I was going to install some VDO gauges I salvaged from the junk yard (applies to new
gauges also) as my next project, but after doing some research I may think twice before
doing so, and so should YOU!
Here's why:
Majority of aftermarket gauges provide such inaccurate/inconstant readings that they are
virtually useless, below I will try to explain exactly why:
Let’s start with
Temperature GaugesThey are most prone to inaccurate/inconstant readings and provide very important
information on the state of your engine (Exhaust Gas Temp or EGT, Oil Temp, Water
Temp, etc.):
The position of the sender/sensor/probe (later sensor) is one of the things to bear in mind,
in e.g. if we are measuring oil temp installing the sensor in the oil pressure sensor
location or after a oil cooler is not such a good idea...
A place near the oil pump pickup may be a better spot (as this area has good flow, and the
oil completes it’s full circle here), but generally requires drilling/welding the oil sump
and depending on the sensor type may require an adequate ground to be wired... Also
airflow around the sump can cool the sensor to the point where its readings may become
off.
Many gauges use a thermistor (
wiki) based sensor (in e.g. VDO oil temp gauge/sensor).
What this means is that the sensor is very slow to react to temp rise, so by the time the
gauge reports a higher temp the oil has already began to break down and/or foam if we
are talking about the oil temp, or the coolant is already boiling if we are talking about
coolant temp and so on...
Thermocouple based sensors (in e.g. VDO EGT gauge/sensor) are almost instant to react,
but also have drawbacks:
Thermocouples (
wiki) consist of two wires made from different material (in e.g. alumel
and chromel for a K-Type thermocouple). The end of the thermocouple that is placed
near the object which temperature we want to measure and is called the “hot junction”,
the other end is known as the “cold junction”. The difference in temperatures between the
cold and the hot junction generates a voltage. The voltage going the gauge indicates the
temp of the object. The problem here is: we either have to keep the temp of the cold
junction constant (which is almost impossible in a automotive environment), or we must
compensate for the variation of the “ambient temp” in which the cold junction is located
(be it inside the engine bay or inside the cockpit) but only very few gauges make this
compensation. Most of the gauges are set to a fixed cold junction temp of ~ 75F (24C).
And here is a quote from a gauge manufacturer CHT (the same as VDO) to show exactly
what I mean:
For COLD JUNCTION temperatures HIGHER than 75 DEGREES F. : The Indicator will read ONE DEGREE LOW for each DEGREE of COLD JUNCTION temperature ABOVE 75 DEGREES F.
For COLD JUNCTION temperatures LOWER than 75 DEGREES F. : The indicator will read one DEGREE HIGH for each DEGREE of COLD JUNCTION temperature BELOW 75 DEGREES P.
So, imagine this: On a hot 90F (32C) summer day your typical non compensating
thermocouple based gauge/sensor will show a temp 15F (6C) lower than the actual temp
of the object you are trying to measure and on a cold 35F (2C) winter day it will show a
temp that is 40F (22C) higher than the actual object temp. How is that for accuracy and
consistency?!
Just to add wood to the fire consider these observations made by a man named Dave Bolen:
I finally got out my new CHT gauge and sender over the weekend and did some
comparisons on the temperatures using the oven and the freezer.
Here is what I found out. First of all, the sender was "mis-wired", red went to the
negative terminal on the gauge rather than the positive one.
The major thing that I noticed was that the gauge reads about 20 degrees difference
(higher) when tilted upward at an angle like it might be mounted in a dash(rather than
horizontal).
Placing the gauge in the freezer for 15 minutes while the sender was held at a steady
temp raised the temp shown on the gauge by about 20 degrees.
The senders were placed between two pieces of sheet metal in the oven I set a pyrex dish
on top of the sheetmetal pieces in order make sure nothing moved. Gauge temps based on
"house" ambient temperature(70 degrees).
All temps are in farenheit, and numbers that aren't "on the mark" on the cht gauge are
approximate.
The meter I used was a fairly expensive METEX with a type k thermocouple probe. The
meter probe in boiling water reads about 210 at 1100 feet above sea level.....which is
pretty much exact. The probe for the digital meter was ordered with the meter when I
bought it.
Metex VDO Error
400F 460F 60F
358F 405F 47F
300F 340F 40F
280F 300F 20F
250F 260F 10F
235F 250F 15F
201F 210F 19F
188F 200F 12F
So, if you graph these, what becomes apparent is that the difference between "real" temps
and CHT shown temps becomes wider and wider, until at a true 400 degrees, the CHT
gauge is off 60 degrees.
You may also read
this autospeed’s article titled “Giant Gauge Test - Part 2” in which
many aftermarket temp gauges are compared. Although the article does not take into
account the changing ambient temp factor from what I see…
Pressure GaugesThese gauges are less prone to inconstant and inaccurate readings provided the gauge and
the sensor is calibrated, but alas, in real life things are much more complicated…
Concerning most oil pressure sensors with a “idiot light”, they have a major problem: the
light comes on with a pressure as low as 7 PSI (0.5 BAR), and the damage is already
done by then. Some advanced gauges have a programmable “idiot light” threshold, so
that can help keep your eyes on the road and not the gauge…
I have no idea what causes aftermarket pressure gauges to perform the way they do, but
you can read the autospeed’s article “Giant Gauge Test - Part 1”
here and decide for
yourself…
AFR/Lambda Gauges These where covered in other threads on this forum, and from what
information I’ve gathered going with products such as LC-1 or LM-1 is a safe bet…
All other gauges (voltage, current, etc.) are less important, and their accuracy does not
play as a vital role as the gouges mentioned above. (Here I can add that the VDO battery
voltage gauge shows the same values as my Mastech multimeter)
P.S. Would be interesting to hear what you people have to say?
What affordable gauges that one can trust are out there?
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maybe leds only for daylight...
