[cjatx]

I've never been able to get the stock temp gauge working right in my car. I replaced the engine harness and the temp sending unit. Prior to starting the car the gauge sits at about 125 degrees. When I start the car, it drops to zero, but in a couple minutes it's at 250 degrees. With the car warmed up, readings at the sending unit with a temp gun show between 175 - 185 degrees. I'm using a 180 degree T-stat.
I'm wondering if I have a bad ground somewhere or a bad gauge. Does anybody have any experience with this?
Thanks,

[67 Lamnas]

Originally Posted by cjatx

I've never been able to get the stock temp gauge working right in my car. I replaced the engine harness and the temp sending unit. Prior to starting the car the gauge sits at about 125 degrees. When I start the car, it drops to zero, but in a couple minutes it's at 250 degrees. With the car warmed up, readings at the sending unit with a temp gun show between 175 - 185 degrees. I'm using a 180 degree T-stat.
I'm wondering if I have a bad ground somewhere or a bad gauge. Does anybody have any experience with this?
Thanks,

It sounds like your sender isn't providing any resistance to ground after the coolant starts to warm up.

Unplug the wire to the sender with the key on, and the gauge needle should drop all the way to cold.

Then, with the wire unplugged and key on, you can touch the wire to ground with the key on, but that can be very hard on the gauge/needle as it will slam into the stop peg on the high end of the gauge. So, a better solution would be to touch the wire to a 45 ohm resistor that is grounded. Doing this should peg the gauge to the hot side (or come very close) without hurting the gauge.

Probably 99% of all the commercially available sending units from parts stores WILL NOT allow your gauge to read properly. They are designed for lates 70's and later cars, even though the application book says theyll work for rthe 67-74 cars.....they don't and are not accurate. The easiest and cheapest thing you can do is to order a temp sender from these guys:

http://www.lectriclimited.com/mainpage.htm

If your gauge still doesn't work properly, it might be the printed circuit for the gauge (they DO go bad) or the gauge itself.....also not uncommon for a 35-40 year old component. After that about all that's left is the wiring in the dash harness.

Try the easy stuff first.

[Pontirag]

before you condem it think of a few things,

First, its critical that the sender be compatable with the guage.

compatability is measured in ohms, best not to mix and match but ohms in sender must e same for what the guage needs.

second, grounding is critical between sender and engine block. tephlon tape can cause grounding interference, as can sevel other types of sealants.

I prefer indian head gasket shellac.If your a neversieze kinda guy then do not use the silver stuff, use the copper colored stuff, it conducts electicity.

third, make sure the sending unit is actually down in the water/coolant. new senders some times do not thread all the way in and can seal up alright but if they are not down in the water. they are sensing hot air or steam. The problem is that the plumbing tap used to tap the hole is tappered and to run one deaper into the manifold is the solution but the taps are expensive for a one time use. the sensor must be down in the coolant.

check this by filling your system, runn it up to temp normally, do some hyway and around town driving. The park the car and wait for it to cool off to where you can touch the engine manifolds with out discomfort or pain. make sure there is no pressure on the system. then unscrew the sensor and look down in the threaded hole. look at the water level. is the level lower than the sensor. then you sensor is measuring air, then steam, then maybe coolant. determine if and why the level is low but also make sure the sensor is down enough in the coolant to actualy read coolant temp. this presumes your sensor is mounted on the water cross over on the intake,

if the sensor is mounted on the side of the head then of corse this does not work. Actually I prefer a sensor in the head rather than the water cross over manifold in the intake. The head is the hotest point in the engine and the sensor being mounted between to exuaste ports will let you know when you reach critical mass. Personally I wouls mount a compatable sensor in each head and in the intake cross over, and read the temp from all three locations over time to get an idea what each head is reading and do comparative analysis to each other and the intake. that way either source will give you a general idea what the system is doing.

forth. Realise what your cooling system is doing.

When you start the car the cooling system must warm up, hence the term warming up the car. when the cooling system warms up, the thermostadt is clossed untill the coolant in the engine warmes up enough to open it up. Then you will get a rush of cold water that the engine must then also warm it up.

How does this show up on the guage? well as you initialy warm up the engine you will see the guage show the engine coolant getting hotter, then suddenly it drops back down into the cold range again. thats proof that the thermostadt has opened.

you can confirm this, especially in winter time but also in summer. while the engine is warming up turn on your heater and feel the cold air slowly getting blowing warmer, when the thermostadt opens you will get a rush of colder air again and it will slowly warm up again and then stay warm. it will happen simultaneously with guage indication. then thats whats "causing" the sudden drop in temp. so thats not a bad thing but you can eliminate it as a symtom of the 'Problem"

,Hope that gets you there with your problem

[cjatx]

Thanks for the help. I do have teflon tape on the sending unit. I'll remove that and try again. If that doesn't work, I'll get Lectric Limited's sending unit. I'll double check the coolant level too.

[Rob B]

Interesting input on the replacement sending units. My original one went bad and I replaced it with one from Auto Zone and it appears to read cooler than what the original one did. For example: my GTO always ran about 3/8 to 1/2 way on the guage before it quit working. The new one sits right at an 1/8th to a 1/4 on the guage when at normal operating temp. Might have to order one from the website listed above to see if what they say and 67 Lamnas said is the culprit with my guage.

[Peter Serio]

Besides the top part where the green wire plugs in (mushroom vs. spade) there is a wide variation of the replacement sending units for all GM cars with a gauge.

The main groupings seem to be 1965-1978 and then 1979 and up.

Within these 2 groups my tests indicate that the ohms can vary as much as 30 to 35 ohms at the same liquid temperature.


Last fall I bought 10 different brands and part #s of new (auto parts store available) GM dealership and restoration reproduction senders and compared them all to an old original GM unit that I've had for years.

The results were surprising.

[Formulabruce]

May want to look at this on senders...We have hashed this hard in the70-73 Firebird forums..
http://forums.maxperformanceinc.com/...ht=Temp+sender