Any way to calibrate a temperature sensor on my Volvo MD22 diesel, in my sailboat?

[cskaplan]

My temperature gauge has read too high since I got the boat. The Volvo thermostat is set at 180, my gauge typeically would read 200 - 210. I assumed I was running hot and checked the impeller, water flow through the exhaust, replaced the thermostat, flushed and removed the heat exchanger core (but it was fine). I got an infared thermometer and checked around the thermostate housing, and apparently I was never running hot at all. I get readings of around 170 - 176 anywhere I check on the engine. Now, after too much expense time and frustration I believe my guauge is incorrect. I replaced the sensor to no avail. I shorted the two wires going to the gauge and the gauge goes full scale. I also ohmed out the wires and they are good.
I can only think that the gauge is bad and not reading correctly. is there a way to calibrate the gague? I don't have a PCB, it's a simple dash board. I see a small screw on the gauge face but I think it just holds the gauge face in place. I may replace the gauge entirely if I can find a replacement that will fit in the dash board. Has anyone else had experience with this? Thanks in advance..

 

[gordmac]

Guessing you are using farenheight so no idea what the numbers mean but do you know what the operating temperature should be? Can you borrow a guage from someone to see if it reads differently? I would be surprised if there is a simple way to calibrate it but could you put the sensor into boiling water and see what it reads?

 

[cskaplan]

My temperature gauge has read too high since I got the boat. The Volvo thermostat is set at 180, my gauge typeically would read 200 - 210. I assumed I was running hot and checked the impeller, water flow through the exhaust, replaced the thermostat, flushed and removed the heat exchanger core (but it was fine). I got an infared thermometer and checked around the thermostate housing, and apparently I was never running hot at all. I get readings of around 170 - 176 anywhere I check on the engine. Now, after too much expense time and frustration I believe my guauge is incorrect. I replaced the sensor to no avail. I shorted the two wires going to the gauge and the gauge goes full scale. I also ohmed out the wires and they are good.
I can only think that the gauge is bad and not reading correctly. is there a way to calibrate the gague? I don't have a PCB, it's a simple dash board. I see a small screw on the gauge face but I think it just holds the gauge face in place. I may replace the gauge entirely if I can find a replacement that will fit in the dash board. Has anyone else had experience with this? Thanks in advance..

I neglected to mention that when I short out the wires going to the sender the gauge fully deflects. All the temperatures are in Farenheit. The original sender was reading about 360 ohms in a warm room, the new sender was reading about 430 ohms in the same room. My most recent thought is to put a variable in-line resistor on one of the wires going to the gauge and adusting the gauge to read the same temperature that my infared thermometer shows on the thermostat housing.

 

[cskaplan]

These are good suggestions. The engine is supposed to operate at 180F. I can run a separate wire to a sender in boiling water and see what the gauge shows.

 

[jamie N]

I don't know the solution to your problem, it may be sensor or gauge, but i'd certainly counsel against putting a variable resistor (potentiometer/pot) inline to adjust it. This will lead to a situation where the pot's adjusted to make the readings appear good, whilst the engine fries.
Something's not right, so I'd avoid trying to make it look right, and concentrate on making it right.

 

[Rappey]

It can only be the sender or gauge misreading. Since you've changed the sender then it has to be the gauge ?

 

[cskaplan]

that's my thought as well. Finding a new gauge for an older boat can be a challenge. I can get new gauge but the resistor range is different. I may try the boiling water test to see how far off it is.

 

[cskaplan]

I don't know the solution to your problem, it may be sensor or gauge, but i'd certainly counsel against putting a variable resistor (potentiometer/pot) inline to adjust it. This will lead to a situation where the pot's adjusted to make the readings appear good, whilst the engine fries.
Something's not right, so I'd avoid trying to make it look right, and concentrate on making it right.

I have heard this before but I am having a hard time believing it. The sender itself is just a variable resistor that changes depending on the temperature. The inline resistor just adds an additional fixed number of ohms to the sender and passes along the value to the gauge. The gague is essentially just an ohm meter. I know that Searay does this to "calibrate" temperature gauge problems. If the Gauge is non-linear in it's readings I can see why you might get a false reading, but I don't think they are that sophisticated. I would welcome anyone who can explain why I shouldn't do this.

 

[robmcg]

I think the gauge on the Volvo is a rebadged VDO unit so shouldn't be too difficult to source. Strangely I have the opposite issue ie the gauge under reads. Replaced thermostat and sender but alas the same result. I will probably swap out the gauge over the winter.

 

[Boater Sam]

They are not made with any great accuracy. Fixes to get a more accurate reading are:-
1 Take the gauge apart and move the pointer back a bit.
2 Fit a voltage regulator to the positive supply to the gauge. They tend to over-read because we charge our batteries at a higher voltage than the car that the gauge was made for years ago. Something like a 3 leg 7812 regulator chip with a preset resistor to calibrate the voltage. Circuit diagrams on the web.
3 Quick, fix stick forward biased silicon diode/s in series with the supply till it reads correctly, each one will drop 0.6 to 0.7 of a volt without getting hot like a resistance.

 

[Rappey]

Finding a new gauge for an older boat can be a challenge

Most guages appear to be 2" or 52mm. Larger appear in 3, 3.5, 3.75 and 4 inch.
I found a 2" temp gauge with sender for under £10. Why can't you buy a gauge with a sender?
Is yours an odd shape or size ?

 

[cskaplan]

I noticed that the guages seem to mostly be 52mm. The concern with simply replacing gauge and sender is that I doubt the sender will be the same size threads as on my volvo / perkins diesel, and I have a two-wire sender so I would need to fiddle with connecting the gauge to a good ground, but I have not checked yet. If I can fix this will a $4 resistor it would be easier and cheaper.

 

[Boater Sam]

A simple resistor will give you varying readings as the battery/charging voltage changes, so you will be back where you started.

 

[PaulRainbow]

I noticed that the guages seem to mostly be 52mm. The concern with simply replacing gauge and sender is that I doubt the sender will be the same size threads as on my volvo / perkins diesel, and I have a two-wire sender so I would need to fiddle with connecting the gauge to a good ground, but I have not checked yet. If I can fix this will a $4 resistor it would be easier and cheaper.

Of course you can get replacement senders for your engine, you already bought one.

If you want to change the gauge, just get one with the same resistance range as the existing one. It will be either:

• Standard American 287-39 ohms Ranges
• Standard European 134-23 ohms Ranges

 

[Frayed Knot]

Has your boat been re-engined? I ask because mine has & I have exactly the same situation. The boat has a very vague history & I believe the replacement was a used unit & the original panel & gauges were retained.
Also, as the oil pressure vastly under reads (I checked it with a test gauge & it’s fine) I’m now wondering if the gauges & senders are incompatible.
Maybe Paul can share some of his wisdom & either confirm this as a possibility or tell me I’m talking rubbish as usual

 

[PaulRainbow]

Has your boat been re-engined? I ask because mine has & I have exactly the same situation. The boat has a very vague history & I believe the replacement was a used unit & the original panel & gauges were retained.
Also, as the oil pressure vastly under reads (I checked it with a test gauge & it’s fine) I’m now wondering if the gauges & senders are incompatible.
Maybe Paul can share some of his wisdom & either confirm this as a possibility or tell me I’m talking rubbish as usual

It's possible that the original engine had EU sender/gauges and the new one has US senders/gauges ( or vice-versa). But, when this happens the gauges usually read backwards.

 

[cskaplan]

Sorry I have been away for a few days. The engine is a volvo made by perkins, but the boat was set up for the U.S. market. I checked the ohms on the senders (old one and the new one) on a pretty warm day and got readings from 350 - 430 ohms, so I guess it would be in the U.S. range. I will do a boiling water test first to see how my gauge works with a known temp.

The other thing I realized, my gauge is reading too high (hot). If I recall correctly, the sender resistance goes up with temperature. If the gauge goes UP with more resistance, by adding a resistor in the line it will make this situation worse, not better. If the gauge goes DOWN when the resistance goes up my resistor idea may work OK. I am surprised someone here dindn't call me on this LOL