Fuel Gauge & Temperature Gauge Calibration (1 Viewer)

[Spook50]

I really should be holding off on messing with my temp gauge indications until I have the fuel gauge itself dialed in, but with a road trip coming up I do NOT want my temp gauge to be sitting right at the red for an 8.5hr drive. Gives me next to zero leeway in the event of a problem, especially with temps in the upper 90s for most of the drive towards the coast.

But, last night while digging through my electronics stash, I found several 22Ω carbon film 1/2W resistors that are 5% tolerance. I paralleled two of them and got 11.1Ω on my multimeter, so I made the assembly a little more stout to be a temporary installation while driving around this afternoon. Heading to Idaho at lunchtime in 98° with the AC on had my gauge sitting right about at the 210° indication in the calibrated image. This actually does look good, so I'm going to see if I can snag a potentiometer from my work and set it to the 29.6Ω that the sender should be at for 239° so that I can see where exactly my gauge would sit with and without the extra 11.1Ω of resistance.

I really wish I had thought to verify the sender's resistance before installation like what was discussed on page 3, but on the other hand I have no reason to believe that a new OEM sender would be faulty.

 

[Spook50]

I may very well have a faulty (or possibly incorrect even) sender. I only say this because I know for a fact my engine has no reason to be approaching the overheat threshold at all (though likely in this heat it could potentially be getting close when climbing hills with the AC on). I still have the sender I replaced, and on checking it using the warm oil method, it reads spot on to what @OSS logged with his sender. EDIT: Actually as the temperature increases, it has just slightly less resistance (by about 2Ω at 192º so far) than recorded on the data log posted, so should still indicate just a hair higher on the gauge than the reference picture. I'm also left to wonder just how much variance there is between new identical OEM parts.

What's even more strange though, is that I wired a 27.7Ω combination of resistors directly to the thermostat housing to simulate ~239º to the gauge, and my needle only climbed as far as the 230º mark on the reference picture. So this leaves the question; is the new sender allowing less and less resistance (on a somewhat logarithmic scale) the hotter it gets and REALLY making the needle show erratic indications?

I'm honestly tempted to just deal with the mess and throw the old sender in tonight so that I can keep a better eye on it before this trip, then set my regulator back up so that the needle shows an appropriate indication at the 27.7Ω.

 

[CruiserTrash]

I know that the couple times I’ve replaced the sender, they’ve made the gauge read differently by as much as 1/3. None of the senders were bad, it was more “I’m doing the manifold gasket so I may as well”. I don’t think these are high-precision parts, and I do think a trimpot is a good idea for calibration purposes.

Love watching this progress by the way.

 

[Spook50]

I know that the couple times I’ve replaced the sender, they’ve made the gauge read differently by as much as 1/3. None of the senders were bad, it was more “I’m doing the manifold gasket so I may as well”. I don’t think these are high-precision parts, and I do think a trimpot is a good idea for calibration purposes.

Love watching this progress by the way.

I agree; I think a trim pot placed somewhere in the sender circuit is a much better idea than a set resistor in cases when the gauge is indicating too high. In cases where the voltage regulator is set properly based on the fuel level and the temp gauge it indicating too low, that could be a challenge. I'd hate to make this even more complex and have trimmer pots in BOTH sender circuits just for folks to be able to have accurate gauges with my voltage regulator.

Funny thing, I managed to swap out my sender with a quickness in favor of the old one with nothing more than a few drips of coolant. So I set up the "new" one in the oil pot and it's been about 13-14Ω lower resistance so far all the way up. It does make sense that there could be a couple ohms of variance among senders, but I imagine greater than a 10Ω difference is a sign of a bad unit.

When I swapped my old sender back in, I reset my voltage regulator to have the temp gauge needle just be reaching the red at 27.7Ω (239º), though I think I ended up having to dial it up considerably farther than I had it set before I started messing with it last weekend, so I'll see over the next couple days how both gauges behave. I'm sure the fuel gauge will indicate higher than it should, but at least I can keep mental notes of my distance and fuel economy to fairly well track when I'll have to refuel.

 

[OSS]

In the manual somewhere, talking about the temperature gauge, there’s a drawing showing where the needle position should be on the temperature gauge when the engine is running.

Guess where it shows?
According to the Toyota manual, any needle position from the lowest mark, all the way up to but not going in the red zone is acceptable. Ha.

 

[Spook50]

In the manual somewhere, talking about the temperature gauge, there’s a drawing showing where the needle position should be on the temperature gauge when the engine is running.

Guess where it shows?
According to the Toyota manual, any needle position from the lowest mark, all the way up to but not going in the red zone is acceptable. Ha.

Yeah I could've sworn I had seen something about that before as well, but had to get to bed so I couldn't sit down to look for it. I agree the critical indication is the approach to that overheat threshold, so that's why I made sure I made my janky "calibration device" with as close to the 26.4Ω for 239º as possible (as it is, what I ended up with using the resistors I had was 27.7Ω so I set the needle to be just barely into the red; knowing now that my old sender was almost spot on with 26.8Ω once the oil pot was at 239º
Edit: Found my copy on my work computer. I had really hoped there would be more info than just that, but yeah there's only cold and hot resistance numbers.

Thanks to all this diving into temp and fuel gauge indications, I think my strategy for properly dialing in the voltage regulator will be to include a resistor assembly that buyers can use to verify (and if need be calibrate the regulator) that the temp gauge indicates that overheat threshold, then include images in the instructions to show where the fuel gauge should be indicating at certain fuel levels.

Driving in to work this morning, everything looked great. My fan controller is set to start coming on at about 190º to coincide with the thermostat beginning to open, and based on the reference picture, my needle showed just shy of the 195º when I saw the LED indicate it was just starting to ramp up. I won't know until I've driven about 140-150 miles if my fuel gauge is reading too high though.