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nickman123

Trans temp sensor

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Bought an older used motorhome that had some issues. Had a mechanic fix some of the issues. One issue that was supposedly fixed was trans temp guage. When we bought it, trans temp guage didn't work at all- just layed on bottom. Transmission itself seemed to work fine though.  Mechanic said he fixed it with new temp sensor. Unfortunately, I didn't check this repair closely when he was done. I just started the moho up, waited till guage started rising off the bottom, and figured it was fixed.  Now I've discovered that when I start the rig up, trans temp guage does rise, but it just slowly keeps rising till its pegged all the way to the right at 320 degrees. Takes about 20-25 minutes for that to happen, regardless of whether I'm driving or just sitting idling. I'm about 95% sure that the trans is not really overheating. It shows no other sign of being hot. I can even crawl under and put my hand on the trannie case and it is warm, but not burning hot. I located the temp sensor and it does look brand new. It's located in a tee where the trans fluid line goes into the top of the trans cooler.

I've tried calling the mechanic but he's supposedly out of town for 2 weeks. I'm thinking maybe this guy installed the wrong temp sensor? Thing is, because the sensor was not actually in the transmission, it's not listed as a part for that tranny. So how do I know which is the right sensor? Would it have to be some thing that pairs with the specific guage in the dash? Any help would be appreciated.

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Shooting in the dark here but I know with auto meter gauges this applies. Maybe there is a short sweep and long sweep temp sensor. 

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i would with the key on engine off have someone ground the wire at the sender and see if the gauge goes to full hot if it reacts its probably a bad sender. also try grounding the outside of the sending unit maybe the cooling line isn't grounded well. cant really tell in the picture but it looks like the t i connected to two rubber lines

Edited by diesel-e

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I found a part number on the sensor and googled it. Says it is a high range 140-230 degree electrical sensor. This seems to match  the temp limits of the guage in the dash but i can't find anything to tell me if the sensor is meant for use with a short sweep or a long sweep guage. Looking at the dash guage, it clearly seems like I need a short sweep sensor.

I had already considered the issue about the sensor not being grounded but, as I understand it, these sensors work by providing a resistance to ground that the dash guage reads. The resistance to ground decreases as the sensor get hotter. Since my issue is that the guage seems to be reading much hotter than it really is,  seems to me that if the sensor was not grounding properly I'd be getting a lower temp reading than actual temperature.

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If the wire is disconnected it should pin in one direction and grounded it should pin in the other direction of the gauge isn’t doing that I would think the gauge is bad or the wire from the sensor is grounding out. I think this is your gauge https://c-a-i.net/DATwater_gauges.php           edited to add link http://www.partdeal.com/gauges/temperature/oil-temp-gauges/datcon-smart-2000-transmission-oil-temperature-140-320f-polished-103180.html?utm_source=google&utm_medium=cpc&adpos=1o1&scid=scplp77114048&sc_intid=77114048&gclid=EAIaIQobChMInN_38I-_4AIVAx-GCh29nAclEAQYASABEgJ9HvD_BwE      also the sensor part number you have on datcon website is the high match sensor which is the right one for the gauge you have

Edited by diesel-e

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The gauge has to match the sending unit.  If you have a 140 - 320 gauge and a 140-230 as you stated that wont work.  They also have to be in the correct range of ohms.  You can test any sensor by subjecting them to hot water and using a meter to read its ohms. 

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Sorry, I  made a dyslexic typo. Sensor supposedly works 140-320, not 140-230.  So it does seem to match the guage range. I tested sensor in hot water. It does seem to work in that it starts wth an infinite resistance across the ends at room temp and the resistance goes down as you heat it up. It seems to me that I either have the wrong sensor, a bad guage, or something wrong in the wiring between sensor and guage. 

I've looked at the guage and I cannot figure out exactly how it works in conjunction with the sensor. The sensor has just one wire connected to it, and it has 12v on that wire when you turn the key on. The guage has 3 wire connections at the back, red, yellow, and black. The red wire is 12v power to the guage. The black wire doesn't seem to do anything. I can disconnect it and it does not seem to affect guage funtion in any way. The yellow wire does have to do with guage funtion (cause if I disconnect it guage doesn't work) but I can't figure out exactly how. I thought about running a test wire directly from the sensor to the guage, but I'm not sure how to do that. Would I just run it directly from the sensor to where the yellow wire is connected? Or does that yellow wire leave the guage and connect to something else before it goes to the sensor?

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a pic of the backside of that gauge may help...........and I just saw that you have one in the first post.

Let me know if you want me to go with you if you go to storage. I need to go anyway to grab something out of the trailer.

Edited by spize909

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Red is power, black is ground, yellow is probably to the sender. 

The gauge and sender need to match. Not just temp range, but temp to ohms.  There are a bunch of senders out there and they all have different ranges, even within a manufacturer.  Autometer:

https://www.autometer.com/sensor_specs

VDO:

https://vdo-webshop.nl/en/temperature-sensors/519-vdo-coolant-temperature-sender-120c-r-3-8.html

And so on...

But before you chase the sender and gauge ranges, first thing I’d check is that ground on the gauge itself. If it’s bad, it’ll read high. Grab your multimeter and ohm it out. Should be 0-1ohm. The fact that nothing happens when you unplug it makes me suspect it. If it checks out, your gauge and sender probably aren’t matched up. I’d buy a new pair and eliminate the headache. 

Edited by Rockwood

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16 hours ago, nickman123 said:

Sorry, I  made a dyslexic typo. Sensor supposedly works 140-320, not 140-230.  So it does seem to match the guage range. I tested sensor in hot water. It does seem to work in that it starts wth an infinite resistance across the ends at room temp and the resistance goes down as you heat it up. It seems to me that I either have the wrong sensor, a bad guage, or something wrong in the wiring between sensor and guage. 

I've looked at the guage and I cannot figure out exactly how it works in conjunction with the sensor. The sensor has just one wire connected to it, and it has 12v on that wire when you turn the key on. The guage has 3 wire connections at the back, red, yellow, and black. The red wire is 12v power to the guage. The black wire doesn't seem to do anything. I can disconnect it and it does not seem to affect guage funtion in any way. The yellow wire does have to do with guage funtion (cause if I disconnect it guage doesn't work) but I can't figure out exactly how. I thought about running a test wire directly from the sensor to the guage, but I'm not sure how to do that. Would I just run it directly from the sensor to where the yellow wire is connected? Or does that yellow wire leave the guage and connect to something else before it goes to the sensor?

The sensor wire should NOT have 12v to it.  The sensor body should have ground, and then the wire that goes to the sensor will read ohms.  If that wire has 12v then I would guess its wired wrong.  With the sensor in place see if you can get an closed reading with your meter if you probe the body and the frame.  This will tell you if your getting ground. 

Hope I am explaining that right.

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this might help   http://www.precisionspeed.com/technical-support/         the yellow wire should be going to the sender if you have 12v+  on that wire something is wrong it might be shorted to another positive wire

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