Strange Temperature and Fuel Gauge behaviour

Hi all,

I have just purchased a 2003 Golf 1.8T 180bhp. I noticed on the test drive that the temperature gauge didn't get to the right temperature, so just thought i'll change the thermostat and sensor.

Another issue I noticed is the fuel gauge seemed to over-read.

Anyway, since owning i've replaced with all genuine parts
TB, all tensioners, water pump, thermostat, green temp sensor, g13.

HOWEVER:

The temp gauge still does not read to 90 degrees and the fuel gauge still over reads.

I have checked in Vagcom, which shows in engine parameters the coolant temp is pretty much 90 degrees bang on. BUT the instrument parameters shows temperature as 72 degrees at the same time.

Fuel tank level shows 53 litres even though i've driven a good 100 miles since filling up.

Vagcom instrument tests shows the gauges work as normal


Can anyone tell me what is going on?

Anyone?

I've cleaned up the earth connections under the battery, checked connectors for coolant migration and corrosion, removed the dash gauges and checked connectors.

I've just measured the resistance of the green sensor at 9 degree (48F) and got the following:
A-B (gauge): 1.56 kOhm
C-D (ecu): 3.76 kOhm

Is this acceptable?

At this point
ECU reading in engine VAGCOM reports : 9 degC (48F)
Gauge reading instruments VAGCOM reports : 5degC (41F)

Has the thermostat been changed?
Cheers

Yes, new genuine thermostat with new housing.

CTS reads 90 (190F) on engine measure in VCDS

CTS reads 71 at same time in instruments measure in VCDS

Have checked all connectors, no corrosion or fretting.

I think you can trust the number you're getting out of VCDS. You could double-check with a hand-held IR thermometer if you have one.

The gauge is "buffered" - in that it should read "straight up" anywhere from about 165-210 (or I think I heard 220?) degrees. This is also true of BMW's, and I know that you can get into the instrument cluster's controller with special software like INPA and re-write the binary map that controls this "buffering" factor to get the gauge to read more accurately. I do not know if there's an equivalent procedure for the VW gauge cluster - it would be cool if that were possible.

(there also used to be a 3rd party device you could buy called "coolant snitch", which you could plug into your ODBC port, and it had a digital readout and would report your ODBC coolant temperature on the screen; without the buffering adjustment of your dashboard gauge. I don't think the manufacturer makes them anymore, unfortunately.)

I think that if you ECU is sending a correct temperature to the ODBC connector, read by VCDS, then there would only be one or two possible reasons for a bad reading on your gauge: Either the gauge has a mechanical problem, or VW is also using a controller in the cluster that handles the buffering, and somehow that binary image has gotten corrupted.

But at this point, I think I'm straying way into the range of the speculative and theoretical, because I'm talking about a known-system on a completely different car.

Ross-Tech might be a good resource to ask on whether there's a controller in the dash cluster that can be queried.

As for the Ohm readings - you might consult the Bentley manual, if you have one, to see if there's a spec. But I'd think that if you're getting a correct reading out of VCDS, then the Ohms coming from the sensor are probably correct.

Wish I could be more helpful, because this is an interesting problem.

The OBD port temp reading comes from the ECU via "the other side" of the CTS. Remember, the CTS has two connections in the connector. One is for the Temp Gauge and the other is for the ECU. However, I'd certainly be inclined to depend on the VCDS reading for the engine temp.

If the CTS side for the ECU goes bad, the default in VCDS will be minus 40c! I've seen that numerous times while the in dash gauge will be straight up on 190 (what ever that is in centigrade).

There is a "common" connection (splice 269) that a number of sensors ground to though the cluster. Example: Fuel gauge, brake fluid level, washer level, and I think maybe a couple more. But, the CTS has nothing to do with that splice. So, if that ground is the problem relating to the Fuel Gauge, most likely the others would be flashing, etc.

Over voltage due to a bad voltage regulator will cause the Cluster to default to zero on everything (been there and seen that several times). But, before the default, gauges will over-read, including the Tach and Speedometer...only for an instant. That default mode protects the cluster components.

Not really helpful, but should shed some thought on a few things!