Trying to diagnose non working fuel gauge. After going through many posts and lots of time on the Internet I can tell you this -my temperature gauge works fine.

So I accessed the fuel sending plug under my rear seat, followed the instructions and ran wire from Power to one plug, which got the orange low fuel light to come on. (By the way how many gallons is to be left in it for that to indicate?)

jumped wire to the second side, and according to my reading should peg the gauge to full… It did nothing.

I thought it was a bad sender and was going to be an easy fix -now maybe the wiring? I’ve checked all the grounds and everything else in the dash works great. My thought process is If it was a sending unit I would be able to jump the connector and get the idiot light to come on and peg gauge to full?. Am I correct in this theory? If not

any ideas suggestions greatly appreciated

THANK YOU!

 

Well, it doesn't actually work that way to my understanding. The Fuel Sender is basically just a variable resistor, with one side connected to ground (it grounds through the metal tank, which grounds to the frame it is mounted on) and the other side connected to the gauge, which is then connected to "power" battery positive through the fuse block. The "power" comes from the gauge side, when your ignition switch is in the "RUN" position. That energizes the instrument cluster, so you have to have the ignition switch in the "RUN" position to test. Applying power to the terminal for the sender won't do anything, even if the wire from the sender to the gauge is intact & good. What you need to do is apply a ground to that terminal going to the gauge.

How it works:
Your fuel gauge has one or more coils in it that act as heating coils. This heat affects a bimetallic strip which is hooked up to the needle of the gauge through a linkage. The bimetallic strip is a piece of metal made by laminating two different types of metal together. The metals that make up the strip expand and contract when they are heated or cooled. Each type of metal has its own particular rate of expansion. The two metals that make up the strip are chosen so that the rates of expansion and contraction are different. When the strip is heated, one metal expands less than the other, so the strip curves, with the metal that expands more on the outside. This bending action is what moves the needle on the gauge.
Some gauges do not use the heater coil/bimetalic strip method. Instead they have two electromagnetic coils placed at either side of a "field magnet" that is attached to the needle.

As for the sender, as I said it is a variable resistor, which depending upon fuel level will resist the flow of current to the gauge coils either more or less. Most basically put, the more fuel you have in the tank, the less resistive the sender, and the more current that will flow to the gauge and the more the gauge needle will move towards "full". The sender works the same way for either the bimetalic or electromagnetic gauges. To the best of my knowledge, Toyotas use the electromagnetic coil setup.

It would be better to test using a Volt/Ohm Meter. Your fuel sender should have a range of 5 Ohms (full) to 96 Ohms (empty). Probe the terminal for the sender at the tank and place the other probe to ground. If the sender is working at all you should get an Ohm reading somewhere between 5 to 96 Ohms. That would show that the sender at least has continuity.

Check for voltage from the gauge with ignition in "RUN" position. Positive Probe to the socket terminal that connects to the sender, Negative Probe to a good ground point. You should be reading 12 volts or so and the gauge should want to peg to "full". If you read no voltage, the gauge is at fault.

If no voltage from the gauge but everything else on the dash is working (that means your instrument fuse is good), you either have a break in the wire from the gauge to the sender or that wire is shorted somewhere to ground, OR you have a broken coil wire in the gauge itself. The coil wires are very small & thin and can sometimes break due to vibration, or one of the solder joints fails, either of which would cause an open circuit & no current flow (empty gauge reading). To get an idea of this here's a pic of a fuel gauge from a MG, which is very similar to the Toyota setup:


MG Electromagnetic Fuel Gauge - many other makes have a similar design.

I can't locate them now but have seen a few write-ups before of people repairing the wire or solder joints themselves. If it turns out the gauge itself isn't working and you aren't that adventurous in trying to fix it yourself, I'd recommend Bob's Speedometer in Michigan. https://www.bobsspeedometer.com/1/120/index.asp
They do good word for reasonable prices & have been around since 1929.

BTW, you specified 88 3VZE but didn't say what model; I assume 4Runner? Adding that info might help with some responses from others.

 

Here's the manual section for diagnosing the sender and gauge. http://web.archive.org/web/201102052.../6combinat.pdf This is the '93 manual, but I expect the '88 to be very similar. Note that the actual resistances to be read from the sender depend on tank volume, but you don't care. (You don't know how much fuel is in the tank anyway). If you have a failed sender, you'll get either infinite ohms (open) or a dead short (0-ish ohms); anything between 3 and 350 ohms means the sender is probably good (of course, it could be stuck at some level, but that's diagnosis for another day). Be careful testing the sender with a full tank; it is pretty difficult to accurately read resistance less than 10 ohms with a hand-held multimeter. Is it 3 ohms or a dead short?

The gauge itself is a "hot-wire ammeter," which uses the technology described by 13swords as bi-metallic. These are very rugged and self-damping. "Electromagnetic" gauges (usually called d'Arsonval movements) aren't suited for automotive applications (so it's no wonder you'd find one in an MG ;^) ). To measure resistance with an ammeter requires a constant voltage source. In your multimeter, there is a tiny 21st-Century chip that provides that. In your 80s technology Toyota the "constant" voltage is supplied by a relay that switches back and forth about 1/second (which is fine for a hot-wire ammeter, which by design respond slowly).

If you care, modern cars use stepper motors for all gauges, and they're driven by microprocessors. Which is sorta cheating.

 

Oh man, thank you so much for all the detailed info!!! time to get testing!!

 

Scope, Thanks for the info! The link will help a ton!!!!!