Can't get Temp gauge to work

[jmmdawg]

I just bought a 2001 Astro 1950 ss. Motor is mercruiser4.3L. I removed the tan wire from the sender and grounded it to the block and the gauge pegged on the hot side. So I bought a new sending unit becuase I was told if the gauge did that it means the gauge worked. Well installed new sending unit and started the boat on muffs and let it run for about 10 min and still no reading at all. I stays below the 120 mark. Now, if I turn the key to the off position and tap on the guage starts to move up as I tap the guage. Any suggestions on what to do next? getting very frustrated

 

[cr2k]

Re: Can't get Temp gauge to work

Do you have the thermostat gasket that has the brass grommets in it? Try grounding the thermostat housing and see if that helps.

 

[jmmdawg]

Re: Can't get Temp gauge to work

Sorry, i'm not all that mechanically inclined. This is my first boat and I'm learning as I go. Looking at the thermostat housing and gasket I don't see any brass grommets or anything sticking out. How would I ground the thermostat. Just run a jumper wire from one of the bolts on the thermostat to a bolt on the engine block? Thanks so much for any help!

 

[achris]

Re: Can't get Temp gauge to work

Sounds like "someone" replaced the thermostat and used a standard AUTOMOTIVE gasket.... :facepalm: Why on earth are people saving 10c to cause themselves so much grief??? I just don't get it....

OK... I understand that you just bought the boat and it was the PO that did the job... The Mercruiser genuine gasket between thermostat housing and the intake manifold has a couple of brass rivets in it so the t-housing is grounded (so the gauge will work). If you don't use a genuine gasket them the housing isn't grounded. To test for that (as you can't see the rivets once the gasket is installed) just run a wire from the t-housing (scratch the metal so you get a good contact point, not the bolts... the housing itself) to a good ground on the engine block and see that the gauge now works... If that's the case go to a Merc dealer and buy a 27-53045-1 gasket....

Chris......

 

[Don S]

Re: Can't get Temp gauge to work

You didn't use teflon tape on the threads of the sender did you?

 

[wellcraft-classic210]

Re: Can't get Temp gauge to work

Sounds like your circuit works up to the sender. As mentioned above the Sender needs a good ground. If you have access to an ohm meter thats a fairly easy check. ( sender housing to engine block or batt gnd-- look for less that 3 ohms)

Once thats verified you can also check the resistance of the sender inernal resistance to ground ( sender terminal to sender housing or known good gnd) and it should vary with engine temp. ( this resistance should drop as temperature rises -- Going from memory I think it may be around 200 ohms or less ( do this with the wire to gage disconnected as it will affect your readings ) If you have access to a heat gun you can vary the temp of the sender without stating the engine.

Is the engine getting warm enough to change the resistance of the sender? Is your thermostat in place and closing to help make that happen? My merc 5.0 barely reads anything without a thermostat except under heavy loads of long duration.

 

[jmmdawg]

Re: Can't get Temp gauge to work

Thanks for all the help guys. I think I'm just going to pull the thermostat, put a new one in and change the gasket just so I will know if that's the problem

 

[Don S]

Re: Can't get Temp gauge to work

Over tightening can also damage senders.

 

[jmmdawg]

Re: Can't get Temp gauge to work

ok, i didn't put any teflon tape on it. I figured that the reason it had the orange stuff on the threads was to help get a good seal. I did snug it up pretty good but i didn't by any means crank down on the sender too much.

 

[Don S]

Re: Can't get Temp gauge to work

Do an ohms test between the sender body (not the terminal) and a good clean (no paint or rust) ground on the engine block. You should see very little resistance.