I put a complete new wiring harness on an 856 I restored a year ago, and remember the air restriction indicator wiring well. Assuming an 856 and 1566 use the same system, this is how the 856 system works. The green / red gauge defaults to the red zone when no power flows through the gauge. The restriction gauge takes power from the ignition switch to one gauge terminal and grounds the other gauge terminal through the sender at the air cleaner. The sender at the air cleaner has two terminals, one is connected to ground near the sender and the other terminal connects to the gauge terminal. The sending uniot at the air cleaner is just an on / off vacuum switch so normally it grounds the gauge which puts the needle in the green zone. When restriction vacuum gets high enough, it opens the sender switch, which opens the gauge circuit to ground and allows the needle to swing to the red zone.
If yours remains in the red zone, look for no power to the gauge, or an open circuit in the ground circuit, through the sender switch to ground on the engine. If the gauge is known to be good, your problem has to be in the 12V supply circuit to the gauge, or in the ground circuit that goes through the sender switch to ground on the engine.
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Today's Featured Article - Restoration Story: Fordson Major - by Anthony West. George bought his Fordson Major from a an implement sale about 18 years ago for £200.00 (UK). There is no known history regarding its origins or what service it had done, but the following work was undertaken alone to bring it up to show standard. From the engine number, it was found that this Major was produced late 1946. It was almost complete but had various parts that would definitely need replacing.
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