Temperature Gauge?

The temp gauge on my '73 had always been accurate (key word "had"). The gauge consistently showed operating temperatures between 205 and 210. An infrared thermometer always read in the same range when taking temps at the thermostat housing and along the upper radiator hose.

Earlier this summer, I replaced my alternator with a CS144. While doing the swap, I unplugged the wire at the temp gauge sending unit. After the first good long test drive, I noticed the temp gauge needle had not moved above the white line that is approximately halfway between 100 and 210. Double checked the connection and found a butt connector hidden in the wiring harness that looked a bit weak so I removed it and soldered the wire back together. Also put a new end on the wire. Didn't help. Every time I drive the car the gauge needle climbs to the white mark between 100 and 210 and stays there. Every time I check the engine operating temp with the infrared thermometer it always reads between 205 and 210 at the thermostat housing and along the upper radiator hose (same as it did before).

The engine is a NOM SB350 the previous owner had built to be the rough equivalent of the LT1's. It has Camel hump heads and Sanderson cast headers. The temperature sending unit is fairly close to the headers.

Any ideas? Is there a way to test the sending unit while on the car? I
am assuming that there is nothing wrong with the gauge.

Thanks,

DC

Your temp. gauge is nothing more than an electrical resistance meter. It seems that when you swapped out the alt that you also changed the electrical values in the gauge circuit. You could try adding a small inline potentiometer (variable resistor) and try tuning the circuit until it reads accurately. This is assuming that the circuit needs more resistance in it to get the gauge to read right.

OK Jeff - let me have it! I'm in your backyard here!

With the ignition key on remove the wire going to the sender---the temp needle should go to full cold.
Now ground the sender wire---the temp needle should go to full hot.
This should mean your gauge and wiring is good.
Check your ohms reading coming from the sender at operating temp.

DeeVeeEight said:

Your temp. gauge is nothing more than an electrical resistance meter.That would be a recalibrated voltmeter It seems that when you swapped out the alt that you also changed the electrical values in the gauge circuit.I highly doubt it You could try adding a small inline potentiometer (variable resistor) and try tuning the circuit until it reads accurately. This is assuming that the circuit needs more resistance in it to get the gauge to read right.

OK Jeff - let me have it! I'm in your backyard here!

Click to expand...

If it read correctly before, and now it does not, the connector is probably shot, giving the system too much resistence.
Rogers test is a valid one.

Double check the ground wire on the back of the alternator too. It could be as simple as it's not making a real good connection. Not too likely, but hey, it's easier than digging thru the whole circuit.

Gauge tested good. Ground at alternator is good. Connection seemed loose at the sending unit so I closed the connector on the end of the wire a bit. If that doesn't fix it, I'll ohm the sending unit next time I drive it and see where I stand. Thanks for all the help.

DC

Wiggle hell outta that firewall connector behind the fuse panel....and also check the connections behind the gauge itself....there are two flat resistors bolted in place to get that meter to read right....I forget the value and configuration.....

:shocking:

Bringing this one back to life

I have been beat down by the cooling system of my car for a couple years now, I finally get my cooling issues worked out and the gauge fails.

It is now reading high, very high, 250* when the engine is only 180*.

I've replaced the sending unit but no change so now I'm looking for an issue with the gauge.

Was wondering if DC3 found the root of his issue and if so what did you find?

Thanks Neal

496BBC said:

Bringing this one back to life

I have been beat down by the cooling system of my car for a couple years now, I finally get my cooling issues worked out and the gauge fails.

It is now reading high, very high, 250* when the engine is only 180*.

I've replaced the sending unit but no change so now I'm looking for an issue with the gauge.

Was wondering if DC3 found the root of his issue and if so what did you find?

Thanks Neal

Click to expand...

Did you do the test in my post above ?
When the engine is warmed to 180 what is the ohms reading coming from the sending unit with the wire disconnected ?

496BBC said:

Bringing this one back to life

I have been beat down by the cooling system of my car for a couple years now, I finally get my cooling issues worked out and the gauge fails.

It is now reading high, very high, 250* when the engine is only 180*.

I've replaced the sending unit but no change so now I'm looking for an issue with the gauge.

Was wondering if DC3 found the root of his issue and if so what did you find?

Thanks Neal

Click to expand...

Try changing the resistor on the back of the gauge. I substituted resistors until I got it in the ballpark. You can see the resistors (colored red, green, blue and yellow) under the printed circuit. I may have some extra's laying around if you need help.

[/IMG]

DWncchs said:

496BBC said:

Bringing this one back to life

I have been beat down by the cooling system of my car for a couple years now, I finally get my cooling issues worked out and the gauge fails.

It is now reading high, very high, 250* when the engine is only 180*.

I've replaced the sending unit but no change so now I'm looking for an issue with the gauge.

Was wondering if DC3 found the root of his issue and if so what did you find?

Thanks Neal

Click to expand...

Did you do the test in my post above ?
When the engine is warmed to 180 what is the ohms reading coming from the sending unit with the wire disconnected ?

Click to expand...

DWncchs; have not done the test, its first on my list.

I'll report back with my findings.

Neal

DeeVeeEight said:

496BBC said:

Bringing this one back to life

I have been beat down by the cooling system of my car for a couple years now, I finally get my cooling issues worked out and the gauge fails.

It is now reading high, very high, 250* when the engine is only 180*.

I've replaced the sending unit but no change so now I'm looking for an issue with the gauge.

Was wondering if DC3 found the root of his issue and if so what did you find?

Thanks Neal

Click to expand...

Try changing the resistor on the back of the gauge. I substituted resistors until I got it in the ballpark. You can see the resistors (colored red, green, blue and yellow) under the printed circuit. I may have some extra's laying around if you need help.

[/IMG]

Click to expand...

Thanks for your response, after verifying the wiring and gauge I may try and add a potentiometer as suggested in an earlier post in an attempt to allow me to calibrate the gauge to my IR gun. I also have two other temp readings on the cooling system to verify operation.

Thanks Neal

Well I spent some time trouble shooting my temp gauge.

Started with a cold engine,

Attached my ohm meter with the sender cold

Starting ohms with cold sender was 700, cranked the engine and watched the meter. Ohms slowly started dropping; I have two temp gauges other than the OEM gauge along with an IR gun. Once the temp got to around 180* on the IR gun and the other gauges I checked the dash gauge, it was showing around 200. Ohms were at 209; this was with the sending wire disconnected. Once the wire was connected the ohms jumped to 333. The ohms changed from the sending wire being connected, I connected and disconnected the sending wire multiple times to make sure it would repeat the change in ohms and it did. Not sure if this is how it should work or not. The three other temp indications were solid at 175* to 180* but the factory gauge just kept climbing and the ohms from the sender kept dropping. I kicked on my oil cooler fans and the radiator fans and the temp started dropping, got down to about 150* but the OEM gauge dropped slightly to around 250* around 180 ohms. Not sure why the resistance keeps dropping when the gauge is connected, the gauge may be failing. The one thing I should have paid a little closer attention to was to see how the sender reacted without the gauge connected.

Should the gauge have any effect on the sender resistance?

I've got a spare sender I'm going to heat on the stove and get some readings.

I still have the SPAL PWM fan controller that is controlled form the intake sender, it was acting odd also. I'm sure due to the sender resistance changing form the gauge.


Neal

When measuring resistance of anything, like your temp sender, you have to leave all other wiring off the unit, just your test gear.....

the voltages applied with the unit trying to function will mess up the test meter and the dash temp readings.....it's ONE or the other, one at a time....

the PWM controller has it's own sender, so it's independent of your dash meter/sender that should have nothing to do with it acting 'funny'....

I wonder if you don't still have some cooling issue....

:surrender:

mrvette said:

When measuring resistance of anything, like your temp sender, you have to leave all other wiring off the unit, just your test gear.....

the voltages applied with the unit trying to function will mess up the test meter and the dash temp readings.....it's ONE or the other, one at a time....

the PWM controller has it's own sender, so it's independent of your dash meter/sender that should have nothing to do with it acting 'funny'....

I wonder if you don't still have some cooling issue....

:surrender:

Click to expand...

The PWM takes its signal for on/off set points form the stock sending unit, the signal wire for the PWM is spliced into the signal wire going to the gauge, per the instructions.

No more cooling issues per the three other temp indicators I'm using.

Neal

496BBC said:

Was wondering if DC3 found the root of his issue and if so what did you find?

Thanks Neal

Click to expand...

I've tested enough to know my gauge is good but have not made the time to check the sending unit. I now think it might be a connection issue. Drove the car a couple of weeks ago and the gauge was reading normal. Drove it last weekend and it was back to reading low.

I think Gene may be on to something in regards to the connector at the firewall (behind the fuse panel). I replaced the lower speedo cable a few weeks back and did some wiggling of that connector just for grins. The next time I drove it was when it read normal.

I'll get around to finishing up the diagnosis eventually but it may have to wait until after the holidays.

DC

I didn't think about it until Gene's post, its possible that the PWM is the problem. I'm going to disconnect the wire to it and see if things change.

Neal

DWncchs said:

When the engine is warmed to 180 what is the ohms reading coming from the sending unit with the wire disconnected ?

Click to expand...

If the ohms reading was 209 with the wire disconnected the sender is bad.

496BBC said:

I didn't think about it until Gene's post, its possible that the PWM is the problem. I'm going to disconnect the wire to it and see if things change.

Neal

Click to expand...

Don't mean to bury you with bullshit, but if any other device hooked to the sending unit has any 'load' presented to the stock hookup, the gauge is going to read funky.....modern electronics is easy enough to present a wide open circuit that will not load anything down....but that's not a given to any particular design.....

pop that controller on/off the gauge wire with a cliplead and note any differences in readings.....

I UNDERSTAND from comments past that the Spal controller is a bit funky, and so maybe in fact there is a problem with it....dunno shit about the nature of the failure mode, specifically....

IMO...a simple on/off relay is fine for the purpose at hand....it's what I have on my Spals run by the FI computer...

mrvette said:

496BBC said:

I didn't think about it until Gene's post, its possible that the PWM is the problem. I'm going to disconnect the wire to it and see if things change.

Neal

Click to expand...

Don't mean to bury you with bullshit, but if any other device hooked to the sending unit has any 'load' presented to the stock hookup, the gauge is going to read funky.....modern electronics is easy enough to present a wide open circuit that will not load anything down....but that's not a given to any particular design.....

pop that controller on/off the gauge wire with a cliplead and note any differences in readings.....

I UNDERSTAND from comments past that the Spal controller is a bit funky, and so maybe in fact there is a problem with it....dunno shit about the nature of the failure mode, specifically....

IMO...a simple on/off relay is fine for the purpose at hand....it's what I have on my Spals run by the FI computer...

Click to expand...

No bullshit here, always value your input. Gona check for changes with lead disconnected and re-connected.

I was sent an on/off sensor when I purchased the Dewitt radiator, didn't use it due to the high on setting, 195* I believe.

The SPAL PWM has worked fine for me till now. Gota do some trouble shooting and pin this issue down.

Thanks Neal