Temp sending unit issues

[ejay]

My engine is running great now, so I am turning my attention to the temp sending unit. My gauge never moves and I have attached a picture. The gauge is getting 12 volts with the ignition turned on and the needle moves all the way over when ground the sending wire at the gauge.

The next pic is the actual sending unit which has corrosion on it, which I cleaned with a wire brush to no avail. I tired an ohmmeter from the body of the unit to the block, but it did not change from 1. I then tried to take off the wire but the nut is frozen and the little inner part of the sender unit turns with the nut.

I figured the wire connection must be impaired so I clipped and stripped the wire and touched it to the unit after the engine had idled for a while and got the needle on the guage to bounce around the 100 mark, but not really what I would call an accurate reading.

So my question is, should I just get a new sending unit? The tiny inner part of the 1/2 inch larger part? I do not want to replace the guage, but MMI says I should only use their brand of guage, which is not what I have now. Thanks so much!

[Dave Neptune]

You can purchase a Stewart Warner sending unit as it goes with your gage. The gage and sending unit are "sets" that work together. There are many brands that share the same resistance scale however unless you know what is what it is better to stick with the same brand or replace the set to be sure.

The gage pictured is an automotive from the looks of it and is not protected as well as a marine grade which is the environment it is in.

Dave Neptune

[B Moyer]

Incorrect statement clarification

I feel the need to clarify a statement you made in the last paragraph of your post. MMI does not say you should only use their brand of gauge. MMI sells Stewart Warner gauges/senders and Isspro gauges/senders. MMI says whatever brand gauge or sender you have, you must use the same brand for the part you are replacing. Meaning Steward Warner gauge and a Stewart Warner Sender or an ISSPRO gauge and an ISSPRO sender. They are designed to communicate with one another. Not using the same brand will/may result in unreliable reading.
Hope this helps you. Whatever brand gauge you currently have, find the same brand sender for reliability.

[edwardc]

I have that exact same gauge on my 1977 Pearson 323, installed by Pearson.
The bezel seems to be high quality stainless and shows no sign of corrosion, unlike the automotive grade oil pressure gauge some PO put in which has rust stains bleeding out from under the chrome plating.

Looking carefully at the extreme bottom edge of the faceplate, you can make out the Stewart Warner part number "826544".

I'd like to determine the matching sensor too. Haven't been able to yet.

[ejay]

I had no idea MMI offered SW units, so I will get a matching sending unit from them.

[ejay]

Update

MMI does not carry Stewart Warner products. I confirmed with Ken today.

[Dave Neptune]

Anybody that sell SW should be able to get you one. They're sold at most auto parts stores and almost any automotive performance parts house like Jegs or Summit and most marine supply houses sell SW as well, they are quite common.. Should not be hared to find. Or just go with our hosts gage and be done with it.

Dave Neptune

[ejay]

You are correct, however evidently I need to get the SW sending unit that corresponds to the particular SW guage I have installed already? OR will any SW temp sending unit work? I know everyone has warned about resistance compatibility between guage and unit.

Taking the guage out is a little bit of a mystery to me because the bezel did not turn easily and I dont have easy access to the rear of the instrument panel. I may end up getting the ISSPRO guage and unit, but I am wary of project creep.

[Dave Neptune]

On the SW gages there is a clamp that holds the gage in place on the back side. Sort of a U shaped bracket with two nuts holding it against the panel keeping the gage in place.
Check with SW using the number of your gage that Edward posted it is probably correct. SW should be able to verify it and even any dealer should be able to too.

I doubt they use multiple sensors for the water temp gages as the scale is limited to what the gage measures.

Dave Neptune

[edwardc]

I have been able to determine that there are at least two different temp sender resistance ranges for stewart warner gauges.

There are roughly three different vintages of SW gauges. The oldest have an elaborate SW logo that has wings on it, and are often referred to as "Wings" gauges.

The next oldest have the letters "SW" in a largish square block and are sometimes referred to as "Big Block" gauges (not to be confused with engine blocks! ).

After that, somewhere in the 70s, came the ones with the letters "SW" in a small block (known, unsurprisingly, as "small block gauges"). The 826544 gauge I mentioned previously is a "small block" gauge. E-Bay listings refer to it as a "Vintage" gauge.

I found a listing for a Stewart Warner temp sender which warned "Will not work with Wings gauges". So this implies that there are at least two different sender ranges.

I am currently becoming convinced (but haven't proven it) that the sender I want is a 280EC (https://www.summitracing.com/parts/sww-280ec).
This sender is described as "100 - 230 degrees", which closely (but not exactly) matches the printed range on my "vintage" small block gauge. It is also a 1/8 NPT with the threads located rather far down the sensor, and comes with a 1/2" adapter. Both of these match the sensor I have installed. But, as usual, no resistance range is specified.

Lastly, assuming it turns out to be the somewhat common 33-240 ohms sensor, I found this series of resistance-to-temperature sensor measurements online:

33 ohm = 240F
55 ohm = 200F
61 ohm = 180F
90 ohm = 160F
190 ohm = 120F

I guess the next step is to head back to the boat with some resistors to put in place of the sensor and see what temp reading each one corresponds to.

[ejay]

Thanks so much for all the research! I went looking as well and I concluded that the sender I needed is part number 280EA, which should correspond to my guage. I was able to buy it from speedway motors online and it should be here tomorrow.

I will try to install this weekend and post accordingly!

[ejay]

Update:

I was able to get the correct sending unit and installed it and now the temp guage actually works! The picture shows the old unit on the right which had the exact same part number stamped on it as the new one, which made me feel at ease.

The curious thing was the black coating on the body end which was obviously impairing the functionality. This coating would not rub off, so I am confused as to what it is and how it got there from a water bath exposure over the years.

[joe_db]

I feel the need to clarify a statement you made in the last paragraph of your post. MMI does not say you should only use their brand of gauge. MMI sells Stewart Warner gauges/senders and Isspro gauges/senders. MMI says whatever brand gauge or sender you have, you must use the same brand for the part you are replacing. Meaning Steward Warner gauge and a Stewart Warner Sender or an ISSPRO gauge and an ISSPRO sender. They are designed to communicate with one another. Not using the same brand will/may result in unreliable reading.
Hope this helps you. Whatever brand gauge you currently have, find the same brand sender for reliability.

I get why MM would say that, it eliminates a lot of confusion and returned parts, but it is not strictly true. By far the most common sender and gauge arrangement in the USA is 240-33 ohm senders and gauges. As long as they match, you should be good to go.
I swapped out my temperature sensor when replumbing the cooling system and the replacement 240-33 ohm sender works just fine with the gauge without being from the same company. If you don't have the ability or desire to figure out the ranges of gauges and sensors, buying a new patching pair will of course be the sure fire way to go. There are a variety of gauges and senders out there including senders designed to run 2 gauges that will misread when used with one, other resistance ranges, isolated ground units, and 5 volt transducer senders. VDO at least and maybe others make oil pressure senders that not only are not the usual 240-33 ohm range, but have an extra terminal for a warning light.