Cant figure this out. Appreciate anyone who has had this happen before or can shed some light.
500 EFI temperature gauge in dash pegged on 280*. Engine cold or warmed up shows 280*
I have one at a time done the following:
- disconnected the temp sensor, still read 280*
- swapped gauges from one engine to the other. still reads 280*
- ran a separate sensor wire from the engine harness plug to the gauge still read 280*
ANYBODY HAVE A SUGGESTION?

When the key is off (no power to gauge) is the gauge still pegged ?

I believe there are two water temp sensors. One for the gauge and one for the ECU. The one for the gauge typically has just one wire. The one for the ECU will have 2-3 wires.

Where you dealing with the correct sensor + wire ?

am electric gauge 'pegging' typically is from grounding out the sender wire or post on gauge...or bad gauge too of course. the sender just sends ohm readings...it has no power.

With key off, gauge returns to the other side of the scale or 0.
I unplugged the 2 wire ecu sensor for the computer - still 280*
then I unplugged the 1 wire sensor which is for the alarm? - still 280*

Where you having a wiring issue: see Anybody have an engine wiring harness for 500EFI??

Other issue back then - turned out to be the map sensor pigtail that Whipple sent me had 2 of the 3 wires crossed over from one plug to the other. Been running great since then.

was temp gauge okay then ? after ?

anything behind dash changed ?

I'd diagnose from back of gauge and go from there. Again, this typically happens when the sender post on gauge is grounded out.

maybe hard to read 'S' vs 'G' or 'I' or hard to see color difference where back of gauges are....lol...........

For easy lookup for any of usm the two ect senders are here:
Genuine Mercury & Mercruiser parts. Senders And Sensors
Gauge should be the single wire sensor/ the ECU sensor two wire

gauge sensor
I believe the one wire is tan.

eCU coolant temp sensor
I believe the two wires are a yellow and a black.

Thanks SB. You've got me thinking that it might be the tan wire is grounded somewhere from the sender to the harness plug. Gonna check that out.

check the cannon plug / connections / and the wires going into the cannon plug as well