The sensor connects to the dash electronics.
The gage connects at a different point in the electronics.
Look on both sides for the issue.
The gage is a voltmeter. The electronics and sensor are a vairable resistance to limit the voltage the meter sees in relation to the temperature.
A pegged meter indicates it's seeing max voltage instead of what the sensor circuit feeds to it.
Thank for the replies. I gave up on this 98 Expedition cluster and am going to add an aftermarket tach to the original cluster.
According to the wiring diagram, the sensor input goes to a microchip which in turn feeds the gauge. Not much diagnosis to do. All I could do was to see if an electrolytic capacitor leaked. But there was none. Individual gauges cannot be replaced.
When the pickup is started, temp stays at the bottom. About a minute or two later, it suddenly pegs to max. Turns engine off and back on. Pegs to max immediately. Wait a day. Returns to bottom. Other gauges work just fine. By the way, temp warning light remains off.
I found a post here someone successfully installed a cluster from Expedition to his F150. His goal was the same, to get the tachometer. I am sure it's the gauge problem, not interchangeabiblity.
Tach is the only difference between the two clusters. Both have a temp warning light and a temp gauge.
Thanks again. I am putting my old gauge cluster back. Anyhow, the Expedition odometer is 50,000 miles higher than mine.
I went to the jy to return the instrument cluster for store credit. I went through the Ford rows one last time and found another 98 Expedition. Removed instrument cluster. Wow, so new. The original must have gone bad.