unknown sensor

oldboat1 · Post by **oldboat1** » Fri Jun 17, 2011 5:07 pm

I've had a 1980 F26 for about a month, and have had some electrical gremlins. The temp. gauge suddenly stopped registering, so I figured I would start with the connection at the sender, thinking some clean up might fix it. That fix seems to have worked -- now shows a temp of 100 like it did before, but at least it's showing something. But I have a bunch of issues.

Before correctly locating the temperature sensor on the top of the engine (brown wire), I mistakenly started working on a sensor sticking out horizontally at the front of the engine, at the water jacket for the raw water intake. The wire and nut were badly rusted and I broke the sensor trying to get the wire off. It had what turned out to be a plastic housing (red covered) with a wire running through to the sensor. I broke off the housing, then promptly broke off the wire as well. Can anybody tell me what this sensor is? The connecting wire is green striped (green stripe over brown or tan, as I recall). There is no reference to a green striped wire in the engine manual I have (Mercruiser 228 and 255 -- believe mine is the 228, a GMC 305).

I decided to fire up the engine and let it idle to see if the temp gauge worked, and to see if anything else failed to work because of the sensor I broke. The other gauges continue to work normally.

So first, what is that sensor I broke, and do I need to replace it? I'm thinking maybe it signals some kind of warning if no water is sensed from the raw water intake, but no idea.

Second, I'm thinking the engine should run at 140 degrees or so, and that somebody may have removed the thermostat or it's stuck in the open position -- tapped on the housing to see if anything would happen, but no change. The engine runs at 100 degrees according to the gauge -- which admittedly could be in need of replacement. I'm thinking I should put in a 142 degree t.stat if there isn't one in there or if the current one isn't working. My assumption is that all of these engines have t.stats, although I don't know that for sure either.

Any help appreciated.

oldboat1 · Post by **oldboat1** » Fri Jun 17, 2011 6:44 pm

should add that the unknown sensor is on the port side of the engine -- sticks out to port from the front jacket that is connected to the raw water intake pump (receiving end) and the hose to the water circulating pump.

randyp · Post by **randyp** » Fri Jun 17, 2011 8:21 pm

I'm not sure about the sensor you broke, but the temp on my 1977 F-26 runs under 140 just about all the time. Raw water cooled, like yours, and upstate NY lake, like yours (I'm guessing, based on your location). Check to see if the t-stat is still in place and if so, leave it alone. You'll get some steaming from the exhaust at higher throttle speeds, but that's not a problem either, unless the steaming happens all the time even at lower speeds. Then, that's another story altogether.

Big D · Post by **Big D** » Fri Jun 17, 2011 10:44 pm

A picture of the sensor would help. 100 degrees is too low IMO. Engine is happier running at proper temp spec; best fuel economy, better for lubrication etc. Get yourself an IR temp gun and check temp at the sender to get a better picture on whether it's actually 100 or higher and just faulty hardware. If it is indeed running at 100 deg and has always been that way, I would find out why. It could be a t-stat but may also be incorrect exhaust manifold gaskets.

oldboat1 · Post by **oldboat1** » Fri Jun 17, 2011 11:39 pm

Thanks for the input. I don't have a pic of the device/switch I broke (and nothing to take a picture of now), but it looks like the switch shown at the lower right of the Mercruiser catalogue page: http://www.marineengine.com/parts/mercu ... g00931.htm

I think it's part of an alarm system, but since it is located at the point where raw water comes in, I think it must warn if there is no water present. I'm guessing there is a horn buried somewhere, and thinking if I just leave the switch in place as a plug (no electrical connection), there shouldn't be any harm. I might buy the switch shown at the website, but can picture problems getting the old one out -- might not be worth it.

Either there is not a working thermostat in there (maybe none at all, like guys like to do with outboards), or the temperature sender needs to be replaced. Based on my old fashioned touch test, I'm sure the t.stat housing as well as the top of the engine in the area of the temp sender is well below 130 degrees or so -- that point at which it is uncomfortable to keep your hand in contact. It could well be running at 100 degrees or so, as indicated by the temp gauge, and that can't be efficient.

I'm in Keuka Lake, out of Branchport, for those who might be in the area -- "The Deacon's Shay," like the one-horse buggy that suddenly crumpled in a heap (but managed to last 100 years, anyway).

Stripermann2 · Post by **Stripermann2** » Sat Jun 18, 2011 7:45 am

Dependent on year and application, it could be either a temperature sending unit or alarm for coolant. On my engine, the coolant sensor is on the port front with alarm at rear of intake manifold near distributor. Very simple test is to ground the wire to the block momentarily with key on. Two things should happen...either the alarm buzzer will sound or temp gauge should move. Keep us posted to your findings.

oldboat1 · Post by **oldboat1** » Sat Jun 18, 2011 1:23 pm

Jamie --

I grounded the switch lead to the block as well as to the switch base (still intact) -- no alarm with the starter switch in the on position, or with the engine running. There was also no effect on the temp gauge, which is still working from the sender with the brown lead. If the switch in question goes to an alarm, it's possible that there is a bad fuse or maybe that the horn is not operating. I'm going to tape off the end of the lead, and move on to other issues.

I've ordered a 142 degree thermostat and gasket, and will install that when it arrives. If there is no change in the gauge reading, I'll replace the sender.

Curt

Torcan · Post by **Torcan** » Sat Jun 18, 2011 10:30 pm

is this the item you are wondering about?
I myself am interested in knowing what it is and for what purpose it is there

oldboat1 · Post by **oldboat1** » Sun Jun 19, 2011 10:07 am

That's it, Torcan! Mine had a green striped wire to it. I put a pliers on the red portion as it was spinning with the rusted nut holding the lead to the switch. Unfortunately, that red tip covers a plastic end that shatters. The plastic cap apparently serves as an insulator for a wire that appears to run through to the sensor, and probably engages with the sensor to complete a circuit with the connected lead.

I see a brown lead in your picture. Does your red switch also have a tan or brown lead connected? My tan/brown lead goes only to the temp sender on the other side. If yours also has a brown lead connected, that would presumably put it in the circuit with the temp sender. If that's true, and if the red device is an alarm switch, it would be testing both the raw water temp (pressure, maybe?) and the internal temp of the water circulating in the engine (?)

I'm still inclined to let mine remain out of the loop, but if it is supposed to be part of the water temperature sensor/sending system -- and if the temp sensor and gauge won't work correctly without it -- I may have to replace the switch. Maybe the brown wire in your picture just happens to run by the red switch -- would appreciate clarification on that.

Has your warning horn ever sounded?

Torcan · Post by **Torcan** » Sun Jun 19, 2011 11:00 am

my horn hasn't sounded
but as I posted, I am at a loss as well as to how it works
I am sure that as soon as some of the more knowledgeable posters log on and see this post and pics, they will chime in with the answers we are both seeking.
Glad I was able to help at least in posting the pic, now we shall wait and find the answers hopefully.

oldboat1 · Post by **oldboat1** » Sun Jun 19, 2011 11:25 am

yes, must be somebody out there who has the exact info. It's a good pic -- looked at it again, and it appears the brown (tan?) wire comes out of a wrapped bundle, then presumably goes over to the temp. sender. (i.e., doesn't seem to be connected to the mystery switch.)

prowlersfish · Post by **prowlersfish** » Sun Jun 19, 2011 6:50 pm

Looks like a temp sensor . Ground the wire and see what happens , look for gage movement or a alarm or light. Your merc dealer od shop manual will tell you for sure

Big D · Post by **Big D** » Sun Jun 19, 2011 10:13 pm

That is the water temp switch for the audio warning system. Nothing to do with temp gauge.

prowlersfish · Post by **prowlersfish** » Sun Jun 19, 2011 10:27 pm

If thats the case the engine could be wired for a "alarm " But the boat may not be . Not uncommon if re-powered

Big D · Post by **Big D** » Sun Jun 19, 2011 11:07 pm

Keep an eye on your gauges!!