need help with coolant temp sensor resistance - IDI Engine - VWDiesel.net The IDI, TDI, and mTDI source.

• need help with coolant temp sensor resistance by 92EcoDiesel Jetta on 19 Sep, 2011 17:13
• My coolant temp  gauge is reading extremely low at the 1/4 mark. I checked the coolant hose at the flange with an IR thermometer on a hot engine and it reads 194 F. I am trying to determine, if the sensor, or wiring or gauge is bad. With a hot engine idling for a few minutes and 194F per the IR thermometer, the sensor is measuring 160 ohms. Can someone measure their sensor resistance with a DVM on a hot engine as a check? I'd hate to spend $30 on a new sensor if it is not bad.
  
  
• #1 by cyrus #1 on 19 Sep, 2011 20:24
• I don't have the resistance handy at the moment, although I did see a chart floating around somewhere.  However, as a quick and dirty test you can disconnect the harness from the sensor and ground the yellow w/ red stripe wire.  This should simulate a high engine temperature and make the gauge move to the top of the scale.
  
  Have you noticed any other odd issues?  Is your fuel gauge behaving normally?  There is also a voltage stabilizer that can fail and cause these types of issues.  Typically the fuel gauge would be doing the same thing though.
• #2 by 92EcoDiesel Jetta on 20 Sep, 2011 12:08
• No other odd issues that I noticed. Fuel gauge is behaving normally. I will try shorting the yellow/red wire to ground and see what the gauge do. If you can dig up a link to that table it'd be appreciated. Thanks!
• #3 by 745 turbogreasel on 20 Sep, 2011 14:28
• Check your Bentley for sender resistance @ temp, then run down to Radio Shack, and buy a resister of  however many Ohms.
  Substitute the resistor for your sender, and see what you get.
  
  I think mine was 1050 Ohm @ mid gauge, but Volvo used a few different senders by year.
• #4 by 92EcoDiesel Jetta on 20 Sep, 2011 15:30
• I grounded the yel/red wire and the gauge pegged to the max. Did the same thing when I jumpered the connector terminals to each other (also grounds the yel/red wire).
  
  My 85-92 Bentley does not not have anything on the resistance /temp relationship of the sensor in the cooling system section. Maybe I missed it? If you find it please post the info. At this point I suspect the sensor. I think they are original sensors from when I bought the car. The one for the glow plug is NFG (open circuited so it was getting max glow plug time and since modified to be manually controlled) . I'll just order new sensors.
• #5 by 92EcoDiesel Jetta on 20 Sep, 2011 15:50
• Autohausaz shows 2 coolant temp sensors. I have the black one. Anyone know what the blue one is for?
  
  https://www.autohausaz.com/search/product.aspx?sid=1w3j1knwvjcdrnecbe5ajs55&makeid=800026@VW&modelid=1283486@JETTA GL TDI &year=1992&cid=sensor@sensor&gid=2188@Coolant Temperature Sensor
  
  
• #6 by R.O.R-2.0 on 20 Sep, 2011 16:10
• i would say its for either a TDI, or a 2.0 motronic..
• #7 by Luckypabst on 20 Sep, 2011 16:31
• I would begin by just checking for a change in resistance with change in temperature to see that the sensor is working. I think most failures would show up as no change as the sensor heats up.
  
  In your case, I would check wiring connections and power supply first if I suspected an inaccurate gauge reading. Apples to oranges for sure but my Vanagon had a temperamental temp gauge, until I turned one of the contactor nuts on the backside just enough to break up any tarnish on the connection.
  
  Chris
• #8 by 92EcoDiesel Jetta on 20 Sep, 2011 20:28
• Quote from: Luckypabst on 20 Sep, 2011 16:31

  I would begin by just checking for a change in resistance with change in temperature to see that the sensor is working. I think most failures would show up as no change as the sensor heats up.
  
  In your case, I would check wiring connections and power supply first if I suspected an inaccurate gauge reading. Apples to oranges for sure but my Vanagon had a temperamental temp gauge, until I turned one of the contactor nuts on the backside just enough to break up any tarnish on the connection.
  
  Chris
  

  
  I did all that already. I am down to a bad sensor. If someone can measure theirs at operating temp so I can confirm my sensor is bad that'd be great.
• #9 by 92EcoDiesel Jetta on 01 Oct, 2011 11:51
• The old (bad) sensor resistance is roughly 2x higher than the new one for any given temp. Installed the new sensor and just went for a drive and temp gauge reading is back to normal. 
• #10 by Luckypabst on 01 Oct, 2011 13:08
• For future reference, what are the resistance values for cold and for operating temperature?
  
  Chris
• #11 by 92EcoDiesel Jetta on 01 Oct, 2011 18:11
• Quote from: Luckypabst on 01 Oct, 2011 13:08

  For future reference, what are the resistance values for cold and for operating temperature?
  
  Chris
  

  
  1.1k ohms @ approx 72 F
  108 ohms @ approx 170F measured by an IR thermometer of big coolant hose off the flange where sensor resides.