Fuel gauge calibration point (2 Viewers)

[cps432]

You know, all this is getting complicated. When I was in high school I used a bamboo stick to check how much gas I had in the ‘66 ford pickup truck. I only ran out of gas once. okay maybe twice…

 

[Spook50]

You know, all this is getting complicated. When I was in high school I used a bamboo stick to check how much gas I had in the ‘66 ford pickup truck. I only ran out of gas once. okay maybe twice…

I don't have the temper (or hatred for technicians) to be an automotive engineer that's for sure

 

[CruiserTrash]

Oh yeah I remember reading through that thread. At least it was a fairly simple fix for that one once you found it.

So it would appear that the voltage regulator is now too low, as my second gauge picture in post #4 is where it sat (after adjustment) with 4.25 gallons of fuel in the tank. here's where it sat after topping off the tank this evening:
View attachment 3398338

Also for reference, here's where my temp gauge was after a hard 20-mile run on the freeway with the AC going in 85° temps. Granted this is just slightly under what I would consider normal, but I know my engine is running warmer than it should be due to I suspect just overdue cooling system maintenance (though I suspect now it's really not as close to overheat temps in 90+° temps like I had considered before). To guesstimate, I expect it should have been sitting right at the third line up from C or perhaps right at the upper edge of that line.
View attachment 3398351

I'm suspecting that there's very little reserve designed into this system and when the gauge is on E, you're sucking air. Seems odd that Toyota in the late 80s would do that, given a lot of other automakers designed a small reserve into their systems. Then again I could still be wrong. I just won't know until I start fiddling with actual resistance readings of the sender and compare those readings to what my gauge shows.

So my next plan of action is a bottom drain, then add fuel a half gallon at a time with a multimeter connected to the sender lead at the gauge connector in the dash and to a reliable body ground. Once I reach 120Ω and see how much fuel I've put in, I'll plug in my gauge and see where it's sitting. If it's bottomed out, I'll dial up the regulator until the needle moves and then slowly back until the needle just barely rests on the lower limit. Then another highway run and fill-up and see where my gauges are at that point. This will also tell us if there's really any reserve designed into the system once the gauge is on E.

I would offer that the bottom LINE of the fuel gauge represents being on your "reserve" and that E really equates to empty. That might be the offset that is messing up your "full" calibration. If that works you could just include a note about that in a spec sheet shipped with each order: "Please note that when you reach the bottom hash mark of the fuel gauge, you have very little fuel and should fill up as soon as possible - E really does mean EMPTY."

That offset error could also be showing as a lower reading on your temp gauge as well since the two are tied together.

Had another thought after yesterdays couple of posts: the calibration procedure could be totally different on a 60. Maybe not, but it's a possibility.

 

[Spook50]

I would offer that the bottom LINE of the fuel gauge represents being on your "reserve" and that E really equates to empty. That might be the offset that is messing up your "full" calibration. If that works you could just include a note about that in a spec sheet shipped with each order: "Please note that when you reach the bottom hash mark of the fuel gauge, you have very little fuel and should fill up as soon as possible - E really does mean EMPTY."

That offset error could also be showing as a lower reading on your temp gauge as well since the two are tied together.

Had another thought after yesterdays couple of posts: the calibration procedure could be totally different on a 60. Maybe not, but it's a possibility.

You're absolutely correct on your second point. Since both the fuel and temp gauges share the same output from the voltage regulator, adjusting the voltage down will result in both needles reading lower; same with higher readings from higher voltage.

You very well may be correct on your first and third points too. I'm hoping to verify that on my next day of messing with the gauges. Another consideration I'm pondering is if the FSM reference that @ChaserFJ60 posted refers to the needle literally sitting on E (the float basically bottomed out) or if that's referring to the bottom line. If zero to very little fuel gives me my 120Ω reading, I'll know that means E; but if it takes a 2 or 3 gallons to reach the 120Ω I'll adjust based on the assumption that it's referring to the bottom line. I'm hoping that the 60 and 62 gauges read close enough to be fairly uniform. I won't know for sure until I get mine dialed in and pop the 60 quad gauge in for comparison. Given that the gauges are technically interchangeable and identical mechanically, I'm hoping they'll be close.

 

[Spook50]

I guess I should've summarized what I'm hoping to accomplish once this is all said and done:

Once I figure out which point to use for setting the regulator and having the gauges read correctly, I'll be able to put my regulator on the test fixture and measure the output voltage so I will have a default voltage to set the gauges to for shipping, and then a procedure I can detail for anyone who orders one to follow for verifying and fine tuning the regulator setting.

 

[Spook50]

Okay so today I again bottom drained my tank and played with my multimeter as I slowly added fuel back in. I removed the yellow/red terminal (fuel sender lead) from the quad gauge connector so I could clamp my multimeter to that and a solid body ground. As it turns out, I put in just a bit over two gallons to get a 119.4Ω indication, telling me that with Toyota's design of this system, once your gauge shows empty you have only about ONE gallon realistically before your pump starts sucking air and cavitating. Not cool, honestly.

But it gets more interesting. I'm now fairly well certain the the lines themselves on the gauge are meant to be the the indication levels, and NOT the letters. With the approx. two gallons in the tank and the 119.4Ω showing on my meter, I reinserted the terminal and plugged in my gauge assembly. This is the reading my gauge showed:

It's important to keep in mind that with my voltage regulator at its current setting, the needle showed well below full on a filled tank, and the temp gauge also read lower than it should've. So my next step was to add fuel gallon by gallon until I got a reading of 40Ω on my meter. I finally got this with about 15 gallons in the tank. I would like to take another day in the near future and verify with EXACTLY two gallons initial and exactly 15 total what my meter will show, but now that I have a general idea, it's a big step in the right direction.

Now according to the FSM, with a 40Ω reading on the meter, the gauge should indicate a half tank. Here is where my needle sat with the current regulator adjustment:

Somewhat lower, but considerably lower than where the needle sat before I started playing around with adjustments this last week. Going off the difference of 8.8 gallons between that and full (keeping in mind these are just close approximations that I intend to verify in the near future) my typical fuel economy would've had me about 122-123 miles travelled, and before I dialed the voltage down last week, my needle would read just under 3/4 tank after that distance.

Now I didn't want to bring the needle all the way up to the halfway mark just yet, as I want to see where it'll read with a topped off tank and where the temp gauge will sit when I drive a ways into Idaho in the 99 degree heat we have forecasted for tomorrow (yuck). I did still raise the voltage to bring the needle up to just on the low side of the half tank indication. In all likelihood I will go ahead and set it to exactly the half tank mark next time I do another drain and fill and pull the dash apart.....again. Here's where the gauge sits now after adjustment with the ~15 gallons and 39.3Ω on the meter:

Something I almost neglected to mention is that these pictures are taken from a "dead center" perspective of looking at the gauge. Seeing it from the normal driver's angle does make the needle appear lower, so that may be something I have to take into consideration as well. Jury is still out on that one.

With what I learned today and still intend to verify, it looks like a 40Ω reading between the connector terminal and a body ground with approximately 15 gallons in the tank is going to be the ideal method for calibrating the gauge via setting the regulator's output voltage to make the needle rest at the half tank position. It's tentative at this point, but it's looking pretty good.

 

[CruiserTrash]

I would much rather have “empty” be at a hash mark than a big letter “E”. What part of the E? The top edge? I’ll run out of gas once the needle passes the E? Maybe empty is at the center of the E?

See what I mean? Way too inexact. A hash mark is much better and it seems like Toyota is telling you this! Even if full ends up having the needle well up by the “F” that’s fine, it’s far less critical than knowing when you’re empty!

 

[Spook50]

I would much rather have “empty” be at a hash mark than a big letter “E”. What part of the E? The top edge? I’ll run out of gas once the needle passes the E? Maybe empty is at the center of the E?

See what I mean? Way too inexact. A hash mark is much better and it seems like Toyota is telling you this! Even if full ends up having the needle well up by the “F” that’s fine, it’s far less critical than knowing when you’re empty!

I agree completely, for the same reasons you stated. I must admit though I'm surprised that's the case, but it definitely seems to be a good thing.

What I DON'T like though is that gives you an only 2 gallon reserve, which with our rigs' low fuel economy doesn't get you far if there's no fuel source nearby. When I next drain my tank to verify fuel amounts to resistance measurements in the sender circuit, I might first experiment with setting the needle at the bottom hash mark with three gallons in, see where it sits at 15 gallons, and see how both gauges indicate when hot/full. If I like it I'll post those indications and see what the consensus is. If the indications are still high enough to look out of whack, I'll then drain again, verify 2 gallons for a 120Ω reading, 15 gallons for a 40Ω reading, and set the regulator to have the gauge show exactly 1/2 tank at the 40Ω mark.

I won't be touching it this week though because I'm still working in my driveway and we're looking at 100+ temps most of the week. That's a hard no for me. I did that enough in the gulf during my younger days.....

 

[CruiserTrash]

@Spook50 I think as long as the 1/2 & full indications are reasonable you should set the empty reserve wherever you want. I assume your product would come with documentation so all you need to do is make it clear to people where exactly "empty" is (the hash mark versus the "E") and how much reserve they have at that point, and you're good. Personally I would agree with you that having more reserve is better. Three gallons seems good: two gallons usable and one gallon below the pickup tube. Two gallons gets you 18-30 miles depending on driving $ motor tune conditions. That's better than one gallon which could be as low as 8-9 miles of range for some trucks.

 

[Spook50]

@Spook50 I think as long as the 1/2 & full indications are reasonable you should set the empty reserve wherever you want. I assume your product would come with documentation so all you need to do is make it clear to people where exactly "empty" is (the hash mark versus the "E") and how much reserve they have at that point, and you're good. Personally I would agree with you that having more reserve is better. Three gallons seems good: two gallons usable and one gallon below the pickup tube. Two gallons gets you 18-30 miles depending on driving $ motor tune conditions. That's better than one gallon which could be as low as 8-9 miles of range for some trucks.

Yes the documentation and installation/calibration manual will be the next big challenge, since I'll want to make it as easy to follow as possible. I still have the list of folks who contributed to my GoFundMe when I first started the project that if I can get a hold of them will get the earliest samples and I have a couple "beta testers" lined up to demo early units and give me feedback on the product and documentation.

I need to correct my statement from my previous post about the needle readings if I set the regulator to read the bottom hash mark at 3 gallons: It will cause the needles to read LOWER, not higher like I stated. Now if they're not too low, that should still be okay. I'm considering making a quasi test rig with my deep fryer (don't laugh) so I can have a controlled test environment for a temp sender and verify readings on the temp gauge as well (engine operating and overheat indications mostly). I want to make sure that with the fuel gauge set appropriately, the temp gauge would have a reasonable indication.

 

[CruiserTrash]

Yes the documentation and installation/calibration manual will be the next big challenge, since I'll want to make it as easy to follow as possible. I still have the list of folks who contributed to my GoFundMe when I first started the project that if I can get a hold of them will get the earliest samples and I have a couple "beta testers" lined up to demo early units and give me feedback on the product and documentation.

I need to correct my statement from my previous post about the needle readings if I set the regulator to read the bottom hash mark at 3 gallons: It will cause the needles to read LOWER, not higher like I stated. Now if they're not too low, that should still be okay. I'm considering making a quasi test rig with my deep fryer (don't laugh) so I can have a controlled test environment for a temp sender and verify readings on the temp gauge as well (engine operating and overheat indications mostly). I want to make sure that with the fuel gauge set appropriately, the temp gauge would have a reasonable indication.

Where was the gofundme? I missed that?

 

[Spook50]

Where was the gofundme? I missed that?

I deactivated it several years ago, but I think it was late 2013 or early 2014 that I had initially done it. I ran it for a while to get up some initial R&D funds for components and such, but once my divorce got heated up I deactivated it because I had to focus on that and the subsequent custody battle. Now I'm just fighting to pay my mortgage and keep my 62 maintained while doing what I can as a single dad to get this project finalized after ten years and keep doing my window relay refurbs.

 

[CruiserTrash]

Very similar situation here, except my $h1t hit the fan in 2016. This is a really good idea and you're really close. Let me know how I can help get it over the finish line.

 

[Spook50]

Very similar situation here, except my $h1t hit the fan in 2016. This is a really good idea and you're really close. Let me know how I can help get it over the finish line.

So far the extra input has been great. Once I finalize this step, the next is prototypes and tweaks to see if I can make it work for 24V Cruisers, then finalize the PCB design and component BOM, documentation, and with any luck an initial production run after that.

 

[Spook50]

So here's where my fuel gauge sat with yesterday's top-off:

It's looking like shifting the needle up the half hash-mark-width to read 1/2 tank at 40Ω (which I want to verify actually IS 15 gallons as soon as I can) will still have the needles sitting lower than they were at the start of this thread before I began experimenting with the regulator adjustments (which is good). When I go to do that I'll initially add an even 2 gallons to see just where the multimeter reads (I'm betting on it'll be at least within the FSM's limits for the 120Ω reading), then adjust the needle up to read on the lowest hash mark. Once that's set I'll take it up to an even 15 gallons, verify my multimeter is at or within limits for a 40Ω indication and check if the needle indicates 1/2 tank. If it's all good I'll keep it there for the duration of a trip across the state at the end of the month to see how the fuel gauge (and especially my temp gauge) behave. Something I'm very curious about and have began focusing on just as much as the fuel gauge is my temp gauge's readings. I'm still sure my engine is running warmer than it should be on long hauls, but suspecting now that it may not be as close to the overheat range as I had initially been seeing. I'd really like to get a T fitting so I can have my OEM gauge and a cheap Autometer gauge hooked up at the same time so I can see my engine's real temps and be able to provide a reference for OEM gauge indications as well for the forum.

 

[Spook50]

Against my better judgement, I went out into the driveway in 100° heat, drained my tank and played around with my regulator again after work. Once empty, I put in exactly two gallons of fuel and saw 119.5Ω on my multimeter. With the gauge indicating slightly lower than I expected (I still expected below the lowest hash mark, but not by this much):

Yes I cleaned my dash the other day. So with this indication I began dialing up the output voltage of the regulator only to see that even with the regulator maxed, the needle only sat between this point and the lowest hash mark. I decided to let it be at this point and began adding fuel to take it to the 15 gallon level. When I saw the multimeter getting close to 40Ω I slowed down, then stopped just in time for the multimeter to indicate 39.5Ω. Once I plugged the gauge back in and let it stabilize, the thing showed a 3/4 full tank! To clarify, it was on the second hash mark down from the top most one. So I adjusted the gauge to read exactly at the 1/2 mark and buttoned everything back up. While I'm now not in a huge rush to do so, I'd like to drain my tank one more time when it's low again and see what the needle indicates with two gallons, especially now that I know two gallons is the magic number for the "Empty" indication on the gauge per Toyota's design.

So the two main things I can take away from tonight's work are
1) 13.5 gallons in the tank showing 40Ω (or within the limits specified in the FSM) is the ideal point for adjusting the voltage regulator to properly calibrate the fuel and temp gauges, assuming both are functioning properly; and

2) Toyota sadly did NOT design the "Empty" indication to be the lowest hash mark as I had hoped. As it is (which I will verify next time I drain my tank), it seems the "Empty" indication is with the needle exactly halfway between the "E" and the lowest hash mark (just a hair above what's shown in the picture), which when looked at from the driver's angle, appears as the needle just barely beginning to touch the "E"