Here is some info I have found and saved for those who are looking to add a gauge.
Several people with LT1/LT4 powered vehicles have noticed that the temperature gauge reading seems to fluctuate greatly, depending primarily on whether the vehicle is moving or not. It has also been noticed that the PCM indicated engine temperature does not always agree with what the gauge shows, leading many to believe that their instrument cluster temperature gauge is inaccurate or faulty. I can assure you that in most cases the temperature indicated by the gauge is absolutely and totally accurate, for the POSITION on the engine at which the gauge sending unit is MOUNTED!!!
To understand this, first note that there are two temperature sending units on the LT1/LT4, one for the gauge and one for the PCM. The one for the gauge is mounted in the side of the cylinder head, while the PCM sending unit is mounted on the front of the engine directly in the water pump. B/D-cars (Impala, Caprice, Roadmaster, Fleetwood) and Y-cars (Corvette) have the gauge sender installed in a tapped hole in the rear of the RH cylinder head, between the #6 and #8 spark plugs. F-cars (Camaro, Firebird) have the gauge sending unit mounted in a tapped hole in the front of the LH cylinder head, between the #1 and #3 spark plugs.
Now remember that this engine is reverse flow cooled. This means that cold engine coolant entering the engine from the radiator is directed to the heads first, which is exactly why the gauge sending unit reads low compared to what the PCM sending unit sees. The PCM sending unit sees actual engine coolant temperature, and is not hit by the flow of cold coolant directly from the radiator.
If there is a problem with the gauge at all, it is certainly NOT that it is inaccurate. The gauge is perfectly accurate, but the SENDING UNIT for the gauge is mounted in the WRONG PLACE to get a "correct" reading. Unfortunately there are no other tapped holes in the water jacket to mount it unless you drill and tap another location. There is a tapped and plugged hole in the opposite cylinder head, but that would offer the same gauge readings, in fact it might read even lower temperatures since the tapped hole in the LH head it is mounted more towards the front of the block, closer to where the coolant first enters the heads from the radiator.
It has been verified with scan tools which can directly read the PCM indicated temperature, as well as a digital Cyberdyne gauge sending unit mounted in the stock gauge sending unit location, that the two stock temperature sending units (gauge and PCM) will read temperatures as much as 60 degrees different while moving at high speeds on a cool night. If you stop and let the car idle long enough, eventually the radiator coolant temperature in the head equals what the actual coolant temperature in the block is, and both readings are relatively the same. As soon as you start moving, the temperature read at the gauge sending unit in the head will drop rapidly, however the actual engine temperature, and that which is sensed by the PCM sending unit, remains about 10 degrees above the thermostat setting.
Note that you cannot run both the gauge and the PCM off of the same sending unit. The gauge sender is a simple one wire sender where the body is grounded to the engine block. I believe it runs on 12v through the gauge and sender to ground. The PCM sender is a two-wire sender with a 5v signal and a digital PCM ground (not chassis ground). The gauge cannot be run off of this sender without messing it or the PCM up.
In any case, this anomaly CAN be corrected, in fact I will be getting around to doing this "MOD" very shortly.
The proper way to correct this temperature reading anomaly is to mount the gauge sending unit in the water pump housing, in the same water passage as the PCM sender is now. There are two ways to correct the problem. One would be to drill and tap a hole in one of the existing unused sending unit bosses in the water pump to mount the gauge sending unit in the same water passage as the PCM sending unit is located. With the gauge sending unit in the water pump, instead of in the LH cylinder head, the gauge will now read correctly in concert with the actual engine temperature, which is also the temperature the PCM sees. This method requires that a proper sized hole be drilled, tapped, and then the gauge sender can be installed. Then the existing sender wire must be lengthened and re-routed over to the new location.
A more elegant method of correcting this problem would be to simply replace the existing PCM sender with a new dual purpose sender that GM has recently been offering on other applications. This is a new GM 3-wire sending unit which includes both the PCM and Gauge senders in one unit, and which installs in the existing PCM sensor location.
GM has begun using a 3-wire dual purpose sender on some new engines. It combines both the PCM and gauge senders in one unit. This is ideal as no additional holes will have to be drilled or tapped, this new dual purpose sender can replace the existing PCM sender.
Here's the information on the 3 terminal coolant temperature sensor.
Part No Description Cost
10096181 PCM/Gauge sending unit $14.81
12102748 Connector pigtail (incl. terminals) $17.29
This connector pigtail is a complete connector with terminals and wires installed, which would have to be spliced to the existing wires.
Another alternative would be to get the connector and terminals plus wire from a junkyard. It's the same connector that is used on the throttle position sensor of the LT1 and many other engines.
The new sending unit is wired as follows:
Pin A = Black, PCM sensor ground
Pin B = Yellow, coolant signal to PCM
Pin C = Dark Green, to Gauge
In summary, the temperature differential in the Impala gauge vs. PCM sending units is the fault of the location of the gauge sending unit, and not the gauge or sending unit itself. I would consider this a design anomaly (nice word for bug or flaw ) which fortunately can be easily corrected, and which of course does not affect
UPDATED INFO -
Auto Zone Pigtail Number 417 is equivelant to GM#12102748,
Auto Zone Sensor TU178 is equivelent to GM#10096181