1977 IVR

[Muddy]

Hey everyone I’m at the point I’m trying to get my gauges to work and in doing so, I put my meter on the IVR and I have 14vdc on input side but nothing on the output side. My question is, is the IRV toast because it has 14v? I don’t want to swap it until I know what the correct voltage should be. Any help and or suggestions appreciated.

 

[Broncobowsher]

14V is fine for the input.
Output will not be stable. It averages about 5V. By average, few seconds at 14V and a little more time at 0V.
The IVR is a heating element and a set of points opened by a bimetalic spring. As the heating element warms the spring, it bends and opens the points.
People have done solid state IVRs that lack the features that the original has. The original design has a long initial high output cycle as it brings the cold spring up to operating temp. This works with the cold gauges to bring them up to operating temp faster as well. It is also temperature compensating. When it is cold, the gauges and IVR loose heat faster. Since the gauge is also a bimetalic spring moved by a heating element, on a cold day it will cool off faster and read lower. But the IVR is also cold and has longer duty cycles. So the gauge isn't affected by cold, even though it is a thermal device. Same on a hot day. Everything is already preheated, the IVR has shorter cycles as it only needs a little heat to be in the cycle range. Same thing for the gauge, it is in the same hot cab. Don't need to add much heat for it to warm up. The solid state IVR is fine for a fair weather ride that doesn't matter if the gauges are off in hot or cold weather.

For the IVR to work, it has to have a good ground. If the ground is weak, or non-existant (as often happens when people try to check it with the meter unbolted from the dash, gauges ground through dash mounting screws) the IVR won't see the ground plane. It will just put out full battery voltage all the time. Without a ground it doesn't see that the voltage is above zero. Like a bird on a power line, it doesn't know that it is at an elevated voltage. The elevated voltage is the ground plane to the bird sitting on it. The IVR is referencing it's output voltage on the elevated voltage of the gauges not being grounded to the body.

Yea, it sounds like your IVR is bad. Generally 2 failure points. Either the contacts get corrosion and can't close the circuit to start the heater heating the spring. Or the heater resistor is burned out. You might be able to clean the points if you open the crimp and get inside of it. If the heater resistor is burned up, no quicky fix, you have to get a new one. If you do clean the points, you might change the calibration. There is a little screw adjuster...

 

[Muddy]

14V is fine for the input.
Output will not be stable. It averages about 5V. By average, few seconds at 14V and a little more time at 0V.
The IVR is a heating element and a set of points opened by a bimetalic spring. As the heating element warms the spring, it bends and opens the points.
People have done solid state IVRs that lack the features that the original has. The original design has a long initial high output cycle as it brings the cold spring up to operating temp. This works with the cold gauges to bring them up to operating temp faster as well. It is also temperature compensating. When it is cold, the gauges and IVR loose heat faster. Since the gauge is also a bimetalic spring moved by a heating element, on a cold day it will cool off faster and read lower. But the IVR is also cold and has longer duty cycles. So the gauge isn't affected by cold, even though it is a thermal device. Same on a hot day. Everything is already preheated, the IVR has shorter cycles as it only needs a little heat to be in the cycle range. Same thing for the gauge, it is in the same hot cab. Don't need to add much heat for it to warm up. The solid state IVR is fine for a fair weather ride that doesn't matter if the gauges are off in hot or cold weather.

For the IVR to work, it has to have a good ground. If the ground is weak, or non-existant (as often happens when people try to check it with the meter unbolted from the dash, gauges ground through dash mounting screws) the IVR won't see the ground plane. It will just put out full battery voltage all the time. Without a ground it doesn't see that the voltage is above zero. Like a bird on a power line, it doesn't know that it is at an elevated voltage. The elevated voltage is the ground plane to the bird sitting on it. The IVR is referencing it's output voltage on the elevated voltage of the gauges not being grounded to the body.

Yea, it sounds like your IVR is bad. Generally 2 failure points. Either the contacts get corrosion and can't close the circuit to start the heater heating the spring. Or the heater resistor is burned out. You might be able to clean the points if you open the crimp and get inside of it. If the heater resistor is burned up, no quicky fix, you have to get a new one. If you do clean the points, you might change the calibration. There is a little screw adjuster...

Wow, thanks for explaining how it works. That helps more that you know. I like to know how and why so down the road I can troubleshoot it better. You also answered my follow up…if solid state would be better if it needed replaced. Thanks for you help!