Temperature Sending Unit

[BlueBaron762x39]

I recently ordered and installed a temperature sending unit from Lectric Limited in my 1979 Trans Am (Olds 403).  I'm not sure how old the sending unit that was previously in the car was (I don't think it was original) or where it came from.  With the old temperature sending unit the gauge usually read between 180 - 220 degrees.  Now it's reading 220 - 240 degrees.  The entire cooling system on this car is brand new.  I'm assuming the temperature gauge in the car is original but don't know for sure.  Should I get a new reproduction replacement gauge for it and see how it reads after that or is it unlikely the gauge is faulty?  Has everyone found the Lectric Limited temperature sending units to be very accurate?

[nUcLeArEnVoY]

I recently ordered and installed a temperature sending unit from Lectric Limited in my 1979 Trans Am (Olds 403).  I'm not sure how old the sending unit that was previously in the car was (I don't think it was original) or where it came from.  With the old temperature sending unit the gauge usually read between 180 - 220 degrees.  Now it's reading 220 - 240 degrees.  The entire cooling system on this car is brand new.  I'm assuming the temperature gauge in the car is original but don't know for sure.  Should I get a new reproduction replacement gauge for it and see how it reads after that or is it unlikely the gauge is faulty?  Has everyone found the Lectric Limited temperature sending units to be very accurate?

That's very odd. Lectric Limited is known for their correctly-calibrated sending units. I use one in my car and you can see the temp rise as the car warms up until around 15 degrees after thermostat setting (160) and then it starts to fall down to around 160 on the dot as the stat opens and flows coolant, which indicates it's pretty accurate to me.

Only way to know for sure is verify engine temps with an infrared gun aimed at where the sending unit is. If it is inaccurate, maybe call Lectric Limited up and get their advice.

[ryeguy2006a]

I'd check the ohm reading of the wiring from the engine block to the temp sensor. I'll be there is a bad connection somewhere that is causing your issue.

[BlueBaron762x39]

I'd check the ohm reading of the wiring from the engine block to the temp sensor. I'll be there is a bad connection somewhere that is causing your issue.

Do you know where the ground is for temp sensor?  There is something else odd that it does.  After the temp gauge reaches between 220-240 degrees, If I shut the vehicle off, wait about a minute, then turn it back on, the temp gauge reads 180 degrees and then slowly climbs back up to 220-240 degrees as the engine runs.  It does this consistently.  Does this pattern help indicate a particular problem I should look for?

[nUcLeArEnVoY]

I'd check the ohm reading of the wiring from the engine block to the temp sensor. I'll be there is a bad connection somewhere that is causing your issue.

Do you know where the ground is for temp sensor?  There is something else odd that it does.  After the temp gauge reaches between 220-240 degrees, If I shut the vehicle off, wait about a minute, then turn it back on, the temp gauge reads 180 degrees and then slowly climbs back up to 220-240 degrees as the engine runs.  It does this consistently.  Does this pattern help indicate a particular problem I should look for?

That is odd. It should read HIGHER right after the car is shut off since the coolant will increase in temperature since it's no longer circulating, and then the temp should start to drop immediately once the car is started and the water pump starts to move coolant.

Those sending units are grounded through the head or manifold in which they're threaded into. That's why you *DO NOT USE* any thread sealant or teflon tape on the threading of those sending units, since it may disrupt the ground and mess up the reading. It's NPT threading: it's designed to seal with just metal-on-metal contact, and if it doesn't, then something is wrong with the threads on your head or manifold. Just in case, it may be prudent to take a wire brush drill attachment and run it along the threads in the head or manifold to clean them up to help support the best ground possible.

You can test your gauge in one of two ways: max resistance and no resistance. First is to test max resistance, and that is simply by inserting your key into the "ON" position and disconnecting the connector off the sending unit. The gauge should bottom out to zero. Then, test no resistance by disconnecting the connector from the sending unit, and then linking a double-ended alligator test clamp wire to the metal female spade on the connector while clamping the other side to a ground somewhere. Key still needs to be in the "ON" position. If your gauge is good, it should then read maxed out (the needle should be pegged all the way at the opposite end of the gauge from the first test, even past the max temp reading - you may have a hard time even seeing the needle anymore). If your gauge reacts as described in both of these tests, it's not your gauge or the wire.

[DynoLee]

I bought one of the high-$ Lectric sending units, and it read just like my cheapo AutoZone sending unit.  Used laser temp gun and a Fluke multimeter to measure ohms/actual temp at various temps, then repeated with the AutoZone unit - VERY close, and both caused the factory gauge to read excessively high. 

I later played around with a bunch of resistors, and found a series that made the gauge read "180" when the temp gun said the motor was 180.  At least I knew if I was over or under 180 at that point.

Then I pulled the gauges ALL out, sold them, and installed a set of New Vintage USA gauges.  I can finally trust the readings on the gauges!

[b_hill_86]

Can you also confirm the gauge is original to the car and reading approximately? Lots can change over 40 years and there is a difference between sensors 79+ and pre 79 I believe. I too recently switched from an u known source sensor to a lectric limited and in comparing my factory gauge to an IR thermometer pointed at the water neck and a mechanical gauge, the lectric limited sensor is about as close as you could ask for.

[BlueBaron762x39]

Can you also confirm the gauge is original to the car and reading approximately? Lots can change over 40 years and there is a difference between sensors 79+ and pre 79 I believe. I too recently switched from an u known source sensor to a lectric limited and in comparing my factory gauge to an IR thermometer pointed at the water neck and a mechanical gauge, the lectric limited sensor is about as close as you could ask for.

I've owned the car the past 10 years but couldn't say for sure if anyone changed the gauge before I bought it.  It appears to have the correct gauge face/markings for a 1979.  Do you know for sure which years were calibrated differently?

[b_hill_86]

In comparing the part numbers on lectric limited’s site, 79 looks to be the change. If you have the factory service manual it tells you the correct resistance values for the gauge in order to test. It’s also possible you have the correct gauge and it’s just lost it’s accuracy for some reason.

Also, as far as your temp reading dropping after you turn off your key then slowly raising once you power on, that may be normal. I say “may be” because I’m only comparing against my gauge, which does the same thing, and I haven’t had another Firebird to compare to. True, when your engine is off after running it will heat soak and should cause the gauge to raise after the engine is off (my mechanical gauge does) but my factory gauge acts the same as yours. I can’t say for sure but I imagine it’s just by nature of its design and how it works.

You could test the gauge with a variable resistor, also called a potentiometer. You’d need to know the correct resistance values for that gauge though. If you haven’t already you could also call lectric limited and tell them your issue. Since they advertise as being one of the few with accurate sensors maybe they would exchange yours for a different part number.

[formula jg]

Is there a way to test the water temp gauge on its own?

[formula jg]

It’ the original gauge.

[DynoLee]

I bought a Lectric a couple years ago, and it was way off with my gauge - reading excessively high.  I contacted them, they didn't seem to care.

I had also bought a sender from AutoZone, for MUCH less money.  I measured resistance (ohms) for both sending units at different temperatures (checked with a calibrated infrared temp gun), and they were both VERY close.

I bought a pack of various resistors from Amazon.  I tested different combos until I found a set that that made the gauge read "180" when I was getting 180 on my temp gun.  Soldered the resistors together, covered them with shrink tubing, then installed between the sender and the gauge.  (later yanked it all, and got a New Vintage USA gauge set that is very accurate).

[firebirdparts]

Is there a way to test the water temp gauge on its own?

If you give it power and ground, nothing to the sender connection, that should read full cold.  Then, as you reduce resistance to ground through the sender connection, you can make it read whatever temperature by adjusting the resistance.  The easiest test is zero ohms (don't need anything but a wire to do that) and when you do that, you'll get off-the-scale hot.  That shows you that the gauge works, but you'd need some test resistances to see just what it points to at different resistances.

[formula jg]

firebirdparts, can I test “full cold” reading using  a 9v battery to connect power and ground to gauge?

[formula jg]

With water temp gauge removed from instrument cluster I tested it to direct voltage and the needle sweeps full hot so I’m fairly certain gauge is good. I found a break in the printed circuit board path for the temp gauge so this must be the reason the gauge stopped working, ordered a replacement circuit board and hopefully that resolves this issue.

[tiggershark]

The factory gauge pictured is what went bad in my car. Driving the car (79 T/A) it read 180 and one day it crept up to 260. I shut it off and coasted. Restarted and turned around. All this in 3/4 of a mile. New temp sender right away. Same thing.   slowly to 260, odd it didnt over heat and the top hose was hot/warm, but not excessively. I even checked water pump for an impellor that might have fallen off, nope.  I had a bad oil pressure gauge needle, it broke off and thats part of the temp gauge. I had an extra gauge cluster from a bone yard that was on my shelf for at least 20 years. I replaced the gauge(s) and it read 180 and a bonus an oil pressure needle that was "full length" The oil pressure needle bounced so violently that it broke, it was a bad oil pressure sending unit *( a new one at that) So a similar problem as yours.