1940 Temperature Sensor

[kirkf]

This post will be a bit long, so bear with me:

As some of your may know I'm trying to put a 1940 Pickup back on the road. It was a basket case project that's been apart for decades and the previous owner was trying to put a Mopar into it.

I'm putting an 8BA in and converting to 12V negative ground with the original dash cluster.
I'm using a VRC-604 to convert from 12 to 6 volts for the dash gauges.
(Same unit used on cars / trucks from the 60s and 70s to convert their gauges)

The 1940 Gauge sits on hot when unpowered and sweeps to cold when voltage is applied.

The 8BA has two temperature sensors on the motor.

As I understand it:

The driver side sensor is a switch. If the motor reaches a certain temperature, the bi-metallic switch opens, breaking the circuit to the gauge and making it sweep to full hot.

The passenger side is used as a sensor. A bi-metallic switch opens and closes as current is applied. The hotter the engine is, the longer it takes for the contact to cool and close. The regular opening and closing of this switch averages out at the dash gauge to show a temperature (This is the same concept used for the VRC-604 to convert to 6V)

My gauge starts off on Hot when unpowered and sweeps to full cold when i turn the ignition switch on, but does not come off cold when the engine is running.

I first assumed the passenger side sensor was faulty, so I changed it out for another one, and I see no difference.

So I'm open to any ideas as to what the problem might be. I believe the temperature sensors were the same from 1940 to 1952. The 59A only had the sensor and not the additional switch, but this should make no difference.

Any ideas?
What is the method to test the passenger side sensor?



Kirk

 

[Ken/Alabama]

Run a wire from the gauge straight to the single pole sender bypassing the double pole sender and see how it works

 

[petehoovie]

 

[JayChicago]

I think it would show full cold on the gauge as you have described if there was a short to ground anywhere in the sensor wire. Could also be that both senders are defective and continuously grounded.

Suggest you try what Ken said in #2, to bypass things and hopefully home in on where the problem might be.

 

[Tom F OHIO]

Kirk, I have a 52 and the temp sensor stayed on hot all the time and never moved sitting still or running. I did as what Ken suggested and after bypassing the sensor the dial moved when turning on the key. I purchased an OEM double clip sensor on ebay for the same price as the repro's. I seen a completely restored 52 on you tube and when they showed the dash and the car was not running, and the dial was on the H and mine does the same thing until I start it.

 

[kirkf]

Step 1:

I tested the drivers side cut out switch with my fluke multimeter:
Terminal 1 to Terminal 2 - Continuity
Terminal 1 to case - Open
Terminal 2 to case - Open


So I didn't believe it was involved in my temperature gauge issues.
I decided to bypass the switch as suggested and the temperature gauge did come off cold and settled at just below full hot.
(Indicating the Passenger side sensor was now functioning, if not accurately)

I can't explain this, as my fluke meter insists there is no path to ground in the Drivers side cut out switch.
I replaced the Driver side cutout switch with another unit that tested the exact same and it now operates correctly.

Step 2:
With the gauge now reading close to full HOT while the engine was COLD, I decided to replace the Passenger side sensor with an alternate unit.
After the sensor was replaced, the gauge continued to read at about full hot. This indicated there was an issue with the gauge and not the sensor.

Step 3
I set up a 6 volt source and wired from 6V to an alternate gauge and then to the drivers side sensor. The alternate sensor indicated a more correct range.

So it looks like my final problem is the gauge in my dash is out of calibration.
There are two adjustments for the gauge that can be made by pivoting small adjusters in the gauge. I assume these adjusters are zero and span.

So this leads to my next question:

I have never come across a Ford gauge calibration procedure, but the existence of these adjusting points means one must exist somewhere.

(See attached picture, the small holes with the adjusting points. You insert a screwdriver and move each one. The one by the needle would be the zero, and the other one the span)

Note: Picture shown is not a temperature gauge. A temperature gauge sits at full right (Hot) in the unpowered state

Kirk

 

[petehoovie]

Step 1:

I tested the drivers side cut out switch with my fluke multimeter:
Terminal 1 to Terminal 2 - Continuity
Terminal 1 to case - Open
Terminal 2 to case - Open


So I didn't believe it was involved in my temperature gauge issues.
I decided to bypass the switch as suggested and the temperature gauge did come off cold and settled at just below full hot.
(Indicating the Passenger side sensor was now functioning, if not accurately)

I can't explain this, as my fluke meter insists there is no path to ground in the Drivers side cut out switch.
I replaced the Driver side cutout switch with another unit that tested the exact same and it now operates correctly.

Step 2:
With the gauge now reading close to full HOT while the engine was COLD, I decided to replace the Passenger side sensor with an alternate unit.
After the sensor was replaced, the gauge continued to read at about full hot. This indicated there was an issue with the gauge and not the sensor.

Step 3
I set up a 6 volt source and wired from 6V to an alternate gauge and then to the drivers side sensor. The alternate sensor indicated a more correct range.

So it looks like my final problem is the gauge in my dash is out of calibration.
There are two adjustments for the gauge that can be made by pivoting small adjusters in the gauge. I assume these adjusters are zero and span.

So this leads to my next question:

I have never come across a Ford gauge calibration procedure, but the existence of these adjusting points means one must exist somewhere.

(See attached picture, the small holes with the adjusting points. You insert a screwdriver and move each one. The one by the needle would be the zero, and the other one the span)

Note: Picture shown is not a temperature gauge. A temperature gauge is the sits at full right (Hot) in the unpowered state

Kirk

 

[kirkf]

I set up a small test to check the zero and span adjustment of a gauge.

Starting at an arbitrary zero, with span set to far left (counter clockwise) adjustment I applied zero volts and then six volts while measuring needle deflection.
Deflection was ~23 degrees

I then set the span to far right (clockwise) adjustment and applied zero volts and six volts while measuring needle deflection.
Deflection was ~39 degrees

Note that as with all analog gauges, an adjustment of span will also affect zero. In our example the zero moved about 5 degrees counter clockwise when the span was widened.

If adjusting span from the back of the gauge you screw driver would be turned clockwise to increase span.

If adjusting zero from the back of the gauge you screw driver would be turned counter-clockwise to increase zero.

This test was performed on a fuel gauge which operates opposite of a temperature gauge. The fuel gauge drops to zero with power off, while the temperature gauge would go to full hot.

See attached pictures.


Kirk

 

[Drbrown]

I did internal adjustments to my temp and gas tank gauges while they were functioning in the car. With the engine running I was careful not to ground the gauge wire out while doing that. For the temp one, I had a new mechanical temp gauge installed on the drivers side head and I had kept the stock sender on the passenger side, knowing that the passenger side runs a slight bit higher than driver's side. Optionally, one could use the coolant temperature in the top of the radiator which would represent a combination of both. Likewise I half filled gas tank and adjusted that dash gauge; it was pretty actuate as was.

 

[kirkf]

I removed the gauge from the dash and I used a 6 volt external source to power through the gauge to the passenger side sensor on the motor. Then I calibrated the zero on the gauge.

I started the engine and let it run for 15 minutes at high idle and then I spanned the gauge to sit at high normal.

Then I cooled the engine down to make sure the gauge again returned to the Cold mark.

However when I installed the gauge in the dash, the gauge sat at the normal mark when the engine was cold.

The problem is the way I was converting to 6 volts at the dash.
I was using a VRC-604 ford gauge voltage reducer. The VRC-604 works by heating a bi-metallic spring and pulsing power on and off.

However the engine temperature sensor is also using a bi-metallic spring to pulse power on and off to the dash gauge. This means the engine sensor doesn't receive the steady 6 volts it expects, and between it pulsing on and off, and the VRC-604 doing the same, the gauge doesn't get the current it needs to move the needle to the cold position.

This left me with the choice of re-calibrating the dash gauge to read accurately with the vrc-604, or replacing the dash voltage reducer with an electronic unit that would provide a steady 6 volts.

I chose to change to the electronic reducer.

When I repeated the test with the electronic reducer the temperature gauge performed as expected.

So this resolves my issues with the temperature gauge. Thank you to everyone for their help.


Kirk