cold start coolant temp sensor

[Jettage1]

Doing some troubleshooting on my cold start system.  Question regarding the above sensor:

The engine is stone cold and I unplugged the sensor to work on the system.  I happened to test the unplugged sensor with an ohmmeter and it shows continuity.  I'm thinking it is a bad sensor.  Seems to me that it should have no continuity if the engine is cold (since you unplug the sensor to test your system, "fooling" it into thinking the engine is cold), but I can't find anything in Bentley about this.

Can anyone help here?  How do I test the sensor properly?

Thanks!

[burn_your_money]

What year car? And what sensor?

[Jettage1]

92 Jetta Eco.  Sensor at front of head, top of radiator hose.  Part of the cold start system (blue/white & brown wires) 

[trev]

You have two sensors there together, or just the one?

 My '90 TD has two identical sensors there, and I can swap back and forth at will. The one that is currently supposed to run the Temp gauge is FUBAR, so I leave the start circuit one connected to the good sensor.

 Maybe check the resistance on the other sensor, and see if they match. Cold should be higher resistance than hot, but it should change in a more or less linear fashion.

 Can't help with actual resistance values.

 Cheers
  Trev

[Quantum TD]

How to test: Unplug sensor. Your glow plug light should stay on 10-20 sec depending on what GP relay you have. If you have fast glow plugs, then you really need a fast-glow relay.

If when you plug the switch back into the system, the GP light does not come on, or only comes on for a few seconds (assuming the car is stone cold), then you know it's bad.

I've never had much luck diagnosing bad electrical components with resistance values. The values I get never seem to match the specified range.

Replacing that sensor is cheap (like $10). Might as well change the AC cutout sensor on the side of the head while you're at it.

Here are the 2 sensors for the front of the head:

http://www.autohausaz.com/search/product.aspx?sid=4dgclb45fnqil455n24mmt45&partnumber=251919501D

The big BROWN one on the side of the head is for the AC/Fan system. I can't seem to find  PN for it.

[92EcoDiesel Jetta]

You have two sensors there together, or just the one?

 My '90 TD has two identical sensors there, and I can swap back and forth at will. The one that is currently supposed to run the Temp gauge is FUBAR, so I leave the start circuit one connected to the good sensor.

 Maybe check the resistance on the other sensor, and see if they match. Cold should be higher resistance than hot, but it should change in a more or less linear fashion.

 Can't help with actual resistance values.

 Cheers
  Trev

On my 92 Ecodiesel, the 2 sensors are NOT the same. one is higher resistance (coolant temp sensor) than the other (controls glow plug relay).

[trev]

Hmmmm.....

 Must look closer, then.

 Same connector. Same number on both, too, IIRC.

 Been wrong before, though. And found wrong parts installed before, too.

 Cheers
  Trev

[Jettage1]

Resurrecting this thread...  I have two identical (at least they are both black) sensors there.  I believe one runs the dash gauge and the other supplies the glowplug relay.

I don't know if they are SUPPOSED to be identical on the ECO diesel.  I do know that if I swap the dash gauge lead into the questionable sensor, it doesn't work right.  That doesn't really tell me if they are supposed to be the same and the suspect one is bad, or if they are supposed to be different. 

I've been trying to search various parts houses, including the VAG CAT site but it is really difficult to figure out an answer.  QuantumTD, I did see that listing on AutohausAZ and both of mine do look like the one pictured.  If I had owned the car from new, I'd say it has to be the right one, but I'd like to confirm that I'm putting in the correct thing now...rather than possibly replacing someone else's mistake.

92ECO, what colors are your two sensors, and which does what?

If anyone has a definitive part number for the glow plug coolant temp sensor for a 92 ECO, I'd be much obliged... 

[Baron VonZeppelin]

On the CE2 wired NA's (at minimum) they seem to be the exact same sensor with same functionings. It would be odd if the 91/92 Eco were any different. But not impossible.

You could try asking Broke VW on Vtex, he has factory parts look up - and can also get factory parts.

[Jettage1]

Thanks for the suggestion, Baron.  I think the odds are greater than not that what's in the car now (2 - same) is correct, but you never know.  I'll try to contact Broke.

[92EcoDiesel Jetta]

It's been a while since I worked on the temp sensors on the coolant flange when I was sorting it out my 92 Ecodiesel. As I recall, they were different, the resistance readings were different, if I swap them the connectors, the temp gauge would read way low.  I can't find the printout from the dealer on the various parts that go into the flange but as I recall it did show a few different part numbers for the 2 temp sensors.

[92EcoDiesel Jetta]

"The big BROWN one on the side of the head is for the AC/Fan system."

Does anyone have a schematic for this big BROWN one with 4 terminals? I can't find it in the Bently- no mention of it anywhere. The p/n is 357919369E. I just bought one from a local VW dealer who had 2 in stock. I got one for $22 tax included (he gave me a good price). Further Google search indicates Bus Depot also have it http://www.busdepot.com/details.jsp?partnumber=357919369E

Of the 2 sesors on the front of the head, the connector with the white/blue wire controls the glow plug duration (via the glow plug relay) and the one with the red/ yellow wire is for the coolant temp gauge.

How to test: Unplug sensor. Your glow plug light should stay on 10-20 sec depending on what GP relay you have. If you have fast glow plugs, then you really need a fast-glow relay.

If when you plug the switch back into the system, the GP light does not come on, or only comes on for a few seconds (assuming the car is stone cold), then you know it's bad.

I've never had much luck diagnosing bad electrical components with resistance values. The values I get never seem to match the specified range.

Replacing that sensor is cheap (like $10). Might as well change the AC cutout sensor on the side of the head while you're at it.

Here are the 2 sensors for the front of the head:

http://www.autohausaz.com/search/product.aspx?sid=4dgclb45fnqil455n24mmt45&partnumber=251919501D

The big BROWN one on the side of the head is for the AC/Fan system. I can't seem to find  PN for it.