How do you calibrate the oil pressure and water temperature gauges?

[djwarner]

The dash is out of my Series I to repair cracks and to install a VintageAir AC.

Pulling out the old gauges, I noticed there are adjustment screws opennings on the back of the gauge.

My water temp is off by about 30 degrees high and I suspect the oil gauge is reading low. Considering the gauge is 42 years old, I can understand a need to re-calibrate.

Does anyone have a procedure that can be done while I got the dash apart?

Which adjustment screws do what?

I have a 12 volt power supply, regulated air pressure to exercise the pressure sender and hot water to perform the calibration.

BTW I've found a way to maintain the fresh air/recirculation control by modifying the blower fan in the VintageAir unit.

[EuroDat]

Hi, I have not tried to adjust the gauges on a Z yet, but I have done it on old MK1 & MK2 escorts.

One thing is important to remember; the gauges work on resistance caused by the sender, but also in the old wiring.

With that in mind I calibrated the gauges in the car using the wiring in the car as follows:

1: Dismatled the dash to access the gauge adjustment screws

2: Removed the senders from the block and used a jumper cable to earth the gauges to the car.

3: Simulated oil pressure with a compressor and accurate pressure gauge. The Z only has 0 and 90 psi maximum.

4: If you want accuacy, I used a 1/2 round disc of paper and marked every tien psi to maximum. Then scanned it in and made a sticker for the gauge. We wanted to know what was going on with the oil and not just a ball park figure in the middle somewhere. Took a couple of goes to get the colour and scale right with the sicker.

I only adjusted maximum pressure, since the sicker would show the rest.

5: The temp sender was a little harder. Then I followed steps 1 & 2 and used a cup of water and a thermometer taped to the sensor to read the temp. Don't just hang the thermometer in the cup. Reading can be off. I used this up to 65degC and then used a nut roaster (small oil deep fryer). That way I could go about boiling point. I had to be really carefull with the oil fryer and ended up using foam to support it between the engine an chassis of the car. Then I followed the same procedure in step 4.

Might not be what you had in mind, but it worked find in the escort. Nice accurate gauges

Edited October 27, 2012 by EuroDat

[240260280z]

The best way is to calibrate on the car because the voltage regulator will put out more than 12V and there will be losses in the wiring to the instrument. (If you bench calibrate then you will lose the accuracy once installed.

For calibrating the temp sensor, fit a thermocouple to one of the fittings on the thermostat housing and run the car to temperature. Adjust the gauge to match the digital readout. You can also check with an IR thermometer to see if the temp on the outside of the thermostat housing is the same inside. You can also plot external temp of the motor to see hot spots and temperature gradients.

For the oil temp, do the same but fit an external pressure sensor gauge to a y fitting with the oil pressure sensor.

[djwarner]

Looking at the inside of the gauge, there is a voltage regulator made out of a coil wrapped around a bimetal strip with the other end of the strip providing a contact point for power to the gauges themselves.

According to the Haynes repair manual: "In the event of the oil pressure gauge and water temperature gauge becoming inoperative at the same time, then the instrument voltage regulator should be suspected."

[EScanlon]

Looking at the inside of the gauge, there is a voltage regulator made out of a coil wrapped around a bimetal strip with the other end of the strip providing a contact point for power to the gauges themselves.

According to the Haynes repair manual: "In the event of the oil pressure gauge and water temperature gauge becoming inoperative at the same time, then the instrument voltage regulator should be suspected."

I'll check my Haynes as well, but this could be a leftover bit of information from the Sports Roadster (311S) since that vehicle did use a separate voltage regulator to step down the voltage to ~8v and made it cyclic.

With the Roadster this is a common problem, if the Fuel and Temp start acting erratically then the Instrument Voltage Regulator was the item to check.

Biggest problem with the Z, is that while the Roadster's Inst. Volt. Reg. was EXTERNAL to the gauges, in the Z it is INSIDE the gauge. That would necessitate a replacement of the gauge.

FWIW

E

[djwarner]

I had a go at calibrating the water temperature gauge this evening. Requisitioned my wife's range and set up shop with a 12Volt power supply digital multimeter with thermistor probe as well a contactless thermometer and jumpers. Before begininng I measured the sensor at 200 ohms at room temperature and dropping as the temperature rose. Recorded 150 ohms at 150 degrees.

While I was getting the sensor out, I replaced the 160 degree thermostat I recently bought with the 180 degree unit the PO had installed. I had previously confirmed that it did open exactly at 180 degrees confirming the 30 degree error to be in the sensor gauge circuit.

When calibrating the gauge, I at first had a difficult time trying to get much of any adjustment with the two slots in the back of the gauge. After no small amount of frustration, I discovered that the two levered adjustment joints were frozen after 41 years. The only movement was slippage around the mounting rivets!

I hit the slip joints with a very small hit of WD40 and finally got the joints loose. The adjustments worked this way: The adjustment slot closer to the gauge center adjusted the the needle left and right - this is equivalent to setting the zero offset. The adjustment closer to the outer edge of the gauge adjusted the distance between the needle pivot point and the pin coming from the bimetallic strip. This adjusted the meter's sensitivity. When the pin was moved farther from the pivot point, the meter's sensitivity was decreased. When the pin was moved closer to the pivot point, sensitivity increased.

There is some interaction between the two adjustments. For anyone trying to do a calibration, I would suggest first moving the pin to the middle of the adjustment slot in the needle. You will need to take the lens cover off to observe this. Supply a temperature/pressure that would be easily recognized. I assumed the center tick mark on the temperature gauge to be 180 degrees, because it is slightly offset from the center. My oil pressure gauge ranges from 0 to 140 PSIG with a tick mark in the center that I assume is 70 PSIG.

With the test temperature/pressure set to the tick mark value as measured by another meter, use the adjustment closer to the center move the needle to match the tick mark.

Then reset the test temperature/pressure to that indicated be the leftmost tick mark (120 degrees or 0 PSIG on my gauge). Note the error and adjust out half of the error using the adjustment closer to the center.

Return the test temperature/pressure to the middle tick mark value. Use the adjustment closer to the edge to again match the needle with the center tick mark. Again take out half the error as before.

Repeat these last two until the gauge hits both tick marks with the need for adjustments.

Normally, calibrating a gauge would be done using the far left and far right tick marks but above procedure will verify the accuracy at the normal operating conditions which should be near the center tick marks.

Some others have indicated their oil pressure gauges only go up to 90 PSIG, since this should be attainable with compressed air, using the right most tick mark would be desirable.

[Wade Nelson]

It's a nice IDEA to have calibrated gauges, but clearly, some of you guys have far too much time on your hands.

About the ONLY thing a water temp gauge can tell you is, if it's running 20-30 degrees hot, you've got a pressure leak in the system. Other than that it's either A) the engine's cold, or the engine has warmed up and the T-stat is open.

Oil pressure? Perhaps you can tell when you're a quart low if the pressure drops off a little, but other than that, the reading is mostly based on oil temperature and the age/condition of the motor. The only thing you care about is if you have low or NO oil pressurr, right?

So how is "calibrating" your gauge going to tell you a single thing more than what you already know with an uncalibrated gauge?

Like I said, SOME of you guys have too much time on your hands. IMHO, of course.

[djwarner]

It's a nice IDEA to have calibrated gauges, but clearly, some of you guys have far too much time on your hands. QUOTE]

Perhaps so. In August I acquired this Series I car and I'm still sorting it out. With 168K on the clock and oil pressure reading 20 psig when I got it, it's been a point of concern. A new pressure sender brought the reading up an indicated 50 psig dropping down to 20 at idle. When I got off the expressway the other day, it went to an indicated 0 psi waiting for the light to turn. Oil levels were okay and pressure returned as soon as I accelerated. I would like to know if my pump is about to buy the farm.

The Z should run around 180 degrees which is not enough to pressurize the system. But as you said, stabilizing around the boiling temperature would indicate a leak to be dealt with. However, through trouble shooting, defect was in the gauging not the cooling system.

I was flying a small plane once when faulty oil pressure caused the engine to eat an exhaust valve. The head of the valve separated from the stem. Shortly thereafter, the valve head fell over and with each rotation, the piston rammed the valve head in to the cylinder head. The resulting shock caused the prop to swing up and down +- 2 inches with each revolution. With each collision the valve head potato chipped a little more. As I watched the engine swinging wildly wondering if the time for the engine mounts to fail was measured in minutes or seconds, the valve head broke into 2 pieces and was ejected out the exhaust port. As they say in aviation, some things can just ruin your whole d__m day.

I got the plane safely on the ground, but I never want to be asked, "How many souls on board?" again.

I am sorting out 41 years of maintenance with varying degrees of competency. My experience so far has taught me to trust little. I've been finding some kind of make-do-work-around or maintenance error about once a week, so yes, I'd like to know what the engine is really doing.

BTW the underlying cause of valve failures in that series of aircraft was unknown at that time. Later it was discovered there was a change in where the oil pressure gauge was tapped into the engine. In the original applications, oil pressure was measured at the far end of an oil gallery from the oil pump. In the design of my aircraft, they tapped the oil pressure at the near end of the oil gallery but the neglected to change the oil pressure specification. As a result, by FAA regulation the oil pressure had to be set at a pressure that did not account for the pressure drop in the gallery and the valves ran too hot.

[Wade Nelson]

http://www.classiczcars.com/forums/thread39295.html

And if your prop starts wobbling up and down, I suggest you pull over and call a tow truck.

[djwarner]

Thanks for the link. It has useful information.

[djwarner]

Just a quick update. The '70 and '71 oil pressure gauges are 0-140 psig. The '72 service manual shows a picture with the 90 psig gauge but the associated text still refers to 140 psig.

While in the service manual I found the idle speed was 750 rpm and oil pressure was supposed to be 14.2 - 17.1 psig. The only other specification was for the pressure relief valve that limits the high oil pressure to 54.0 - 59.7 psig.

That being said, my oil pressure gauge is probably fairly accurate.

[EuroDat]

My gauge is 0 - 90psig in 1977 280Z. It seems to work fine, so Im leaving it at that. Ive got to many other projects to begin playing with the gauges.

Cheers