Gfellas

Just put my 489 stroker motor back in, everything works great except I get no temp reading. Pulled the sender out n tested it with a lighter and the gauge read, touched the wire to a ground and the gauge maxed out, so both of those are in working order. Let the boat run for any amount of time, gauge doesn't budge. BUT.. Turn it off and let it sit for a few and turn the ignition on and the gauge reads like normal, soon as the motor starts, gauge drops down to nothing again. Any ideas?

NHGuy

Shoot a temp gun around sender. If the temperature is correct, check for ground to the t stat housing. Usually the ground is carried by a metal piece in the thermostat gasket.

Gfellas

The more I talk to people it seems like a grounding issue, and the thermostat gasket is a simple paper one, only ground would be from the bolts (which could have some paint in the way) ill check that out. Also read to try and jump a wire from the sender to an engine ground for a quick answer also.

jwp

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If your thermo housing looks like this...... Make sure the upper gasket (3) is there and (6) is in good shape

Without them in good shape it can run so cold the gauge will not move.

You can just touch the sending unit with your finger as it warms up to see if you are building heat.

(It could be why it works after it sits the water heats up)

Griff

You need to use a Merc gasket and no sealer. It has metal crush rivets to complete the ground needed for the sender.

sceptre

Can you tell me what the fix was here? My boat is doing the same thing.

zz28zz

OP's last post was 5 years ago (to the day!). Not likely he's going to reply any time soon.

Assuming the eng is actually getting warm, you need to verify temp sender body is grounded with a multi-meter. Teflon tape on threads of sender and improper gasket on thermostat housing (assuming the temp sender is on the thermostat housing) can both interrupt the sensor ground.

If that's all good, briefly ground sensor wire and see if the dash gauge jumps to full scale hot. If so, the gauge and wiring are probably OK.

sceptre

Thanks. I did that. Grounded the tan wire- gauge pegs. Checked continuity of both tan wires on both motors to both gauges- .8 ohms. Grounds are good on both gauges. Merc gaskets are installed with the brass rings. Weird thing is temp gauges seem to work fine when keys are on- just not with the engine running- they go to 0. All other gauges are working fine. Tested it with temp gun on hose and thermostats are working. Turn engine off- then key on- and temp gauges work...

zz28zz

Need to verify gauge isn't loosing power (or ground) when eng is started. Is there more than 1 gauge not working?