Loudgoldwing

'98 Cabriolet 2.8L 12V

Got a wandering Temperature Gauge and I'm getting wandering fuel economy, so I figured I would replace the Engine Coolant Temperature Sensor.

Problem is, I don't seem to have the sensor that people are telling me I'm supposed to have. Here's a picture of what I've got:



It's a brown plug, with a four-wire setup. (Blue/white, Blue/yellow, Blue/black, and Blue/brown)

It looks like the sensor itself is about a 1" hex-head. I can't get far enough into the area it's mounted in (passenger side, screwed into a black gloss-painted metal pipe connecting the two sides of the head like a foot-long shower "handicapped handle.")

I can't tell if it's held in with a clip, or if it's supposed to screw out.

The parts store sold me a 2-prong unit that's a LOT smaller than this 4-prong in the photo. Admittedly, my ELSA system is showing me a two-pronger that lives on the DRIVER side.

I'm going back out to the garage to look on the driver side now... Somebody give me a hand here, would ya?

CRAZY_RUSSIAN

take that one out and read the part number that should be stamped on the brass side of the sensor, take the part number to a dealer or an independent shop that dos VWs and Audis ;-)

maineman

I believe the one for the gauge is on the passenger side.
Multi function sensor

http://www.12v.org/engine/index.php?...n=cl&view=rear

coriard

Yep, I actually replaced both of those on my 12v 90 quattro. But it turned out that I just needed a new thermostat.

When you say driver side and passenger side, I'm assuming US style, not UK?

The Multi Function Temperature Sensor (MFTS) is the one on the passenger side (US style) with the 4 prong wiring harness connection. The new ones only have 3 prongs, but fits just the same. The other side, and this is my understanding, is the one that is actually called the 'Engine Coolant Temperature Sensor,' but I think they both play similar roles. I got a MFTS for $54 online and a ECTS for $12 or something. I pretty much bought the first 2 prong and 4 prong results I found, stuck them both in, and they made a small difference even though it wasn't my problem.

I'll stop there. Did that help?

Loudgoldwing

My issue, though minor, is in two parts. #1, my temp gauge will be right at the thick line that appears to signify "This is the temperature you should be running," and then it will drop (over the course of about five seconds) to just over the "warm-up" area that comprises the first 20% of the range of the gauge.

This is while I'm doing 73mph on the freeway...

Also, I posted a few days ago about wierd fuel economy. 23 mpg in regular daily driving through town, then 21 mpg on a 400-mile freeway road trip. People suggested that the Engine Coolant Temperature Sensor was starting to fail, causing the ECM to mix fuel and air into a less efficient ratio.

So, what I need to know is: which sensor do I need to replace? I've already been to a dealership, and the parts guy was a complete moron (and before you judge me on my assessment of his skill set, please understand that he also didn't know what fluid he was supposed to sell me in order to top off my power steering reservoir, either).

Is there one sensor for the gauge, and a separate one for the ECM? That seems to be what I'm feeling from you guys. The ELSA says the driver-side two-pronger is the one I need to replace. I looked, and I found the one they're talking about. And the picture ELSA shows of the sensor DOES match the one I bought...

But, if ELSA and the auto parts store are both correct, that would mean there is one sensor to control the gauge, and a SEPARATE sensor that sends readings to the ECM for fuel/air mixture calibration. Am I just supposed to assume that the failure of BOTH sensors has happened coincidentally at the same time? Or am I just barking up the wrong "gas-mileage-tree" to begin with?

I have a set of NGK plugs and NGK wires I've been considering putting in over the weekend, too. And yes, before you ask, I am mildly OCD about the precision with which I expect this vehicle to run and drive. Once you own a 55-powered MB, you become a perfectionist.

maineman

The sensor on the passenger side (US) is the gauge sensor.How do you know it's not your thermostat ? Could be the gauge also?Could be the ground on the back of the cluster?
The one on the drivers side is for the ecu.
My cabrio runs cool like that also ...............

Loudgoldwing

Thermostat, water pump, and timing belt replaced immediately upon purchase 2,000 miles ago.

Loudgoldwing

Suppose I should add that I have about half a can of MAF cleaner on my garage shelf, as well.

I'm going to start small. Clean the MAF, change the plugs, and swap the sensor. I'll start it up and see how it feels. If it's still rough, I'll swap out the plug wires.

All of this stuff is what I consider preventative maintenance anyways. It's kinda what I live for on weekends, and I was really just hoping somebady would be able to say, "Hey, I had that problem, too. This is what I did, and it worked like a charm..."

Problem is, I just don't think there are as many "wrench-educated" owners of this vehicle as there are other vehicles I've owned/own. Not to say that people here aren't smart. Quite the contrary... I've received some really good advice here. It's just that there aren't that many of these vehicles on the road in order to create a swarm of "four-ring fanboys."

So, I'll just load up the ol' parts shotgun, give 'er a pull, and let you know how she goes. Who knows... maybe my "happy fun time" I spend this weekend will actually prove really useful to somebody down the road.

For the $150 bucks we're talking about here, I guess the combination of peace-of-mind and posterity will end up being worth it. We'll see...

In the meantime, if anybody else has any suggestions, keep tossin' 'em in the pile!

Thanks, guys.

hubturn

Bentleys has a test procedure for this sensor. You need a good quality multimeter and if you do, you could test it first and possibly not have to replace it. My bentleys is not installed yet (replaced hard drive) or I'd give you the procedure. Somebody else surely can call this up.

Loudgoldwing

Ok Maine, now you've got me thinking... My gas gauge has been screwy ever since I bought the vehicle. If both fuel and temp gauges are having issues, is that indicative of this cluster ground fault you're talking about? I just figured the fuel level sensor was bad.

Geez, what a great tinkerer's ride I bought. This thing is like a set of Lego's. You just keep buying more stuff, and once you install the stuff you bought, that stuff makes the other stuff seem not quite right...