Water temp sensor - help with identifying?

[patience]

Hi folks, acquired a 2005 SV this summer, and have been enjoying getting to know its driving characteristics (what a hoot!) and fixing numerous issues (not so much of a hoot...)

 

My water temp gauge on the dash went dead on my last drive. Thought I smelled a burning smell, but that could be just coincidental.

 

I've tested the gauge (grounded the input), and the gauge pegged high, which I assume means its working.

 

Tried messing around with the temp sensor on the thermostat housing on the SVT Zetec, to no avail, only realizing afterwards that there is a temp sensor screwed into the back of the radiator as well. See attached image.

 

Two questions:

1. The temps sensor measure infinite resistance across it's two terminals (short circuit) when cold and hot. Assuming this means its dead?

2. I can't find a replacement sensor (even on Caterham's online store) that looks suitable, and they haven't responded to my inquiries, so wondering if this is a US-specific part? Its stamped 6.0201. It also has 04 15 stamped on it and 92/87. The last is likely the opening and closing temps. Anyone know who made this part and where I could get one? Checked all the parts interchange search engines, and came up blank.

 

Thanks in advance!

Edited February 27, 2016 by patience

[yellowss7]

I seem to recall that the sender you are showing was a VW part. I'll try to search blatchat to find a part number. But I also seem to recall that there were two sensors on my Rover K powered car, both in the water rail, one to the gauge, one to the ECU and that the sensor in the radiator was a swich to turn the fan on. As I said, that was the set up on my 2000 RoverK powered car. But I bet its the same on your car as well. Tom

 

Glenn beat me to it. here is the caterham part, http://www.caterhamparts.co.uk/product.php?id_product=38

 

Here is the link for the sender to the gauge and it's in stock.

 

http://www.caterhamparts.co.uk/product.php?id_product=17

Edited February 9, 2016 by yellowss7

[glenntwincam]

this switch controls rad fan only. i replaced mine with vw part 823 959 481F/92. you're correct about 92/87 it's temp when fan turns on/off. switch is used on older water cooled vw's .check threads , my car has rad with plastic tanks.

[patience]

Well, you were both right, and I was very wrong. Yellowss7, you're right, the sender is a VW part (and also used on Saabs!) from the 80's. My local auto part store helped me track it down. While I was waiting for the part, I came to realize what glenntwincam said was true. It just controls the radiator fan. So I went back to the temp sensor on the thermostat housing on the back of the Zetec block. Traced the wiring back to where it terminates into a plug that goes into the main wiring harness. Wires have continuity, so no breaks in the wire. In frustration, took the car back out for a spin again, got it nice and hot, pulled the gauge and measured the voltage output from the sensor signal wire that plugs into the back of the gauge. And it's reading 150mV, and slowly descending, as the car cools off. So it's gotta be the gauge (amirite?). Now maybe the burning smell is making sense to me - could the gauge have burnt out? Because I grounded the gauge and it pegged high, I assumed it was working, but that may not be the case. I can't get the housing off the Caerbont gauge, so I can't see what's going on inside, but I'll take it to a local gauge repair shop (turns out we have one of the world's best in my backyard), and see if they can fix it. Unfortunately, this was a special JPE edition gauge (yellow fluorescent face), and I"m assuming I won't find a replacement. Tell me I haven't lost my mind, and this rationale makes sense. In the end, I'm a mechanical engineer by training. Electrons don't make sense to me.

[yellowss7]

I'm not good with electrics either, but from my early days with my first car a Triumph Spitfire, If the smoke came out then it won't work anymore. You have to keep the smoke in the wires or gauges to keep them working. Here is a link for a Lucas Smoke Replacement kit. http://www3.telus.net/bc_triumph_registry/smoke.htm :rofl: Tom

[glenntwincam]

temp sensor on top of stat housing sends info to ecu , sensor for gauge is located on bottom left of same housing is for gauge.

[patience]

I'm not giving up on this. (Tom, you're cracking me up with the smoke bit. I wasted a good half hour reading all that.

 

Well, I had to step away from the car for two weeks to take my family on vacation (priorities, I suppose, ha). Glenntwincam, you are right, there is an additional sensor on the bottom of that housing. I pulled that sensor, measured the resistance across it cold (about 2,000 ohms), then boiled it, and it dropped to about 600 Ohms which is consistent with correct operation (see here:https://csr175.wordpress.com/2013/12/21/recalibrating-the-water-temperature-gauge/ ).

 

Also, I pulled the gauge again, and set up a 12 volt source (8 batteries in series at 1.5 V each), and then applied voltage to the sensor wire input to the gauge. And lo and behold, it works just fine. At 1.5 volts to the sensor wire, it goes full scale, at 3 volts, about halfway, at 4.5 volts, no real movement (again consistent with the above link).

 

So at this point, I have confirmed the right coolant temp sensor, confirmed that the temp sensor works, and confirmed the gauge works. So it's now somewhere between the sensor and the gauge, for which I could still all use your collective intelligence. (I searched for a Zetec wiring diagram for Caterhams, no luck.)

 

The temp sensor on the engine has two wires coming from it. One is just a grounding wire (because the housing its mounted on is plastic, so doesn't provide chassis ground.) The signal wire disappears into the wiring loom. The wire that emerges under the dash that plugs into the gauge is a different color. I'm assuming in between these two wires is a unit that converts the voltage to something that the gauge can respond to. I don't know if that's a separate unit or the ECU. Can anyone help here?

 

The burning smell that I smelled on the drive when all this stopped working is starting to concern me. Anything you can enlighten me on would be much appreciated. I'm gonna go start pulling fuses and checking to see if any of those are burnt out.

 

Thanks to all, you've all been extremely helpful.

Edited February 27, 2016 by patience

[patience]

Well, obnoxiously, after having reassembled everything, the gauge is now working. :banghead:

 

I suppose I could have cleaned up a corroded ground and that was all. Ha, my introduction to notorious British electrics.

 

Thanks to you all. Hope this was at least entertaining.

[yellowss7]

Caterhams are notorious for electrical issues that are ground related. Glad you found the issue, Enjoy the car! Tom

[Off Road SHO]

While not a Caterham, my Superformance S-1 had an intermittent headlight problem that was driving me nuts. They worked, they didn't work, they worked 15 minutes later. Arrrggghhh! Turned out to be the silliest thing.

 

http://www.usa7s.net/vb/showthread.php?9212-Superformance-S-1-Headlight-problem&highlight=Superformance

 

Tom