Could the resistor be used for inductive kick from start solenoid?

Surge suppression is my guess.

Last edited by Nos681 (4/22/2026 10:33 AM)

It is an odd-ball circuit to be sure. Ford used it for 5 years from 69 to 73.
The 10-ohm resistor is not surge protection. The gauges and IVR are all electromechanical. They use a bi-metal wrapped with a heater, and don’t care about the normal level of electrical noise generated by the cars electrical system. They are simply immune.

When initially turned on, an OEM IVR takes time to warmup, then it starts to pulse. During this warmup time, you have the full output voltage until the first pulse. After the first pulse, the subsequent pulses come at a much faster rate. This gives the meters a little kick, and allows them to come up to their operating range a little faster. This is probably only important for the oil gauge- I always like to see the pressure come up quickly.
 
The 10-ohm resistor is part of a voltage divider circuit. They have the 10-ohm resistor in series with three parallel resistors (the gauge heaters and the senders). When all the senders are low, the voltage across the IVR and gauges is 8.35V. When all the senders are high the voltage across the IVR and gauges drops to 5.23v. This changes the IVRs pulse time pretty dramatically. When the voltage is 8.35v, the time to the first pulse is 6 seconds. When the voltage is 5.23v, the time to the first pulse is 44 seconds.
6 seconds at 8.35v makes the needle climb to its “low” operating point in 11 seconds. The needle doesn’t have far to go, and it gets there pretty quick.
44 seconds at 5.23v makes the needle climb to its “high” operating point in 45 seconds. The needle has a long way to climb, so it takes a while.
BYW, an OEM IVR always makes the gauges respond significantly quicker than an aftermarket IVR having a constant 5v output. I bought a used and operating IVR for $15 plus $9 shipping from Ebay.

Mach1Driver & MS; thank you for your responses.
I didn’t include all the facts leading to these gauges not working.  The car is in a restoration stage, the dash panel was completely taken apart for refurbishment and the printed circuit was replaced, as was the wire connector to the printed circuit. 
As suggested, we tried to get the gauges to work in the car, and couldn’t.  With all the work that was done, and not being sure it was put back together correctly, we removed the dash panel and I took it home to check out on the bench and found the three gauges worked.  I used a 27ohm resistor to simulate half-scale on the gauges, and connected a 12 supply directly to the CVU.  I hadn’t realized that there was a resistor installed in series with the supply to the CVU. 
We haven’t gotten back for further troubleshoot, but seems the 12v fed to the CVU is the issue.
I was just wondering why Ford placed this resistor in series with the supply … and hoping that I didn’t damage the CVU.   

Steve69 wrote:

I took out all that crap and with my first project installed gauges in my 69 dash.   Like MS said the printed circuit had crumbled and didn't work.  A few years back I went Dakota Digital set up.  I like it but as I get older harder to see the gauges since there smaller.   Steve69

Interesting, my friend had considered installing the Dakote Digital dash, but decided the funds were needed in other places. 
 

Reading all of this just confirms I'm not very smart when it comes to electronics.   Lol

More details, finding that there are intermittent shorts to ground on the gauge circuits.  The flexible printed circuit was replaced prior to my involvement, and I’m finding out that there seems to have been extra pieces that weren’t reinstalled! 
My suggestion now to my friend is, let’s take the dash apart and start over. 
 

Steve69 wrote:

BobE wrote:

Steve69 wrote:

I took out all that crap and with my first project installed gauges in my 69 dash.   Like MS said the printed circuit had crumbled and didn't work.  A few years back I went Dakota Digital set up.  I like it but as I get older harder to see the gauges since there smaller.   Steve69

Interesting, my friend had considered installing the Dakote Digital dash, but decided the funds were needed in other places. 
 

   I think it was around $725 back in 2019.  There probably a lot more now.   
 

When I looked into the Dakota dash for my 65 a couple of years ago, it was over $1000.  I didn't do the upgrade.