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VDO pyro question
Posted: Mon Oct 05, 2009 6:36 pm
by pongo
I just bought a 2nd hand one and looks like the red cable ( the one that goes from the thermo couple to the gauge) has been shortened. you can see the red cable in the link to give a better idea what im on about.
http://cgi.ebay.com.au/VDO-PYRO-KIT-BRA ... 911.c0.m14
Will this affect the accuracy of the gauge?
can i just lengthen it to suit?
opinions?
Posted: Mon Oct 05, 2009 6:58 pm
by bigbrowndog
My son shortened my cable when I fitted my VDO Pyro. He took a heap off it. That was a few years ago now. I put the probe in the dump pipe and put a hole in the dash while he did the wireing. He is an electrician. Should be ok I think.
Posted: Mon Oct 05, 2009 7:14 pm
by ISUZUROVER
It is not a good idea, it can affect the reading.
Posted: Mon Oct 05, 2009 7:27 pm
by bigbrowndog
ISUZUROVER wrote:It is not a good idea, it can affect the reading.
Yep, I seem to remember being told my 2H motor would not last as it had 246,000 ks on it at the time I fitted the turbo and 3 inch exhaust. I got all the cracking heads, throwing rods out the side of the block stories, LOL still going strong 100,000 ks later. It is about the only thing on the truck not rebuilt.
Posted: Mon Oct 05, 2009 7:38 pm
by ISUZUROVER
bigbrowndog wrote:ISUZUROVER wrote:It is not a good idea, it can affect the reading.
Yep, I seem to remember being told my 2H motor would not last as it had 246,000 ks on it at the time I fitted the turbo and 3 inch exhaust. I got all the cracking heads, throwing rods out the side of the block stories, LOL still going strong 100,000 ks later. It is about the only thing on the truck not rebuilt.
OK smart guy, have a read at the bottom of this link.
http://auberins.com/images/Manual/EGT%2 ... al%201.pdf
Also, please show it to your son, so he doesn't make the same mistake again.
It may only make a few degrees difference, but how do you know???
And just because it reads correctly at ~30oC after cutting the wire, does not mean it won't be inaccurate at 750oC whener you want to use it.
EDIT - same here from autometer - see 2. on page 2
http://www.hpheaven.com.au/uploaded/pdfs/0593A.pdf
Posted: Mon Oct 05, 2009 7:58 pm
by dogbreath_48
I asked the bloke at Ringwood Speedo shop the same question - he claimed that it was ok to shorten the cable for a new VDO gauge, but not the older models. I suppose the new ones use a higher sense voltage or something. This guy seems to know his stuff.
-Stu
Posted: Mon Oct 05, 2009 7:59 pm
by fester2au
Seems to be common knowledge by those in the know that pyro leads should not be shortened as the whole assembly is calibrated.
VDO's web site is a little average for the day to dfay consumer but in their installation instructions that can be downloaded they do say the same thing but without explanation.
Geberkabel nicht kürzen.
Do not shorten measuring lead.
Ne pas raccourcir les câbles de
mesure.
¡No recortar la línea de medición!
Posted: Mon Oct 05, 2009 7:59 pm
by bigbrowndog
You are the smart alec. He does this sort of thing for cold rooms and industril/domestic airconditioning . Also has data licence and has done optic splicing. It made no difference regardless of what is written in your post. But I would assume you would know better.
Posted: Mon Oct 05, 2009 8:04 pm
by dogbreath_48
bigbrowndog wrote:...He does this sort of thing for cold rooms and industril/domestic airconditioning . Also has data licence and has done optic splicing....
None of which has anything to do with automotive pyrometers
Posted: Mon Oct 05, 2009 8:09 pm
by fester2au
dogbreath_48 wrote:bigbrowndog wrote:...He does this sort of thing for cold rooms and industril/domestic airconditioning . Also has data licence and has done optic splicing....
None of which has anything to do with automotive pyrometers
Hmmmmm, mind reader.
Posted: Mon Oct 05, 2009 8:19 pm
by bigbrowndog
dogbreath_48 wrote:bigbrowndog wrote:...He does this sort of thing for cold rooms and industril/domestic airconditioning . Also has data licence and has done optic splicing....
None of which has anything to do with automotive pyrometers
LOL The thermistors that are throughout the airconditioning and cold room refrigeration do much the same thing as I understand [give temperature reading].
Posted: Mon Oct 05, 2009 8:21 pm
by weeman
i wouldn't shorten it.. When i worked at Siemens i spoke to the tech VDO guys and they never shorten the pyro cable as its calibrated..
So the bloke with no issues may just have it conservatively tuned, but id rather have a bit of extra wire then for my motor to go pop...
Posted: Tue Oct 06, 2009 12:51 am
by Z()LTAN
Don't shorten it.... effects the resistance in the circuit...
After all the Pyro gauge isn't reading temp its reading "RESISTANCE"
Posted: Tue Oct 06, 2009 7:21 am
by ddr
Ok I understand how resistance of wire works, but I cant understand how shortening a length can cause inaccurate readings but getting a longer length wont.
Posted: Tue Oct 06, 2009 7:46 am
by chimpboy
ddr wrote:
Ok I understand how resistance of wire works, but I cant understand how shortening a length can cause inaccurate readings but getting a longer length wont.
I think they send you a longer one with the same fixed resistance.
It would be interesting to know if it would read hotter or colder due to a shortened wire.
Posted: Tue Oct 06, 2009 7:53 am
by want33s
Does anyone know where I can buy just the probe for the VDO kit in the link above?
Posted: Tue Oct 06, 2009 8:50 am
by dogbreath_48
fester2au wrote:dogbreath_48 wrote:bigbrowndog wrote:...He does this sort of thing for cold rooms and industril/domestic airconditioning . Also has data licence and has done optic splicing....
None of which has anything to do with automotive pyrometers
Hmmmmm, mind reader.
I'm getting good. I read boggeds mind once
what sort of thermocouple does the vdo pyro use?
Posted: Tue Oct 06, 2009 9:02 am
by ddr
chimpboy wrote:ddr wrote:
Ok I understand how resistance of wire works, but I cant understand how shortening a length can cause inaccurate readings but getting a longer length wont.
I think they send you a longer one with the same fixed resistance.
It would be interesting to know if it would read hotter or colder due to a shortened wire.
If anyone has one not installed it can be checked if the Thermocouple has more or less resistance when it heats up.
Posted: Tue Oct 06, 2009 11:45 am
by Guy
bigbrowndog wrote:You are the smart alec. He does this sort of thing for cold rooms and industril/domestic airconditioning . Also has data licence and has done optic splicing. It made no difference regardless of what is written in your post. But I would assume you would know better.
The people that make em most likey do .. they have said "DONT SHORTEN THE CABLE" .. More than likley they know their own product.
Posted: Tue Oct 06, 2009 12:02 pm
by coxy321
All of the installation sheets i've seen (3, all different brands) have said NOT to modify the length or connectors on the thermocouple harness. Much the same as O2 sensors.
Posted: Tue Oct 06, 2009 5:16 pm
by pongo
want33s wrote:Does anyone know where I can buy just the probe for the VDO kit in the link above?
ring acdc
http://210.9.123.2/acdc/content.aspx?St=true&pageID=3
they have an aftermarket probe and wiring to suit for $99
Posted: Tue Oct 06, 2009 5:59 pm
by want33s
Thanks Pongo.
Posted: Tue Oct 06, 2009 7:22 pm
by fester2au
dogbreath_48 wrote:fester2au wrote:dogbreath_48 wrote:bigbrowndog wrote:...He does this sort of thing for cold rooms and industril/domestic airconditioning . Also has data licence and has done optic splicing....
None of which has anything to do with automotive pyrometers
Hmmmmm, mind reader.
I'm getting good. I read boggeds mind once
Five seconds of your life you'll never get back.
what sort of thermocouple does the vdo pyro use?
Posted: Tue Oct 06, 2009 11:49 pm
by krusty182
I am not 100% sure, but I am under the impression that they use a type "K" thermocouple for their pyrometer, this is reinforced by the fact the sensor wires are red and yellow, which is common on this type of sensor. If that is the case, then it is most likely that the red sheathed cable they supply would be compensating thermocouple cable.
This type of cable compensates for the length of cable (By the way, this type of sensor does not measure the resistance of the probe, but the minute voltage generated by the thermocouple junction when the two dissimilar metals in it are heated).
If you wish to extend it, you can, but it must be more of the same type of compensating cable and polarity is very important. You cannot extend it with speaker cable or anything like that if that is what you were asking.
If it were me, I'd put a whole new piece of cable in, rather than join it, if it is too short.
Posted: Wed Oct 07, 2009 5:57 am
by KiwiBacon
krusty182 wrote:I am not 100% sure, but I am under the impression that they use a type "K" thermocouple for their pyrometer, this is reinforced by the fact the sensor wires are red and yellow, which is common on this type of sensor. If that is the case, then it is most likely that the red sheathed cable they supply would be compensating thermocouple cable.
This type of cable compensates for the length of cable (By the way, this type of sensor does not measure the resistance of the probe, but the minute voltage generated by the thermocouple junction when the two dissimilar metals in it are heated).
If you wish to extend it, you can, but it must be more of the same type of compensating cable and polarity is very important. You cannot extend it with speaker cable or anything like that if that is what you were asking.
If it were me, I'd put a whole new piece of cable in, rather than join it, if it is too short.
Krusty has it.
They're K type thermocouples, the length of wire is irrelevant, but the type of wire and connectors are very important.
The reason they tell you not to shorten or lengthen the wires is because you'll be using the wrong connectors.
I run my thermocouples with mini k type plugs, hence no loss of calibration when lengthening or shortening the cable.
Posted: Wed Oct 07, 2009 7:47 am
by dogbreath_48
Cool tech
However, if connectors are so important why does the vdo loom come with eyelets only on one end? Is it acceptable to crimp run-of-the-mill spade connectors to the gauge end?
Posted: Wed Oct 07, 2009 7:55 am
by KiwiBacon
dogbreath_48 wrote:Cool tech
However, if connectors are so important why does the vdo loom come with eyelets only on one end? Is it acceptable to crimp run-of-the-mill spade connectors to the gauge end?
My VDO came with eyes on one end and spades on the other.
The eyes fractured the wires off within a year, that was the driver to finally wire in those little connectors you see above.
If you use the wrong material connectors you get additional thermocouples (aka "cold junctions"). Basically a change in the temperature of any other junction will influence the gauge reading.
The gauge itself I think will measure and compensate for it's own temperature and the error resulting from the spade terminals on the back of it. So it's the loom-probe connector in the engine bay that remains the biggest problem.
Posted: Wed Oct 07, 2009 9:41 am
by chimpboy
KiwiBacon wrote:Krusty has it.
They're K type thermocouples, the length of wire is irrelevant, but the type of wire and connectors are very important.
The reason they tell you not to shorten or lengthen the wires is because you'll be using the wrong connectors.
I run my thermocouples with mini k type plugs, hence no loss of calibration when lengthening or shortening the cable.
Excellent stuff, that explains a lot!