5ths of the way to the H while IR show the engine at 170 175F.

Changed it out again with another one at room temp its a needle or so above the C and it's a bit better at 1/2 way between the N and the H.

At this point I switched to the Standard Motor Products TS233 sender and at room temp its now where I would expect it to be with the ignition on however it's still reading high at 1/3 or the way between N and H at 175 degrees.

Ohm red top 450 at 75f 90 at 175f
Ohm TS233 540 at 75f 120 at 175F

Any Ideas on what's going on? Gas gauge is working normally and is accurate.

 

[JPSmit]

Might be as simple as reproduction senders being calbrated differently. Can you test them in a pot of boiling water? Or alternately buy an infra-red thermometer and check engine temp.

 

[Stewart]

Engine temp is right at 170 175

 

[DrEntropy]

Put a potentiometer in line with it and "tune" the resistance to match the normal position of the gauge when temp is at 170°~175°f. Ohm the pot at the appropriate position and get an equivalent resistor to put in line with the wire behind the gauge.

:thumbsup:

 

[Stewart]

Took a shot before messing with resistors and put in the wrong sender from '77 on. Well guess what its now reading dead on right go figure. At least I know the whole system is in great shape as I went through everything including checking the calibration of the temp gauge.