Temp sender and gauge accuracy

[80spiddy]

While trying to figure out why my fan wouldn't come on, I pegged my gauge at 260. Not to worry, my heat gun showed nothing got hotter than 220. I had already replaced the radiator sender with a new sender to come on 10 degrees cooler. My problem it turned out was the wire for the sender wasn't getting a ground. So, I'm going to splice into that and run a wire to the fender pod ground. It works as it should now if I jump that ground. Can a bad temp sender in the head cause the gauge to read hotter than it actually is. I also assume this has nothing to do with the bad ground in the sender in radiator also? I assumed the temp sender is just a ground wire that goes to the gauge and the gauge has a power wire. Nothing is interconnected between the other sender? I plan on buying a new head temp sender if this is my problem with the gauge. The front temp sender did nothing when the wire was pulled off. I was expecting to see a red light to come on the dash. Is there something else I should troubleshoot there. Sorry for so long and so many questions. But, truly appreciate everyone's help.

[spider2081]

In the head there are 2 sensors. One is a temperature sensor the drives the gauge needle linearly through the temperature range to about 230. The other is an over temp switch. When it reaches a certain temperature it drives the needle to peg into the red area of the gauge. If your gauge was indication a temperature greater than the actual temperature of the head it could be the over temp switch is causing the issue.

[80spiddy]

I was thinking the front sender was a light. I was only laser reading the rear one (220) which never read what the gauge was (pegged 260). I would think they would both read the same, but am probably wrong. The hottest anything laser read at close to the gauge temp was the sender at the bottom of the radiator (254). The coolant coming out of lower hose was about 160. When the front wire was pulled off terminal nothing changed. When pulled off rear sender, gauge went to 0.

[spider2081]

The green/white wire is the temperature gauge wire. The Gray/black wire is the over temp switch. Sometimes people find the wires on the wrong sensor. Usually the temperature sensor is the one closes to the firewall.

Removing the gray/black wire should not change anything on the gauge if the engine is operating in the normal range. Grounding it should peg the meter.
Removing the green/black wire should cause the gauge to read "0" grounding it should peg the gauge.

If you have an ohm meter you could check the temperature sender when the wire is removed.

Cold the sensor should read over 1000 ohms
at about 190 degrees 150 ohms
At 260 degrees about 50 ohm.

Hope this helps

[80spiddy]

Thanks. I'll see what I have with your info.

[Rdumont]

It appears that I have a gauge sender in the place of where the temperature switch should be. The other has a sender as well. Does this matter or do I need to replace the sender that is supposed to be a switch?

[18Fiatsandcounting]

If I am understanding correctly, you have two temperature gauge sending units in the cylinder head rather than one sending unit and one overtemperature switch?

If this is the case, then you have two sending units operating electrically in parallel. I'm thinking that would give incorrect temperature readings on the gauge. The simple solution would be to just disconnect one sending unit and put an insulator on the loose wire.

I'm not sure if it matters whether the sending unit is in back and the switch is in front (or the reverse), and I've seen it both ways so I don't really know which is correct.

-Bryan

[Bjorn68]

Hi,
Sorry for stepping into this thread. I just wanted to test if I could link to pictures on this forum.
If the link works (or if the picture actually shows, let's see) you should see two temperature guage sender units. The one on the left (black) was sitting in the rear end of my cylinder head. It was reading several kOhms in cold condition, so the guage would hardly move even when the engine was hot. The red unit on the right is new, and has the correct resistance, as so well explained on the forum. The guage now works perfectly.