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Old 28-04-2013, 10:29   #1
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Bad temp guage or sending unit?

Heading up the Okeechobee today and my temp gauge on the 89 Yanmar HM35F is showing 230 degrees at 2500 RPM. The thing is, a laser gauge shows the hottest portion of the cooling system is 175 and the huge coolant reservoir for the hot water heater shows the same. She's full of coolant.

Do temp gauges or senders become inaccurate that quickly?

Is the best spot to check actual temp at the coolant outlet before the heat exchanger?

Thanks,

KM
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Old 28-04-2013, 10:37   #2
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Re: Bad temp guage or sending unit?

Hard to say, but I replaced alot of a Yanmar system over a period of a couple months once, only to find out finally the sender was bad!
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Old 28-04-2013, 11:39   #3
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Re: Bad temp guage or sending unit?

Thanks, Cheechako. Just can't get my head around it being anything else based on the laser readings. Continuing on to LaBelle.
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Old 28-04-2013, 19:20   #4
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Re: Bad temp guage or sending unit?

Pull off the electric stuff and put on a good old fashion bordon tube gage with a long sensing bulb coil ... problem solved even if all your electric system fails. You can also do this with the oil pressure gauge.
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Old 28-04-2013, 19:42   #5
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Re: Bad temp guage or sending unit?

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Originally Posted by kmaz View Post
Heading up the Okeechobee today and my temp gauge on the 89 Yanmar HM35F is showing 230 degrees at 2500 RPM. The thing is, a laser gauge shows the hottest portion of the cooling system is 175 and the huge coolant reservoir for the hot water heater shows the same. She's full of coolant.

Do temp gauges or senders become inaccurate that quickly?

Is the best spot to check actual temp at the coolant outlet before the heat exchanger?

Thanks,

KM
Point your infrared (not laser) guage at the temperature sender.

If you take off the radiator cap at 230 degrees, you will get a geyser on the order of old faithful, whereas if the temperature is 170 you might burp a bit of coolant--another cheap check.
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Old 28-04-2013, 19:48   #6
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That. Is a good idea.

Won't remove the electric as I can see it from the cockpit but adding a mechanical in the engine room will be a nice fail safe.
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Old 28-04-2013, 19:54   #7
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Point your infrared (not laser) guage at the temperature sender.

If you take off the radiator cap at 230 degrees, you will get a geyser on the order of old faithful, whereas if the temperature is 170 you might burp a bit of coolant--another cheap check.
Thanks, Don. This is my first diesel and it was driving me crazy not knowing exactly where to measure. The infra showed 170 at the sender.

Very strange behavior. I've been motoring down the ICW and into the Caloosahatchee for the past several days and she was rock solid. Then today the gauge slowly climbed to 230 while the block and cooling system stayed constant. Checked the gauge when I got to LaBelle this afternoon and it appears functional. Will replace the sender next and report back.
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Old 28-04-2013, 19:58   #8
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Re: Bad temp guage or sending unit?

You can also put the sender in a cup of boiling water to check it--should read 212 degrees.
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Old 28-04-2013, 20:11   #9
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You can also put the sender in a cup of boiling water to check it--should read 212 degrees.
Didn't think of that. Decent way to be certain. It needs a ground, yes?
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Old 28-04-2013, 20:16   #10
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Re: Bad temp guage or sending unit?

Yes, it needs a ground. They generally supply a variable ground to the gauge depending on the temperature so make sure you have a great ground or it will give you inaccurate information.
EDIT- Actually, every one that I've ever seen supplies a variable ground. I qualified because around here if you don't some bonehead will say that in 1922 Dulut & Smiggins made a 19 cylinder steam engine that measured corn flakes with the sender.
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Old 28-04-2013, 20:37   #11
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Re: Bad temp guage or sending unit?

You measure the temp before the cooler. This has always left me in a bit of puzzle but that is what "they"say They in this case are the FAA. I know they are not relevant to boats but there you have it.
This might explain a lower external temp? Are you pointing your temp meter at the sending unit?
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Old 29-04-2013, 05:31   #12
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Re: Bad temp guage or sending unit?

See ➥ Basic Engine Gauge Theory and Testing
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Old 29-04-2013, 17:45   #13
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Re: Bad temp guage or sending unit?

Thanks for all the reassurance and help! Wanted to follow-up for the curious and in case someone stumbles across this in a future search.

Definitely the sender. Tested it per Don's recommendation and the results were inconsistent. Registered 212ish no problem but over 30 mins began to show a higher temp. Cleaned it up and resintalled as I won't be able to find another for a few days. Found another issue others should check if they encounter a similar situation. Brushed the connection clean when reinstalling and it is still not precise but it is consistent. This will make my Okeechobee crossing tomorrow much more relaxed. Keep in mind the gauge is measuring resistance so it makes sense a questionable connection could produce questionable results.
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Old 29-04-2013, 17:55   #14
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Re: Bad temp guage or sending unit?

You might also check the entire wire, connectors and all, that goes from the sender to the gauge. I had one like that that ran me nuts with its inconsistency, turned out to be an intermittent connection where the wire was crimped onto the connector. Replaced connectors and wire with new parts....fixed the problem. The old wire didnt' register as open with a meter, but evidently the resistance varied as the engine vibrated and heated.
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Old 29-04-2013, 18:13   #15
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Re: Bad temp guage or sending unit?

There is bad wiring or sender or even the gauge. If the IR thermo reads 170 I would trust that.

On my Perkins I had the same problem. The gauge was reading 240 or so and the IR was reading 185. I traced the wires and tightened connections till I located a loose connection on the second gauge (there are 2 gauges from onoe sender, designed that way). Once it was tightened up the gauge plopped right down to match the IR reading.

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