C2 Temp Sender Caliburation?

[Sheldon S.]

I have a GM temp sender in my 327 350HP motor and I remember reading on a post that the originals had a resistance of 700 ohms. I recently took mine out and measured the resistance from tip to bottom and got about 500 ohms. how will this affect the reading on the guage and is there a way to calibrate it or where can you get a corret one?
Thanks in advance
Sheldon

[Rich G.]

Re: C2 Temp Sender Caliburation?

Sheldon

There has been a lot of discussion about this over the last 6-8 months that helped educate me. Check the archieves. I think "cold" resistance doesn't mean much. There is a temp vs. resistance curve that has been published, but I don't have it electronically, so I can't send it. The few points I remembr are at "red line" the sender is about 40 ohms. At 210 degrees it is about 80 ohms. I think 150 degrees is about 160 ohms. The curve is NOT linear and the number of degrees/ohm increases RAPIDLY at the hot (low resistance) end. I'm sure others will add useful information. Are you having indication problems or do you suspect overheating?

Rich Giannotti

[Scott Butville]

Re: C2 Temp Sender Caliburation?

There's tons of good stuff in the archives, but the best single piece is an article in the Rocky Mountain Chapter's newsletter (Fall 2002 I think). Hit their website. It starts on Page 4. Paragon also now claims to have a replacement sender ($20) that will give correct readings, so you might give them a call. I bought the Wells TU5 sender which is mentioned in several posts, but won't get around to installing it until Thursday night.

[Jack H.]

Re: C2 Temp Sender Caliburation?

Your 'cold' resistance reading of just over 500-ohms is within the factory original 'accept' loci for this part. I wrote the article in the RMC Newsletter you were refered to and can testify that the 700-ohm cold=correct hueristic that's been circulated for so long is an old wive's tale. The article mentioned will tell you/show you why this is so. Plus, in the recent archieves is an excellent post on this subject from John McGraw.

He has a straight-axle car, looked high/low for a 'correct-original' AC sender with 700-ohm nominal/cold characteristics, installed it and was disappointed to observe his dash temp gauge read LOWER than true. His post documents how he went about testing/verifying the REST of the car (generator, voltage regulator, and installed temp gauge) were correct (within factory original spec) BEFORE he began to blindly swap temp senders to obtain a 'correct' dash gauge reading....

[Jack Layton]

Re: C2 Temp Sender Caliburation?

Might we have a url link to the above mentioned article ?
Thanks.

[G B.]

Testing used senders

You know, I seldom get the opportunity to buy a used temperature gauge sending unit while it's installed in a running car. I usually have the bad luck to find them in junk yard engines or sitting on swap meet tables. In those instances, they are often at ambient temperature. I confess: I'm one of the idiots who tests them cold with an ohm meter for an indication of their calibration.

Should I carry around a pot of boiling water at swap meets in order to test them hot, or should I just buy them from sellers who drive up with the engine running? I feel so stupid for buying them after only a cold test. I would much rather base my next sender purchase on a resistance graph drawn over a range of temps. However, the local junk yards don't seem to have such graphs. The last time I asked for a sender resistance graph, the yard owner turned his dog loose on me!

[John M.]

Re: Testing used senders

Jerry,
Just buy a lot of them and cull out the ones that dont pass muster on a hot test! Someone will be willing to buy them blindly just like we did! I tested a number of sensors in the last couple of weeks and it was a real eye-opener. I had 2 sensors that tested 700 ohms at room temp and they had widely different curves when hot and neither matched the factory curve well enough to give me a reading that was within 40 degrees. I finally ended up with a no-name generic sensor that was about 580 ohms cold giving almost pefect readings over the entire scale, but put an old Delco inverted text sensor in because it will judge well and was within 10 degrees. While you will find some sensors that are accurate with a cold resistance of 700 ohms, you would find an equal number that are accurate at a cold resistance close to 500 ohms. Delco did not even plot the curve below 120 degrees. There just is no sure way of telling how accurate a sensor will be when measured at a cold temp. Someone will buy these old 700 ohm sensors from me at a swap meet, but they will have to use an external resistor to get the proper reading at temp!

Regards, John McGraw

[Jack H.]

Re: Testing used senders

Finding one in a running car isn't a guarantee of 'goodness' either. Who knows who's done what upstream in the gauge or 'tacked' a series/parallel resistor into the wiring harness to 'calibrate' a given system.

Room temp resistance reading WILL give you a 'feel' for sender accuracy so long as you (1) define/control what's meant by 'room' temp (standard definition = 69F), and (2) you recognize that a factory original sender that AC would have shipped and Chevy would have accepted vs. rejected could have varied from the mid 400-ohm range to the low 800-ohm range at this temp....

Last, what's important about a temp sender's thermal profile is NOT how accurate it is under mid-range, or 'normal' engine temperatures, it's the sender's accuracy at the HIGH end of the gauge (abnormally hot engine temperature) that's important. The alternative to the temp sender and an analog temp gauge in the dashboard in those days was the famous 'idiot light' sported by the less expensive cars sold. My guess is this is why the temp sender's specified allowable variance was as great as it is/was--just give a general indication of thermal trend/drift, but be ACCURATE when it counts and do all this CHEAPLY.

Has anyone failed an NCRS Performance Verification test because their temp guage was off by 20F?????

[Scott Butville]

Re: Testing used senders

Last night I replaced my no name sender in my '64 with a Wells TU5 (Car Quest #85208)that measured 544 Ohms @ 70F. Also borrowed an IR gun and verified that the no name sender read high by about 35-40 degrees. With the Wells/CarQuest sender and the IR gun reading 175 - 180 on the manifold next to the sender, the gauge now reads on or slightly past the zero of the "180".

[Gary J.]

Re: Testing used senders

Jack,

Don't be giving the PV judges any ideas. They have enough of their own.

Gary

[Jack H.]

Teaser....

I added that tag line as a teaser to see if any judge or owner could/would come forward to testify a PV test had been failed by MINOR imprecision of the car's temp gauge under NORMAL operating conditions. I doubt anyone will, because those who conduct PV tests are VERY experienced judges and they know how sloppy the factory original temperature monitoring system actually was with it's compound tolerance stack-up error potentials!

Heck, the original Delco Remy assy specs for installing the pointer needle onto the temp gauge shaft allow a band of 15-degrees in terms of accuracy! And, here we are today trying to use a BIG HAMMER to FORCE our dash gauges to read dead nuts on 180F, running amuck adding series/parallel resistors to the sender wiring harness to 'trim' a given system for accuracy and exchanging information as to what the 'correct' room temperature resistance reading is for a 'good' temp sender!!!!!!!!!1

I sit back and laugh, and laugh, and laugh. The whole thing is IDIOTIC. We're trying to force precision into a system that never had it in the first place! We want our 1950-1970 classic Corvette to operate and perform like a current day production vehicle....

It IS possible to do this, from the temp gauge standpoint, IF you execute the system level science that John McGraw demonstrated in a prior post: (1) verify and control the gauge's B+ voltage rail, (2) sweep the in-dash gauge's response profile using an external potentiameter to verify the gauge is in-spec compared to the original AC temp sender response profile, (3) obtain a selection of 'correct/original' AC temp senders and hand sort to get one that 'perfectly' matches your gauge (or remove the gauge, remove the pointer needle and re-install it to match the specfic temp sender you have in hand).