Engine Coolant Temp sending unit - need some help!

[jslayde1]

I need some help guys - have an issue with my temp gauge.

Short version - the guage pegs out on the high side when I turn the key. Seems to be shorted out.

Long version - Last weekend we took the boat out for the first weekend (of course no issued with fake-a-lake) and as soon as it fired up I notied the gauged was pegged. 2 seconds later and it returns to normal and starts heating up as it always has...only starts passing normal and slowly gets to 200 ish. So I shut down and started feeling around the motor. It never really seemed hot but I didn't want to chance anything. Let it cool down and tried again. Same thing - slowly climbed up to 200 ish and I shut it down. Here's the part that got me to thinking - the guage would slightly "flicker" from time to time and then of course the first startup having it pegged made me think it was the sensor.

Last fall I replaced the raw water pump (due to leaking) which came with a new impeller. Also replaced the thermostat last year.

I pulled the thermostat off to inspect - it did have a slight bit of corrosion and felt alittle sticky. I cleaned it up and it defintely could have been the problem. However, I decided to check the Engine Coolant Sensor out. And of course taking it out was a pain and I could have damaged it. I put it back in and now it seems to be shorted - pegs the gauge as soon as you turn the key on. I even threw a new wire from the sensor the gauge just to make sure it wasn't the wire and it had the same result - pegged gauged. BTW - you take the wire off the gauge and the gauge returns to 100*.

What do you think? What am I missing?

[jslayde1]

Need some help if anyone has any info. I don't have any temp reading.

Now I have replaced the coolant sending unit and as soon as I turn the key on the gauge pegs out. If I take the wire off the sensor the gauge returns to the left side. I have also put a separate wire on from the sensor to the gauge with the same results.

So, now I'm thinking it's the gauge. Anyone have a similar issue?

[lee]

I think you may have two senders, one for the ECM and one for the gauge.

[jslayde1]

Yes, that is correct. The sensor for the computer seems to be fine. It's the gauge or sending unit (2nd sensor) that's seems to be the problem.

[boatwakes]

Where did you get the sending unit, there are different resistance senders for different manufacturers. Also, the gauge will only peg if the sending unit terminal on the gauge sees ground so check the back of the gauge and the negative terminal.

[jslayde1]

Thanks Boatwakes - I bought the sending unit at Advanced Auto parts. It was the only sending unit that I could find locally that "looked" identical. No specs on the box.

I will check the negative terminal on the gauge out today. Is it possible the gauge is bad?

[ChrisSnow]

If you know how to use a multimeter then check out these links
http://faria-instruments.com/site_manuals/IS0085E.pdf

http://faria-instruments.com/site_manuals/IS0345A.pdf

Should help you figure out what the issue is.

[boatwakes]

Possible the gauge is bad but not likely. Ive seen ground terminal ends stretched over the sender post on the back of the gauge but that would peg with no sender connection. It's usually the sending units that go bad but also important to note some of the senders may ground through the terminal and others (like what we have on these engines) are isolated. You can test the new sender you purchased by checking continuity between the sending unit post and a good ground with no wire attached. If it reads continuity, you have the wrong sender and the original problem. If it reads isolated, connect the wire and test the gauge side for continuity the same way and narrow down where the short to ground is happening.
There should be an isolation washer at the base of the sending unit post where the wire connects to keep an accidental ground from happening when the wire terminal accidentally crosses between the post and the body of the sender. Check that too.

[jslayde1]

Thanks for the info guys. I didn't get a chance to dig into yesterday - my wife was jumping up and down to surf () so I bought a $15 gauge at the auto parts store that plugged into the block. Verified normal temp!

@Boatwakes - after your info it sounds like I may for sure have a bad sending unit (and purchased one that doesn't work for our application). I will troubleshoot this week and report back.

Thanks again for the help - much appreciated.