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Abrupt failure of fuel gauge and temp gauge, 1987 Toyota truck

I seem to have gotten myself into an awkward spot. I borrowed a friend’s small Toyota truck for a few days of easy chores, and made a point to check the fuel at the beginning: the gauge showed 3/4 full. Four days and only 25 miles later, when I got in to return it, I noticed the fuel gauge was not working, I’m certain there’s no fuel leak, I would have noticed that: the truck had been parked on my property the entire time, driven only a few hundred feet from one spot to another moving firewood around, so I would have smelled the gas if there was a leak.

When I added a couple gallons of fuel, the gauge didn’t budge. Once on the road, I realized that the engine temperature gauge was also dead - no change on the gauge after 10 miles. So I have two non-functioning gauges which failed on my watch. This may not be directly my fault, but as my friend is kind enough to lend the truck whenever I need it, my contribution is to provide maintenance.

Is there anything in common between the two gauges which would explain the synchronized failure? Or is this total coincidence? This one is outside my experience, so don’t know where to start. It’s a 4 cylinder, 5 speed manual, base model truck, about 150k, generally runs just fine. There are no other gauges, and I don’t recall what warning lights there might be, but I didn’t see any after I realized there was a problem. Would the whole instrument cluster die at once, or could this be something like a bad ground for the cluster? I don’t mind investing effort into tracking down and solving a problem, but if it needs a new cluster, that will cost more than I’d like to contribute, yet the problem occurred while I was using the truck (gently). My total use of this truck for all the times I’ve borrowed it is certainly less than 300 miles, so I’m reluctant to fund the entire repair if this is something that just comes with age.

Can anyone suggest how to begin troubleshooting this? Thanks very much!

Check the fuses? The gauges get power from a common source and from a common fuse. The speedo is mechanical, so no power is required for that.

Thanks Busted…I didn’t have a chance to check anything the day I returned the truck, was on a tight schedule, so that was my first thought. But someone I spoke with today was pretty sure gauges aren’t fused…and since the truck is now 10 miles away and my friend is on vacation, I need to gather some good ideas before I get back there to check it out. I just remembered also that two of the five days I had the truck we had steady heavy rain, which might have some role in the problem. I just have no actual experience with instrument issues.

Gauges are always fused and the wiring schematic shows a 15 amp fuse as the power supply to both fuel and temp gauges. It also shows a number of other things run off of the same power circuit.

Is anything else inoperative? If not, I would think the problem would be an aged instrument cluster connector with an iffy connnection or the possibility that both fuel tank sender and temp sender failed simultaneously. The odds of the latter are very slim though.

You might locate the temp gauge sending unit, disconnect the wire, and ground it to metal somewhere. Turn the key on (no engine start) and if the wiring/connectors/etc are good then you should see the temp gauge go to full H. That would point to a failed temp sender and the same thing could be done with the fuel tank sender and fuel gauge.

Offhand, I don’t see that this is your fault at all. The truck is a quarter century old and anything on it is subject to hiccups at any time. Hope that helps and good luck.

@ok4450
Thank you.

The truck is a quarter century old and anything on it is subject to hiccups at any time

I agree that I’m almost certainly not directly responsible for the gauge failure, but since my friend is generous with her truck, I want to be generous with providing help in ways she’s not capable. Thus I’m willing to do routine maintenance, and repairs within reason, in exchange for the use of the truck.

Is anything else inoperative?
Nothing that I noticed, but then, I didn't do any troubleshooting at the time I returned the truck. Didn't think to check lights, radio, etc. Can you tell me what other items are on the same circuit? I'll start with checking the fuse of course. Then if necessary, test sending units, but as you wrote, simultaneous failure is very unlikely. I was more suspicious of the cluster and its connections, but that'w where I have no prior experience....time to dive in.

Your instructions are clear and helpful. I hope to get a look at the truck on Sunday. Won’t know anything before then.

Thanks,
–Roadtripper

The diagram shows that 15 amp fuse as being the power source for the oil pressure gauge, the oil pressure lamp, the dashboard brake light, and the voltmeter along with a 4WD indicator light if so equipped.
That fuse is referred to as an “engine fuse”, which means it’s probably under the hood. Hope that helps.

Great, thanks!

You only drive 25 miles total? Are you sure the fuel gage isn’t working? You’ve used less than a gallon of gas.

Same with the temp gage. Are you sure it isn;t working? Perhaps his T-stat is stuck open.

If it were me, I’d mention to him your concerns and offere to help pay for them if it eases your conscience. Neither is likely to be expensive.

@the same mountainbike
Thanks for the reply but I think you mis-read my post. As I mentioned, the gauge read 3/4 when I borrowed the truck, and it was about 10 miles to my house. After that, I only ran it a few times on the property, and on the way back, the gauge never came up above empty, even after I added a couple gallons of fuel.

The t-stat is a possibility, though the temp gauge should have moved a little on my 10 mile return trip, which it did not. I’m going to take a look at it over the weekend, hopefully it will be just the fuse.

Thanks for eth followup. You are correct, I misread your post.

One thing you should check firt on that temp gage is coolant. If the coolant has for some reason vacated the system, the temp sensor will be in air, and it won;t read properly. If that turns out to be the case, the problem is bigger than a simple T-stat. And, by the way, if that’s the 22R engine (the four banger), the T-stat is super easy to change.

Good point about the temp sensor not being immersed in coolant. I had changed coolant for my friend six months ago, and actually checked it the day I borrowed it-it was full. But didn’t check it on returning because the engine was at operating temp. I’ll look at that when I go back. Thanks.