Temp gauge reading low; gauge checks “ok”, replaced sending unit, no change... - Page 2 - Vintage Mustang Forums
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post #16 of 28 (permalink) Old 02-05-2019, 03:38 PM
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Originally Posted by Grimbrand View Post
All of the new electric temp and oil pressure senders are pretty much junk inside, including the 'expensive' ones.
The stock gauges are basically idiot lights with a dial. Racing caliber sensors are much better though you'd have to find a way to adapt them into the vintage Mustang and use an appropriate gauge. My stock is still in the manifold for the dash gauge but I mounted a VDO at the heater hose near the stock location. If you can get a good Stewart-Warner, AIM or Bosch it's able to give you pro grade readings. That's overkill for these old cruisers. You really only need to know two things to make sure your engine doesn't tank. Oil pressure and water temp. (well, rpm if you have a stick). Most of us don't race these anyway so as long as we know we aren't going to boil over and have good oil pressure we're golden.

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post #17 of 28 (permalink) Old 02-05-2019, 04:58 PM Thread Starter
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I think I may have stumbled upon the solution to my problem in another thread. @Woodchuck posted something about gauge calibration, and said that the resting position of the gauge (ignition off) should be about two needle thicknesses to the left of the C’s hashmark (the hashmark is to the right of the letter C). My gauge needle is actually to the left of the letter C while at rest, which is a similar amount of sweep to it’s low reading at operating temp. I was unaware that the needle should be further to the right at rest; it was obvious that somebody had been into this cluster at some point in it’s life, and they must have messed with the calibration adjustment. I already ordered the potentiometer, so I’ll do a proper gauge calibration once I pull the cluster (back out), but I think this might be my answer.
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post #18 of 28 (permalink) Old 02-05-2019, 10:10 PM
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What is CVR? And did you mean to post the IVR output as ohms? I thought I was looking for volts there?

Thanks. I feel like you may be pointing me in the right direction here...just having a hard time with the gauge being the culprit here...

Also, for those following along, I’m seeing essentially the same resistance at the gauge as I am at the sender, +/- 2 ohms.

CVR is a constant voltage regulator as opposed to an IVR instrument voltage regulator. The IVR pulses a voltage, so it is cyclic, not constant. It cycles between 0 and 12 volts a couple of times per second so that the effective output is 5 volts (not ohms, sorry 'bout that!) A CVR outputs constant 5 volts with no cycling. You can find these on the solid state/digital regulators. One like this allows you to turn a screw to adjust the output.

Your gauges will act differently depending on whether or not you have a CVR or IVR. With the IVR, the oil pressure gauge will swing quickly to the normal position when you start your engine. So will the fuel gauge and the temp gauge if your engine is warmed up. But with a CVR, the needles will swing slowly, taking 15 or 30 seconds to get to the same points that takes a couple of seconds with an IVR. This is because the CVR is only putting out 5 volts. The IVR is really smart: when you apply power, it actually outputs 12 volts, then after several seconds, it begins to clock 0 to 12 volts. (This is one reason why your fuel gauge doesn't bounce all over the place.)

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Color: Y
Trim: 22
Axle: 6
Trans: 6
Build Date: July 18, 1966


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post #19 of 28 (permalink) Old 02-05-2019, 10:26 PM
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... it was obvious that somebody had been into this cluster at some point in it’s life, and they must have messed with the calibration adjustment...
The adjusting holes on the back of the gauge give access to gears. I thought that it was a screw. Use a small screwdriver and move only one pitch and see how much the needle moved. It doesn't take much crank on the gear to get a large needle movement. One gear changes the zero, one changes the sweep. They work together, so be patient and make small adjustments, verifying the output.

Use the pot to simulate the LO and HI sender outputs. I ran my engine at normal operating temp and measured the temp sending unit ohms, so that I could dial that resistance into the pot, to see where the needle should rest. I used a mechanical pressure gauge to match my oil pressure sending unit's resistance to my normal operating oil pressure (55 psi according to the mechanical gauge.) Dialing in this resistance allowed me to see where the needle should rest at normal oil pressure.

Bill

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Color: Y
Trim: 22
Axle: 6
Trans: 6
Build Date: July 18, 1966


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post #20 of 28 (permalink) Old 02-05-2019, 11:33 PM Thread Starter
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With the IVR, the oil pressure gauge will swing quickly to the normal position when you start your engine. So will the fuel gauge and the temp gauge if your engine is warmed up. But with a CVR, the needles will swing slowly, taking 15 or 30 seconds to get to the same points that takes a couple of seconds with an IVR.
While I’m waiting on my Amazon potentiometer (thanks for that link), what would you make of the fact that when I turn the key on with the engine warmed up, the temp gauge swings quickly to about mid range, and then slowly swings back down to the lower than normal reading that I get with the engine running. It never reads as high as it does when I first turn the key (and consequently apply 12v). While I know I have an issue with my gauge calibration, I’m still wondering if there isn’t some sort of voltage issue as well...

Also, I tested the old temp sender this evening (the one I had replaced), and it was in fact bad. It showed over 400 ohms at room temperature, and over 200 ohms at about 180°. It was an original Ford sender and it had a “260” stamped on it, so perhaps it was incorrect for the car...

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post #21 of 28 (permalink) Old 02-06-2019, 04:48 AM
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turn your electric fan off so your engine can heat up. Start engine and let idle. Monitor the engine temp with your IR gun aimed at the thermostat housing and the intake manifold right by the sending unit. As the temp increases, see what the needle position on the gauge is. Now you'll know what the engine temp is when you look at your gauge in the future.

t
t



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post #22 of 28 (permalink) Old 02-06-2019, 07:43 AM Thread Starter
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turn your electric fan off so your engine can heat up. Start engine and let idle. Monitor the engine temp with your IR gun aimed at the thermostat housing and the intake manifold right by the sending unit. As the temp increases, see what the needle position on the gauge is. Now you'll know what the engine temp is when you look at your gauge in the future.
No electric fan, to start with. The first IR temp gun I bought; the $40 one (Made in China) failed half way through the first warm up cycle, so I guess I’ll have to go back and buy the $80 one (also Made in China), but my question is, how accurate is an IR temp gun aimed at an aluminum manifold near the T-stat housing. I mean, I know they’re great for examining relative temperatures, but is 180° observed with an IR temp gun a the T-stat housing the same as 180° coolant temp? Has anybody with a good gauge ever done any comparison!?

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post #23 of 28 (permalink) Old 02-06-2019, 08:45 PM
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I use a meat temp probe. $15 at a big box store. It is fairly accurate between 140 and 240, the temps that interest me the most. Yeah, Chinese, dang it.

From your description, it sounds like your temp gauge sweep is too low. It needs to swing wider so that hot reads hot, not mid-range.

When I turn off the engine after getting it to full operating temperature, let it sit for a few minutes, then turn it back on, my temp gauge swings full right, "hot". Then it slowly drops to normal/mid-range, once I get coolant pumping through the block.

Bill

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Color: Y
Trim: 22
Axle: 6
Trans: 6
Build Date: July 18, 1966


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post #24 of 28 (permalink) Old 02-06-2019, 08:47 PM
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No electric fan, to start with. The first IR temp gun I bought; the $40 one (Made in China) failed half way through the first warm up cycle, so I guess I’ll have to go back and buy the $80 one (also Made in China), but my question is, how accurate is an IR temp gun aimed at an aluminum manifold near the T-stat housing. I mean, I know they’re great for examining relative temperatures, but is 180° observed with an IR temp gun a the T-stat housing the same as 180° coolant temp? Has anybody with a good gauge ever done any comparison!?
oops, got CDs68's post confused with yours. Well you could always take the fan belt off to stop the fan. As for the accuracy of the thermostat housing temp compared to the coolant, I would think the heat transfer is very good and should be fairly accurate (within a few degrees). As for your gauge going up to halfway after you shut off engine, and then down again when you start it. Think what happens to the engine temp when you shut it off. The water stops circulating and the heat from the cylinders would probably raise the block temp a little more than your normal operating temp. Then when you start the motor, cooler water circulates and the gauge goes back down. Sounds normal to me.

t
t



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post #25 of 28 (permalink) Old 02-06-2019, 11:49 PM
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I mean, I know they’re great for examining relative temperatures, but is 180° observed with an IR temp gun a the T-stat housing the same as 180° coolant temp? Has anybody with a good gauge ever done any comparison!?
With an IR thermometer you are measuring the surface temp of what you're pointing at and not the temp of the coolant. In many cases it gets you close but if it's critical there are better ways to do it. It's not as accurate as using a thermal imager or a good temp gauge in the sender port. A lot of guys do it but comparing an Extech IR thermometer, ( 42500: Mini IR Thermometer | Extech Instruments) Snap On thermal imager and VDO water temp gauge the imager and gauge are a lot better at getting an accurate temp. There is a pretty wide window of temp range so it's possible to use an IR gun but if you really need to know you should measure the coolant. There was 10*-20* difference between the IR temp and the gauge temp.

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post #26 of 28 (permalink) Old 02-11-2019, 10:47 PM Thread Starter
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So...I think the gauge is junk. I tested the gauge in place with the potentiometer hooked up to the gauge sending unit wire. At 24 ohms I was just shy of the low line. It took 11 ohms to get the midpoint and 6 ohms to get to the H line. I pulled the cluster and adjusted the stops, as both upper and lower limit were completely maxed out in their respective directions, and it made no difference in the gauges readings at any given ohm level.

I was really hoping that was all it was. Now to find somebody with a good used 1968 Mustang Temp gauge...

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post #27 of 28 (permalink) Old 02-12-2019, 01:01 AM Thread Starter
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Does anybody know if I can steal the internals from a spade oil pressure gauge to fix my temp gauge!?

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post #28 of 28 (permalink) Old 02-12-2019, 02:01 PM Thread Starter
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Does anybody know if I can steal the internals from a spade oil pressure gauge to fix my temp gauge!?
FWIW, YES, you can simply pop the face right off your oil pressure gauge, replace it with the temp gauge face, and bolt it into your instrument panel. The calibration is identical! Yay spare parts for the win!

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