The Stovebolt Site Forums The Shop Area The Electrical Bay Temp gauge blues

My temp gauge has alway given a "cold" reading under all conditions. I replaced the entire wire to the sending unit with no change. I replaced the sending unit today and got the "cold" reading again. BUT, after about 15 minutes the needle swung abruptly to full "hot" where I've never seen it. I wonder if the change in reading coincided with the thermostat opening? It's still hard to believe the reading was accurate. The bottom of my radiator was barely warm after a half-hour. I used teflon tape on the sending unit, I wonder if that was a mistake? Not sure how to verify the problem.

Did you ground the wire and watch the gauge? When grounded the gauge should read full hot. If it doesn't we can assume a bad gauge since it has a new wire. If it does then we need to check the sending unit with an ohmeter. Cold it should have resistance. Record that value. Warm the engine and check the resistance again. The value should be lower. If there is no resistance or the resistance is not changing right you need a new sender.

You shouldn't use tape on the sending unit. The circuit gets its ground through the threads and the tape will act as an insulator. Maybe the new wire got against a manifold or exhaust pipe and melted to get the gauge to read hot.

Are you shure you got the correct sending unit. The one for a light can do that.

Today I removed the tape from the sending unit. I grounded the wire and the gauge needle went all the way past hot. I hooked it back up and went for a ride. As yesterday I got my typical cold reading and after 10-15 minutes the needle swung quickly to full hot. Not believing it, I drove another 20 minutes at full hot on gauge. Immediately after parking I stuck a thermometer in the radiator and got just over 170. I ordered my sending unit from truckandcarshop.com for a '56-'58 V-8. My '58 has a 283 V-8. That was an interesting point about the wire which may be against the intake manifold but I never got anything above a cold reading before yesterday when I changed the old sending unit. So, I have a new wire, new sending unit, new 180 degree thermostat and a radiator that was flushed two months ago and a 48 year old gauge and I'm still not sure what's the problem. Maybe I need to get an ohmeter to check the sending unit as suggested. Scratching my head.

Could the location of the sending unit be at fault. Sometimes the head/block can fill with gunk and restrict water flow to a area.

As all seems well with the components, maybe you are getting a true reading, but in a funky location.

Stuart

My '58 has an intake manifold that is I believe a '67 or later so I'm not positive about the exact location of the original set-up. My sending unit sits in the thermostat housing probably less than an inch above the thermostat which I've had disassembled recently and I think everything is pretty clean.

the 283 sender is normally right beside the thermostat gooseneck, just behind the oil fill tube - you may have the manifold style found on the 350 .... should work the same

the wire normally runs along the manifold side of the drivers valve cover, where there are a couple clips to hold it, shouldn't touch the maniflod, tho' I doubt that'd affect it anyway - I'd say the gauge is the problem, a 283 should show hotter than a 235, but still a bit below halfway

Bill

Confirm your reading with a temp test at the radiator. Take its temperature at the fill neck.
If the temp is above 190 it may be getting too hot. Then you will have to remove the thermostat and test it. To do that use a cup of water put it in the microwave and heat it to the temp. stamped on the thermostat (usualy 160 or 180 deg.F.) drop in the thermostat and see if it opens.

Here is a question I should have started out with. Those who are blessed and have a functioning temp gauge, where is the starting point of your gauge needle when you turn the key?
Mine begins as far below the cold dot as the neele can go and takes about 10 minutes of warming to even reach the cold dot. It seems like it should start at the dot. I'd like to hear what your normal gauges are doing?

The gauge in my 66 rests far left and jumps up about 1/8" when the key is turned on. it reaches 1/3 at normal driving in about 3 minutes.While idling for several minutes the guage reads 1/2 and actual temp of the thermostat housing is 190 degrees. On this 283 fan is about 7" away from radiator. I just put on a fan shroud so I'm hoping it will run 180 or less.
The gauge on my 57 would never get above 1/4, even idling at 190+ until I changed the 235 to a 7.4L and installed a 50 ohm resistor in series.

winchester - 235's are hard to warm up - 283's run best hot, 190 is better than below 180 - the shroud is critical to keep the temp around 180, and will make a difference .... the original v8 setups had a shroud if the fan was more than 1/2" from the rad

Pat - on my 58 w/ 283, the gauge starts "buried" like yours, I don't think that's a problem - when the key's turned on it does jump up a bit, after a few minutes it starts moving up, and at full normal temp it's a twitch below halfway, say 3/8 - my 57 w/ 235 is about at the dot off, jumps a bit when the key's turned on, and when fully warmed up it's up to about 1/8th ... once when I popped a leak and lost a lot of the water, the temp gauge went way up quite quickly, so I know it works - you might fix the situation with a different gauge

Bill

I installed a new temp gauge a couple weeks ago and took the truck out for a drive. After warming up the needle consistently moved from the center dot to just slightly above it. As with the original gauge the new one had the number "100" below the cold dot and "230" below the hot dot below the gauge face. If those numbers are degrees, then the center dot would be 165 degrees. Anybody know if that's correct? If that is right then it looks like I'm about where I need to be.

sounds right, but I never trust the degree accuracy of the average factory gauge, I always check a new gauge [or 'new' vehicle] against an actual reading in the rad to "calibrate" what the gauge reads - 175-185 is really best for a 283, 165 is a little cool, especially in the winter with the cold air the carb's sucking

I don't recall seeing any TF gauges with numbers on them tho ... hafta look at the stash

Bill