additional water temp gauge

[Troutman]

not sure if this belongs in this forum or not, but here goes.

im looking at running an additonal water temp gauge in my 240z (w/ carb' L28) under the hood. i am currently runnng 2 electirc fans on a thermostatic switch, and would like to get a more accurate reading on when to have them turn on. im having trouble locating a mech. gauge that will work with my thermostat housing, (the port is too small, and the probe on the gauge is too long.) so i was wondering if i could wire in an electric gauge under the hood and tap into the wire going from the sending unit in the thermo housing to the stock gauge inside the car? im not sure if this would screw up the signal going to the stock gauge ...or even work for that matter.

Thanks

Todd

[geezer]

Another gauge accomplishes nothing in my view. The operation of the fans is totally dependant on your thermostatic switch(s). What they are rated at will determine at what temperature they supply ground signals controling your fan relays. Here's a really crude drawing showing how to run two fans, each controled by a different valued switch. You only need 1 accurate temperature gauge in my opinion.

[ktm]

Just buy the radiator cap from Summit Racing that has a temperature gauge built into it.

[ktm]

The second gauge is going to much orders of magnitude more accurate than the old stock gauge. Just because the fans are thermostaticlly switched does not mean that you know what the operating temperature is.

You could be operation at 100 degrees C/212 F and your fans would be on all the time. All you know is that your fans are on, not what the actual temperature is.

[geezer]

The second gauge is going to much orders of magnitude more accurate than the old stock gauge. Just because the fans are thermostaticlly switched does not mean that you know what the operating temperature is.

You could be operation at 100 degrees C/212 F and your fans would be on all the time. All you know is that your fans are on, not what the actual temperature is.

Huh...Why do you need two water temperature gauges...so you can watch it overheat in stereovision? I'll stick with the statement, you only need 1 accurate temperature gauge. Unless there is more information on the configuration or intended use of this car that we don't know about, such as the need for manually switching the fans on & off. A visual aid such as a water temperature gauge will tell you the operating temperature but has no control over anything.

[sblake01]

Yeah, I didn't get that either. I've actually used my laser thermometer to check against the guage reading and it's close enough for the purpose of knowing what temp the fans come on at. But the guage isn't part of the fan circuit so I don't understand the gist of Troutman's question or ktm's answer.

[ktm]

You qualified your statement by the word "accurate". I was referring to using a second, more accurate gauge in the engine bay in addition to the stock temperature gauge in the car. If you can tell me within 5 degrees C what your coolant temp is with the stock gauge, you are better than I.

I use my stock gauge as a point of reference ONLY. If it is in the middle, I am doing fine. If it moves past the middle, I know something is not quite right. At that time I fire up my Wolf EMS software and get an ACTUAL temperature reading from the second temp gauge in my T-stat housing connected to Wolf.

I stand by my statement. If you only have a stock temperature gauge in your car, it is worthwhile to have a second means of measuring the exact temperature.

[EScanlon]

You qualified your statement by the word "accurate". I was referring to using a second, more accurate gauge in the engine bay in addition to the stock temperature gauge in the car. If you can tell me within 5 degrees C what your coolant temp is with the stock gauge, you are better than I.

I use my stock gauge as a point of reference ONLY. If it is in the middle, I am doing fine. If it moves past the middle, I know something is not quite right. At that time I fire up my Wolf EMS software and get an ACTUAL temperature reading from the second temp gauge in my T-stat housing connected to Wolf.

I stand by my statement. If you only have a stock temperature gauge in your car, it is worthwhile to have a second means of measuring the exact temperature.

I'm at a loss to understand the legerdemain you're describing.

The 5°C difference is about 9°F; while that can be the difference between boiling and not, or freezing and not, in normal operation if your water cooling system temperature is close to either one of those values you have a problem.

A second temperature gauge isn't going to alleviate that problem nor will it avoid it, regardless of it's accuracy.

A more accurate temperature reading is important if you have a process which must occur at a specific given temperature, or be avoided otherwise. Thermostatic switches for use with the majority of electric fans are NOT calibrated to the umpteenth decimal, so if it actually kicked on at 187.7°F vs 185.036°F vs approximately 180°F is of little to no value other than data. The important thing is that it DID kick on at approximately the temperature it's rated for. If it does NOT, that is a different problem.

If you're having a problem with it kicking on/off at more than 10% of it's value off from the temperature it's supposed to be operating at, then replace the switch.

If the stock gauge indicates a rise or drop in temperature, having the second gauge would be beneficial only to confirm that the stock gauge is or isn't operating properly. But having the "actual" temperature down to the 6th decimal.... to what end?

Then again, there are folks who jog with those watches with the miniature EKG, Pulse, O2, and Breathing sensors so that they can accurately describe at what point they ..... to what end?

But to answer Troutman's original question:

The second gauge would likely require it's OWN temperature sender. Connecting it to the OEM sender may in fact render both totally inaccurate or non-operable. The OEM gauge works on resistance through the sender, by adding a second wire to the sender's wire you are going to change the resistance reading. Additionally, the resistance range in the OEM sensor may be incompatible with the sensing range of the new gauge.

Ron's pointed out a schematic for operating a pair of relays to run two fans at different temperature settings. That sounds different than what you are currently doing, but maybe not.

HTH

E

[sblake01]

And I still don't get it. To me, E is right. Linguistic legerdemain:rolleyes:

[geezer]

Fair enough Bo. After a carefull re-read of Troutman's 1st post, I should have picked up that because of his intent to mount the gauge underhood, his plan was to use it for diagnostic purposes only. Even so I don't see the need for it. For the purpose of turning the fans on & off I have never had the need for a precise decimal pointed water temperature readout. I have found the thermostatic switches, that are available in a wide variety of parameter settings, to be reliable enough for me, to control rad fans reliably. Maybe you have more insight or are privy to Troutman's plans or problems, but not a lot of information was provided. All I noticed was "1973 Z" in his signature, no other details. Is it overheating, fan runtime out of wack? After reading this taken from post 1 - "like to get a more accurate reading on when to have them turn on", I simply offered up the basic staggered run time two fan, independantlly controlled system that has always worked for me, even with a mediocre gauge.

[sblake01]

I don't know. The factory gauge will tell you if the engine is hot and the extra gauge will tell you exactly how hot. I can do the same with my laser thermometer without having to have software or figuring out how and where to mount extra gauges. Especially if you have to go under the hood anyway to do the comparison.

[ktm]

I don't know. The factory gauge will tell you if the engine is hot and the extra gauge will tell you exactly how hot. I can do the same with my laser thermometer without having to have software or figuring out how and where to mount extra gauges. Especially if you have to go under the hood anyway to do the comparison.

Stephen, you are using a laser thermometer. That is the SAME THING as using a more accurate gauge underhood.

I gave a very simple solution to the problem yet everyone seems to be ignoring it: a thermometer equipped radiator cap such as this one http://store.summitracing.com/partdetail.asp?autofilter=1&part=MRG%2D2471B&N=700+115&autoview=sku

I said 5 degrees C as an example. The fact remains you can not tell what temperature the coolant is at with the stock gauge.

Thermostatic switches on a fan are good indicators for the coolant temp, but not one of you as recognized the fact that once they switch on, you have no idea of the true coolant temperature. Put aside your laser thermometers and rely solely upon the stock gauge. If the fans are constantly running, you only know that they are above the off temp (unless the on and off temp are the same). Is the temp 180? 190? 200? The difference between 180 degrees and 200 degrees on the stock coolant temp gauge depends on which side of the printed F you are on.

I was in a situation where my electric fans would not turn off even when cruising. All I knew was that I was above my on point. Turns out I was running at a steady 190 degrees which was too hot for cruising. It only stayed at 190 because my fan was on all the time. I then checked my cooling system and found out my lower rad hose was cool to the touch as well as the bottom of the radiator. Turns out my thermostat went bad.

I never said anything about measuring the temperature to the tenth decimal place. I stated that I check my temperature with my EMS software when I know I am running slightly hot. Once I see what temperature I am at, I then do a quick survey of the cooling system (feel the hoses, top and bottom of radiator, check coolant level, etc.) if I feel that it is too high. I am fortunate that I have two sensors.

Finally, he asked a simple question yet everyone is telling him that he does not need a more accurate gauge. He should not have to justify why he wants a more accurate reading.

I love this site, but at times the nitpicking is unbearable.