NOLA Speed and Style

Author Topic: Engine Temperature Sender Test  (Read 335 times)

November 01, 2019, 05:13:23 pm

ftm1776

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Engine Temperature Sender Test
« on: November 01, 2019, 05:13:23 pm »
My coolant temperature sender is mounted on the side of the radiator hose flange on the cylinder head.
Brown and Yellow/Red wires coming from it.
I'm getting some weird temp readings, so step one is to check to see if the sender is working.
For now, to keep this simple, let me just note that the temp readings are unusually low.
Is there an easy test to verify that the sender is working?? Troubleshoot??
« Last Edit: November 01, 2019, 05:15:12 pm by ftm1776 »


Thomas, Original and sole owner since new:
1991 Jetta NA 1.6 diesel, Engine Code ME, 5 speed, AWY transmission, Hydraulic Lifters
293,000 miles
LOOKING FOR A GOOD VW DIESEL ENGINE BUILDER ON THE WEST COAST

Reply #1November 02, 2019, 11:42:43 am

ORCoaster

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Re: Engine Temperature Sender Test
« Reply #1 on: November 02, 2019, 11:42:43 am »
To verify the sender is working you should pull it off the flange and find a place to heat some water in a pan.  If you have a camp stove or a burner on the BBQ that will keep the EVIL EYE of the wife off you.  If you are like me you just wait till she leaves the house and use the kitchen stove.  Just don't use her best cooking pan for this. 

Add some water to the pan enough to get it to boiling without boiling it away, a couple of inches.  Then put a thermometer in the water or use one of those sensing guns if you have it.  You also will need a multi meter to hook up to the leads and watch the resistance change as it heats up.  It should swing from one reading when cold to another when hot.  I can't tell you the exact values but those might be in the Bentley manual and I might find them later if you need them.  But I do believe, someone tell me otherwise, that the value goes down as the heat goes up.

My logic on that is the voltage on the actual gauge goes up as an indicator that the temperature is going up.  You might try measuring the voltage at cold and then pulling the wire on a hot engine and measuring that.  Just to see if it is changing and by how much. 

What happens if you just run a jumper to the wire from the battery?  Does it show super hot? 
Looking to add fender trim, LED lights and a bumper push to the truck soon.

Reply #2November 11, 2019, 11:13:18 pm

thomas m

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Re: Engine Temperature Sender Test
« Reply #2 on: November 11, 2019, 11:13:18 pm »
I ran some temp vs resistance tests for the sender. It's a thermistor that changes resistance with temp having less resistance as it heats up. It seems to function that way although I can't verify if the numbers are correct as I have no specs on it.

I did the Bentley test, grounding the temp gauge wire direct to ground, bypassing the thermistor; that is, simulating a high temp (low resistance) condition. My gauge read  only about 3/8 full deflection when it should have read full deflection for a simulated high temp condition. To me, that says the gauge is faulty. But this is electrical stuff and it always baffles me or melts something.

I think there is a small solid state "voltage stabilizer" that converts 12 volt to 10 volts and feeds the temp and fuel gauges. I read only 8.6 volts at the sender until I jiggled the wires and the #16 fuse that protects that circuit. Volts jumped to 9.6 volts. That helped a little with the temp gauge and got it to read about 1/2 full deflection at operating temp.

So I'm still stuck where I started. Some time ago I installed an auxiliary temp gauge. I screwed its sender into a tapped boss on the front of the engine block. Its readout correlates very closely with a temp gun reading of the radiator outlet flange on the head. So I have a reliable temp read out.

I think I will replace the sender and see what happens. I have no solid reason to do that and expect different results, but, as I said, electrical is not my strong suit and sometime magic happens or smokes!!!!!
Thomas, Original owner since new:
1991 MK2 Jetta Non-turbo 1.6 diesel, Engine Code ME, 5 speed 020 AWY 04120 transmission, Hydraulic Lifters,
315,000 miles
Location: Vancouver, WA
LOOKING FOR REBUILD OPTIONS IN PORTLAND, OR OR ON THE WEST COAST

Reply #3November 11, 2019, 11:24:19 pm

ORCoaster

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Re: Engine Temperature Sender Test
« Reply #3 on: November 11, 2019, 11:24:19 pm »
Good that you have something that you trust that works.  Gauges and senders do age and go wonky at times.  Not sure a new sender will be the cure.  May need a new gauge.  PM me if you do. I have several.  And those stabilizers are not cheap either.  Just buy a spare cluster on Ebay or Craigslist for 40 bucks and get both of them.   
Looking to add fender trim, LED lights and a bumper push to the truck soon.

 

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