Old Oil Pressure Gauge was reading low then fianally died. Odometer was broken so just got a new cluster. The new gauge read low then quit working. Pulled the cluster, which is always fun, pulled the gauge and applied direct power it works, engine has 70 lbs of pressure. Then, replaced the sending unit. So I figured it has to be the wire. Out of the truck it works. Ran a new wire, put the gauge in cluster and it doesn't work, I don't get it. Anybody have any ideas? Am I missing something?

Thanks,
Rick

The FSM (Factory Service Manual) has testing procedures for both the gauge and sender. Usually applying direct power to the gauge is not a good idea. They work by applying a regulated voltage across the variable sender resistance and measuring the resulting current that flows.

The oil pressure gauges in the mini-trucks are very delicate. The wire to the sender is a variable resistence ground as 4crawler mentioned. If the wire is fully grounded...either from a bad sensor or more likely, a chaffed wire rubbing against ground somewhere, the needle on the gauge will go full sweep the second you turn the ignition on. You may not notice it do this unless you're looking at the gauge.

In the process of going full sweep, the mechanism inside will continue to rotate as the needle hits the stop putting it out of adjustment. Once power is cut, the gauge will go back to zero, but the next time power is applied, the gauge will read extremely low or at zero.

My suspision is that you have a chaffed wire that was grounding the gauge but not all the time.

If the gauge has not been damaged, it might be possible to fix this by taking the gauge out and resetting the needle to zero with the mechanism at rest.

If the gauge is fully grounded for any lenghth of time, the very tiny wire inside the gauge that heats up and causes a piece of metal to move, will melt and break circuit, rendering the gauge useless.

The fix may be a new gauge, with a fresh wire and make sure that wire is NEVER fully grounded. Only hooked up to the sending unit or taped off.

Another thing to remember is that Toyota used two different suppliers for gauges and senders. Sometimes the sending unit from one supplier is not compatable with the gauge from the other supplier. When you go to order a new sending unit, they used to ask for a specific part number. But if you got a different cluster, it's hard to say if the cluster is compatable with the sending unit you're using.

The supplier should be marked on both the cluster and the sending unit. Make sure they are the same company.

Looks like one of the companies that made the toy gauges is called ... YAZAKI METER and they are still in business in Japan.

What was the other company?