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Old 05-24-2012, 11:21 AM   #1
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Question Extra watertemp sensor reads different then OEM sensor... Help me out...

Hi guys,

with my DIYgaugcluster with the Maxigauge (see thread) I installed a second watertemp sensor. The sensor sits also inside the coolant reroute adpater - just like the OEM sensor - just about 3cm (a bit over an inch for you Americans...) away.

The OEM sensor gives me values of ~85-95C (185 F- 203F) in Tunerstudio. Just normal driving, some short boosted runs.

The VDO sensor gives me values of 95C - 115C ( 203-239 F) in the Maxigauge-display.

I tested, if a separate ground to the sensor helps - but there is no change.

About the sensor:

I got the sensor from this webshop -

http://www.ezt-autoteile.de/VDO-Sens...TF::23409.html

They even have a site, where all the values for the sensors are shown:
http://www.temperaturgeber.de/vdo_te...ber_daten.html

The values are similar with the 92-027-004 charateristic curve, which is the curve that Hakan programmed into the Maxigauge.

The sensor is a little hidden behind the engine, but I can see the max. 120C stamp on it - so I think it's the correct one.
(BTW The sensor reads perfectly match with the oiltemp, OEM watertemp, Manifold air temp at 20C when I start the car.)

My OEM Mazda sensor sits in exactly the same spot of the engine in the same aluminium adapter only about 3cm away.

I am about 90% sure that the OEM sensor reads correct, because the values for my engine are reasonable 85-90 at cruise and up to 95C at higher speeds.

The VDO sensor reads up to 115C... which I don't see happening with my cooling system... (coolant reroute, 86C thermostat, huge cooler)

So what do we have:
- Maxigauge programmed correctly - check
- Sensor correct - from what I can see with it in the car - check
- wiring directly from the sensor to the Maxigauge - check
- sensor has good common ground - check

I can't wrap my head around that...

Please help me out - what could possibly be wrong?
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Old 05-24-2012, 12:13 PM   #2
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So both sensors are body grounded correct? Only one electrical pin, and they ground through the threads. My guess is either you have a bad ground on one of the sensors causing them to read funny. Or you have the wrong calibration curve in either TS or your digital gauge. My guess is your VDO sensor is incorrect. I have the opposite situation. My VDO gauge usually reads a few degrees cooler than my OEM gauge. But my VDO has a dedicated ground pin so I don't have to worry about it ground through thread sealant so I trust it a little more.

My advice says to pull both sensors out of the car, put them in boiling water and check the resistance levels. That way you know they are both at the same temperature and you can check your calibrations.
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Old 07-14-2012, 11:24 AM   #3
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As I still have to resolve this problem. I had a look at wiring schematics for the 99-00 cars. If I understand this correctly the ECU sensor itself has ground through the ECU - only the also included sensor for the gauge panel is grounded through the sensor body... Could someone confirm this?

That would imly that only my VDO sensor could be reading wrong because of the thread sealant...
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Old 07-14-2012, 12:44 PM   #4
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That is correct.

How did you "run a separate ground wire"?

With the engine warm and at a known temperature, try disconnecting the VDO sensor from the gauge, then measure the resistance and compare it against the chart. If you're handy at math you can look for the standard thermistor equations and figure out the resistance vs temperature equation of the sensor you have so you know the resistance for every temperature.

If the above checks out OK, next, reconnect the VDO sensor and measure the voltage at the sensor. If it is lowish like 1-2V then it will be sensitive to the voltage drop on the threads. See if you can measure the voltage between the sender body and the cylinder head. If this is very low (like <0.05 V), then you may have a ground shift between the sender and the VDO gauge. Measure the voltage difference between the maxigauge sensor ground pin (does it have one?), and the cylinder head. If this checks out OK too (<0.05V say) then the Maxigauge needs to be reprogrammed.

BTW you can check the Maxigauge by taking a 51Ω 2W resistor and using it in place of the sender. See if it reads 90 C.
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Old 07-14-2012, 02:02 PM   #5
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Thanks Jason,

Separate ground - I attached a ground wire to the back of the head and grounded the back part of the seneor body.

I already checked the Maxigauge with a resistor - works fine.

I am going to check the voltages, as you suggested.
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Old 07-14-2012, 09:00 PM   #6
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all the sensors actually ground to the block under the booster. the head is the main ground.
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Old 07-15-2012, 04:45 AM   #7
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Negatory. A ground isn't a ground isn't a ground. It's all relative.

Sensors are returned to ECU sensor ground, the ECU high currents are returned to the head. Sensors are not connected to the chassis nor head. They are connected to the head through the ECU. The battery is connected to the chassis, as are the lights, stereo, power windows, etc. The chassis is connected to the block via the strap near the booster. All these "grounds" are at slightly different potentials.

The sensor signals, because they are sensitive, have to be returned to a point that the ECU will explicitly measure against, and that is the sensor ground pin.
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Old 07-15-2012, 05:00 AM   #8
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I am with Jason here, I had alook at the 99-00 wiring schematics and there you can see that all the sensors have a common ground to the ECU ground pin. Only the wt sensor for the gauge cluster is grounded directly at the head...
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Old 07-16-2012, 12:19 PM   #9
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Disconnect the coolant sensor from everything and measure the resistance at operating temp. If it's 51Ohms, you know the sensor is OK. Then use a 100Ohm trim-potentiometer and hook it up to your maxigauge instead of the coolant sensor and adjust it until you see 90C. Install it between the maxigauge ground and the input and not any other ground.
Once adjusted to 90C, remove it and measure resistance. If that's 51Ohms, you have a ground problem somewhere. If it's something else your gauge reads wrong.

ST
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Old 07-28-2012, 04:39 AM   #10
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O.K. yesterday I got some time to test some more things

1. I swapped out the sensor and cleaned the threads (just in case) - no change, Maxigauge still reading high.

2. I measured the resistance at the sensor to (head) ground -

at 78 C (according to my OEM sensor in Tuner Studio) I got 77ohm which is spot on

(Here are the characteristic values of the sensor for this spot:
75.0C 82.36ohm
80.0C 70.12ohm)

But Maxigauge is reading ~85C...

at ~86C I got ~56ohm - again spot on

But Maxigauge reading ~ 94C

3. So I tried a new cable between sensor and Maxigauge - no change

Today I am going to give the Maxigauge some separate +12V and ground (not from the gauge cluster) and check the Maxigauge with some resistors.
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Old 07-30-2012, 12:43 PM   #11
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Just check if there's a voltage difference between the head and the various ground pins of the Maxigauge.

It sounds to me like the Maxigauge software is wrong.
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Old 08-10-2012, 04:28 PM   #12
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Just a small update sent from my phone,
I grounded the Maxigauge directly at the head and...

All is perfect now. The two coolant sensors read within 1 degree difference.
Thanks for all your help.
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