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Old 11-04-2012, 08:34 PM   #1
Sly Vette
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Inaccurate temp sender...Again!

I'm on my 3rd temp sender in 2 months and have yet to get one that is accurate.
The one I have in now is from Rock Auto and with the engine temp. at 185(Verified with an IR gun) the gauge reads 210. I'm fed up with changing senders only to find the one I install is less accurate than the last.
As I understand it our gauges operate from 0-90 ohms. With a gauge that reads 210 when the true temp is 185,would it be possible to add a resistor between the gauge and the sender to "Calibrate" the sender to send the proper signal to the gauge?
I'm thinkin' that at zero degrees on the gauge the sender is @ 90 ohms resistance and 0 ohms with the gauge pegged @ max hot.
Do any of you think this strategy has any merit or am I simply pissin' into the wind with this idea?
I am in no way even close to being an electrical engineer so I'm hoping someone with a bit more knowledge could give me some insight.
Thanks in advance.
Sly
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Old 11-04-2012, 08:54 PM   #2
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[QUOTE=Sly Vette;1995232... What I have in now is from Rock Auto and with the engine temp. at 185(Verified with an IR gun) the gauge reads 210.[/QUOTE]

Hi Sly,

I'm no expert, but I'll chime in w/a couple of questions. I'm an EE, and deal with temperature readings somewhat often.

Did you try the IR gun at the same location as the temp sensor? Different places on the engine are probably at differnt temps.

Also, IR temp sensors work better on some surfaces than others due to the emissivity. Some surfaces can give false readings. What was the color of the surface you pointed it at?
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Old 11-05-2012, 03:34 AM   #3
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Does the sensor have a ground wire - or is it expected to ground through it's case? If it's a one wire - and you mount with Teflon tape - or other sealant - won't you lose the ground?

Just thinking...

Cheers - Jim
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Old 11-05-2012, 09:03 AM   #4
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Quote:
Originally Posted by mbwalker View Post
Hi Sly,

I'm no expert, but I'll chime in w/a couple of questions. I'm an EE, and deal with temperature readings somewhat often.

Did you try the IR gun at the same location as the temp sensor? Different places on the engine are probably at differnt temps.

Also, IR temp sensors work better on some surfaces than others due to the emissivity. Some surfaces can give false readings. What was the color of the surface you pointed it at?

There may be something to this. Inaccuracies of IR guns and such. First off, the sender is reading the temp of the fluid moving through the passage, and the IR gun is reading the temperature of the cast metal. Could possible be some difference there.

Second, when the engine is at operating temp, try reading both cylinder heads at both corners each, and mark the temps. I'll bet there is quite a difference from side to side and front to back.
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Old 11-05-2012, 09:18 AM   #5
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None of the replacement senders match the stock resistance range. I ended up repairing the connector on my original sender after trying multiple replacements. The only way to make a replacement sender read correctly is to adjust the resistance at the back of the temperature gauge.

Here are two good threads from the "other" forum:
http://forums.corvetteforum.com/c3-t...and-check.html
http://forums.corvetteforum.com/c3-t...e-success.html

Stock is about 79-82 ohms at 212 degrees. The gauge works comparing the resistance on the back of the gauge to the sender resistance.
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Old 11-05-2012, 10:47 AM   #6
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Bwd wt203z

Quote:
Originally Posted by SteveG75 View Post
None of the replacement senders match the stock resistance range. I ended up repairing the connector on my original sender after trying multiple replacements. The only way to make a replacement sender read correctly is to adjust the resistance at the back of the temperature gauge.

Here are two good threads from the "other" forum:
http://forums.corvetteforum.com/c3-t...and-check.html
http://forums.corvetteforum.com/c3-t...e-success.html

Stock is about 79-82 ohms at 212 degrees. The gauge works comparing the resistance on the back of the gauge to the sender resistance.
Don't do the resistor unless it's your last option. Check out my thread here where I found a sender that falls in pretty close to the range needed. Can you give us a list of the senders you have tried?

I got my BWD WT203Z from Advanced Auto.
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Old 11-05-2012, 06:51 PM   #7
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no, you cannot add a resistor inline and expect consistant accuracies.

the temperature to ohms table is not linear, it is a curve.

get a ohmeter, and a thermometer.. put it in a pot of water, turn on the stove and test/write down the ohms at specific varying temps.. all the way up to boiling. you will see it is a curve and not a straight line.

your temp gauge and sending unit need to be a matched set in order to expect accurate readings and I am not sure the spec is even published. make sure your wire is good and clean/new or you are then introducing more ohms to the equation, causing your reading to be off also.
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Old 11-05-2012, 10:16 PM   #8
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I have checked the eng. temp at many different locations and have found that near the sensor seems to give me the most accurate readings. This was determined by reading the temp at the temp switch for my electric fans which should come on at 210 degrees and off at 195. The readings I got with my IR gun were very close...within 5-10 degrees. Also with a reading at the sender I get a reading of near 180 just as the stat starts to open. I do have a 180 stat.
I have no tape or sealant on the sender to interfere with the ground.
I do not have a list of the senders I have tried however I may have to try this one more time with the BWT WT 203Z sender seeing that they seem to be the most accurate as stated by some who have done some testing on various senders.
I understand the Linear and curve aspect...somewhat.
I do not have a stock wiring harness. It is a replacement from Painless that I feel is more than adequate. I also have aftermarket gauges. They are Autometer Phantoms. and I feel these are of good quality also.
If after replacing my current sender with the one listed above,if I still have a similar problem I will have to live with it. I know that the car is not overheating and I can tell when the temp hits 210 due to the amount of noise made by the dual Spal fans. One thing I refuse to do is I will not be diggin' into my dash to install a resistor on my gauge. I hate dash work and this is something I will apparently have to live with. At least I was on somewhat of the right track with the addition of a resistor. Would have been nice if I could have run one directly from the sender.... Very seldom does there seem to be an easy solution.
Thank you for your replies,guys. I have learned something and I appreciate the time all of you took to respond to my question.
Thanks Again...
Sly
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Old 11-06-2012, 10:39 AM   #9
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Quote:
Originally Posted by Sly Vette View Post
I also have aftermarket gauges. They are Autometer Phantoms.
Not sure if I missed something, but since the auto parts store senders have been giving false readings, have you looked at the Autometer webpage to see if they have a sender that is matched with your gauge?

Something I pulled from the Autometer page...

Quote:
The Auto Meter 100 to 250 degree, short sweep temperature gauge uses a specification of 1123 ohms of resistance to ground to = 100 degrees F, and 65 ohms of resistance to ground to = 250 degrees F. You may contact our Service Department for further specs.
I'm not sure if your Phantoms are the 100-250, short sweep and I don't know that much about electrical stuff. However, I'd have to wonder if the ohms for the Autometers are different than stock...
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Old 11-06-2012, 11:13 AM   #10
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Originally Posted by Sly Vette View Post
I also have aftermarket gauges. They are Autometer Phantoms. and I feel these are of good quality also.
Aftermarket gauges require the correct aftermarket sender. The OHM range is totally different from stock. Call Autometer and get the correct sender.

Helps to have all the info when troubleshooting a problem.
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Old 11-06-2012, 05:20 PM   #11
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Quote:
Originally Posted by joewill View Post
get a ohmeter, and a thermometer.. put it in a pot of water, turn on the stove and test/write down the ohms at specific varying temps.. all the way up to boiling. you will see it is a curve and not a straight line.
Joe - Good advice, I was going to suggest that myself.

Sly - if you don't have an ohm meter, you can get one for well under $10 at Harbor Freight, if there's one near you. If not, Radio Shack, Walmart, Lowes. They are all more than accurate for what you are doing. For just a little more, you can get one w/a temp sensor too.

Keep us posted!
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Old 11-08-2012, 10:38 AM   #12
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Well...That all makes sense! It never occurred to me that different gauges would require a different sender. I guess sometimes the obvious gets overlooked. The oil press and fuel gauges are electrical also...Time to get the ohm meter out and start checking resistance. Fellin' kinda' at a loss for over lookin' something so obvious. I'm thinkin' the vac gauge should be OK...LOL!
Thanks Guys!
Sly
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Old 11-10-2012, 08:51 PM   #13
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As an update....When I changed from the "original" sender I was wondering why the heck did I use an adapter for the sender rather that using a sender that was the proper dia. in the first place. Now after purchasing the correct sender for my Autometer Gauges I see that I had a Autometer sender in there in the first place! They don't sell the correct size sender for a Gen I SBC.
It appears now that the original took a crap and must of shorted out. Boy the things you forget after 5 years or so when you get a bit older. Also I've been married for over 30 yrs. and she is drivin' me crazy!!! That's my story and I'm stickin' to it!
Thanks again guys for puttin' up with my lapses in memory.
I ordered a new sender from Jegs and it will be here next week.
There should be no problems after that!
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Old 11-11-2012, 06:33 AM   #14
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Glad it's worked out.
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Old 11-26-2012, 03:07 PM   #15
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Well with the sender from Autometer in it still reads 210 at the gauge. It is accurate however seeing that the temp with the IR at the head next to the sender is 210 also. The temp at the T-Stat is 180. The temp switch for the electric fans is accurate also in as much that they come on when the temp at the switch hits 210 and off at 190.
So the bottom line is that the engine apparently has some areas that are hotter than others. Naturally the areas where the sender/switches are located in the head are a bit hotter than the intake where the T-Stat is located. Never was aware of this prior to getting an IR gun.
As long as I know the eng. is not overheating(it never has) I guess the reading I get from the head is what I will learn to live with.
Any one else find these varying temps in their eng.?
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