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That Pesky Little Guage Voltage Regulator


Banjo

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Got up on Saturday morning & started my KE30 Rolla, but no fuel or temperature gauge movement. Everything else worked fine, & no blown fuses. As the fuel tank & level sender is at one end of the car, & the temperature sensor is on the engine, it was a fair bet it was not a wiring issue. Then I remembered it will be that little voltage regulator, that sits on the rear of the dash assembly, & powers the fuel & temperature gauges. These two gauges run at 7 – 8 volts instead of 12Vdc, so that the two gauges read accurately, whether the battery is fully charged, or very low.

So out with the dash assembly, to have a look at the regulator. Now I had one go bad years ago, & I knew where it was located, but on my dash rear, there was no regulator. There was a little bit of blue plastic sticking through the PCB, where it should be mounted, but not on mine. The PCB had 12V IGN & 7V marked on it, with the posts & nuts for locating the meters, but no regulator. I hooked up 12V to the dash, & measured a big fat 0Vdc on the 7V posts to the meters. So I knew what the problem was, but where was the regulator. I looked in the garage & found two other dash assemblies, both out of KE55s, & both had regulators visible on the back of the dash PCB.

I noticed the fuel gauge had three posts; 12V IGN - 7V - Chassis/Ground, whereas the temp gauge only had two.

I took the PCB off the plastic dash frame & removed the fuel gauge. Careful study of the fuel gauge working, indicated that the 7Vdc regulator, is built into the gauge, which then supplies 7Vdc to the temperature gauge. However the regulator is not an electronic one like the KE55s, but a rather simple electro-thermic pair of contacts that maintain an “average” 7Vdc to power the meters. This works because both gauges are electro-thermic, & have very slow response times.

This contact is constantly opening & closing at varying on/off ratios, dependent on what the battery voltage is. The lower the battery voltage is, the longer the points will remain closed.

So even if it was working perfectly, you will not get a steady voltage when you check the 7Vdc point, with your multimeter. You get 12V or 0V, but never exactly 7 volts.

My fuel gauge regulator had pitted & worn contacts, & I could have delicately cleaned them, & possibly got it working again. However, after seeing how basic & crude the regulator was, I decided to insert an electronic LM317 DC regulator. This regulator will handle DC voltages up to 37V, & can be set to output basically any voltage between 2V to 35V, using just two (2) resistors.

Altronics sell a little Silicon Chip kit using this regulator, called a model K3220. You can buy then on line for about < $ 10.00 + P&P.

However, my fuel gauge had never read quite full, when the tank was topped up. I now had the chance to fix that, so that the gauge would read full properly. I filled my tank up at $ 114.9/ltr, & I put my variable power supply across the 7Vdc terminals. The fuel gauge indicated about 90% full. I wound the power supply up to 8.0Vdc & the gauge read almost 100%. I thought, if I make one of the resistors variable in the K3220 kit, I could then adjust the DC gauge supply to make the fuel gauge read exactly 100%. I used a 500 ohm multiturn trim pot, & to cut a long story short, the gauges work just like they did when the car was new; maybe even better. The little regulator can be mounted anywhere behind the dash, with only three (3) wires to be connected. If you fit Utilux post lugs to these wires, you don’t have to even solder it to the PCB. 12Vdc input to the regulator goes under the post nut for 12V on the fuel gauge. 7-8Vdc lug goes under the 7V post on the fuel gauge, & GND goes under any of the outside edge screws holding the PCB to the blue plastic frame, that pass through the ground plane on the PCB.

However, you must disable the electrothermic regulator inside the fuel gauge. Don’t butcher it, as it may effect the operation of the gauge itself. I simple placed a strip of thin cardboard (manila folder) between the contacts, and stuck it with some double sided tape so that it would never fall out.

Probably the hardest part of this little exercise is getting those two round multipin plugs on the wiring harness to plug back onto the back of the dash assembly.

If anyone wants the resistor values for the regulator to produce a 7-9Vdc adjustable output, just give me a yell.

 

Cheers Banjo

Edited by Banjo
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