MPG readout adjuster update

I

Iowa4Runner

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With 15 gallons under it's belt, it appears the adjuster's program was a little overzealous in relaying fuel usage to the MPG computer. The indicated MPG was 1.5% LOWER than actual. Actual MPG was calculated as miles since last fillup divided by gallons used since last fillup. I know CMiYC has concerns with this method, but after reading the manual and watching the MPG computer respond in various driving situations, I really have no reason to doubt the computer calculates MPG based on miles traveled and fuel used since reset.

Changing program line 24 from "movlw 7h" to "movlw 8h" should put the MPG computer within 1/2% of actual MPG under typical driving conditions. :)
 
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Cool...thanks for the update!

In case you're interested, I took apart my trip computer to take a look at what MCU they were using. The part number is NEC D780306Gr 055. This maps to a NEC uPD780306 MPU. You can get the data sheet, app notes, etc for that chip here.

http://www.necel.com/cgi-bin/nesdis/o002_e.cgi?aliascode=143580&langcode=E

Based on a quick glance of the data sheets, I don't think there is a flashable version of this part. That's a bad sign for those hoping that Toyota might simply reprogram it.

However, it's a fairly large and complex MCU given the functions that it implements. There are few other ICs that I couldn't see on board. It's possible that they are small serial EEPROMS. The MCU has several serial buses that could be used for that purpose . Such an implementation might would allow this part to be field programmable -- and therefor flexible enough for Toyota to put in several vehicles. Who knows?

Do you have the pin-out for the trip computer? I couldn't find anything about it in the service manuals / wiring diagrams. I'd like to see if we can account for every pin on the connector.
 
Interesting info... What did you take apart, the "Combination Meter" (gauge cluster) or the "AC Amplifier" (MPG display module)? Looking at pages 05-1694 and 05-1730 in the service manual, it appears a processor in the Combination Meter calculates the fuel mileage and sends the information serially to the AC Amplifier. Have you checked out the goods in the Combination meter?

I don't know of any good pinouts for the AC amplifier, but the Combination Meter is shown on 05-1693,4.

05-1233 might be of interest.
 
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The A/C controls / Trip computer display module. No I haven't cracked open the Combo meter. I didn't even realize that there was any smarts in there

I see what you mean about the UART in the combo meter. If MPG is calcualted there then the whole premise behind your circuit is faulty. But since you have it working, I think we can rule that out.

Now I'm thoroughly confused about what is going on here.

UPDATE:
Oh never mind. I just realized that you were tweaking C9-8 all along.
 
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When I removed the combo meter to modify the wiring for my MPG adjuster, I made a half-hearted attempt to open it up and check out what was inside. I didn't get very far before I couldn't see any more screws and all that was left to do was yank, pull and pry at the assembly. I chickened out before I got very far into it. I can only imagine how much it would cost to replace that thing.
 
Yah, I've had the combo meter out before but never considered taking it apart. That's probably at least one of the places where your vehicles mileage is maintained -- so it wouldn't suprise me to find out that it's tamper resistent.

In any case, those serial lines look interesting. I might have to toss those on a scope to take a look at what is moving back and forth between the the combo meter and trip computer display/buttons. Presumably it's just the calculated values (combo->display) and button presses (display->combo). In that case, the PIC inside the display/buttons is just there to render the LCD display and perhaps maintain the time of day.
 
rando said:
That's probably at least one of the places where your vehicles mileage is maintained -- so it wouldn't suprise me to find out that it's tamper resistent.
Click to expand...

Heh, I don't know why that didn't occur to me at the time, because it makes perfect sense. :bonk: If you have any breakthroughs in deciphering the serial info, I'd be curious to hear the details...
 
Bump...

Reactivating this thread now that there are more '05s out there.
Has anyone with an '05 calculated how much error they have seen in the MPG and Range readouts? Just wondering if Toyota made any corrections since the '03 & '04 have a high error.
 
Very interesting thread... I do believe the vehicles mileage is stored in one of the ECM'S. Not in the combination meter. One way to find out is to swap it from another vehicle and see what the odometer says. I would love to know this answer, but I dont know of anybody that has one around here.
 
Yah me too..

ToyotaDoodz said:
Reading this thread gives me a headache. :bowdown:I think I'll stick to the how to change a flat threads. :D:D

Good luck guys. I hope you figure this out.
Click to expand...

I used to wish it was simple to get an accurate reading too. But I finally started ignoring the mileage meter, and I'm happier for it. Just fill up and ignore mileage.

I do know what most of the acronyms (UART, PIC) stand for, and I recognize hex code when I see it, but it's been soooo long since I have graduauted from school with my AASEET degree (1987), that I can't apply much of that information anymore.

All I'm good at anymore, mostly hobby, is discrete analog and digital circuitry in simple devices. I ended up working in the spring manufacturing industry, mostly real world mechanical and physics problems to solve at the machines. Go figure.

How about trying to shut off the traction contol feature without the warning light coming on, so someone can try some burnouts with their 4x2 or 4x4 V6 in 2WD mode??
 
It is a more complicated problem determining gas mileage then indicated above.

The first problem is getting accurate measurements of the fuel added at refueling.
The gas pumps are not that accurate, some are long, some are short of actual measure,
this is why they are state inspected. Then, there are the stations that have the secret
switch which short change you. I remember in SoCal one time of putting 26 gal. in a
22 gal. tank when I had 1/8 of a tank from empty.

Unless milage is averaged over a large number of miles, it can't be even close to correct.
Every different setting of the throttle and vehicle speed gives a different reading.

I don't know the intended accuracy of the indicator, but I would guess +-5% would be good.

Cliff s
 
Cliff s said:
It is a more complicated problem determining gas mileage then indicated above.

The first problem is getting accurate measurements of the fuel added at refueling.
The gas pumps are not that accurate, some are long, some are short of actual measure,
this is why they are state inspected. Then, there are the stations that have the secret
switch which short change you. I remember in SoCal one time of putting 26 gal. in a
22 gal. tank when I had 1/8 of a tank from empty.

Unless milage is averaged over a large number of miles, it can't be even close to correct.
Every different setting of the throttle and vehicle speed gives a different reading.

I don't know the intended accuracy of the indicator, but I would guess +-5% would be good.

Cliff s
Click to expand...

Yes every throttle setting results in a different injector pulse length. The "MPG adjuster" can handle the full range of pulse durations. The results so far look pretty good. After visiting a variety of gas pumps over the last 5 tankfulls, I calculated the average MPG error at 0.89%. That's Good enough for me...
 
Toyota's reply

I emailed Toyota about why their MPG readout is always higher than the actual MPG. This was their reply:

Thank you for contacting Toyota Motor Sales, U.S.A., Inc.

We apologize for your dissatisfaction with the accuracy of the fuel economy display in your 4Runner.

The system used to determine the fuel economy figures displayed in the multi-information display mainly uses the fuel injector pulse rate to project the fuel economy. Therefore, the fuel economy information displayed should be treated for reference purposes as it will not provide 100 per cent accurate fuel consumption information. If you believe there is a genuine concern with your vehicle, please contact the Customer Relations Manager at your local Toyota dealer for further assistance.
 
The funny thing is that my previous BMW, my GFs Volvo and all the BMWs in my family who had MPG or fuel consumption indicators were very accurate. Now Toyota makes one some 15 years after the first BMW we had with it and they still dont get it right. Go figure, I used to use the indicator on my BMW, now I'm back to milage and gas pumped :( Oh well, can't always win.
 
Here's something I noticed with my MPG reading: Everytime I disconnect my battery, the MPG computer gets completely reset and reads significantly higher for a few tanks. Here's an example:

After I disconnected my battery recently, my reading was 27.1mpg, but the actual was only 24.4mpg for the 2nd tank of gas after disconnecting. Before disconnecting it was reading 26.2mpg and I was getting 24.1mpg actual. I reset all the reading everytime I fill up.

My point is, could extended usage without disconnecting the battery eventually cause the computer to become more accurate? Mine certainly seems to. Has anybody else noticed this?
 
Do what I do

Ignore it. You already know the truck will get somewhere between 14-17 mpg city and a bit more highway. All the electronic gizmos are clever and nice, but really, has anyone bought their truck because it had a mpg readout calculator?

I haven't looked at mine in over a year. I keep the display set on outside temperature. I fill up when the fuel gage gets near E . Very easy.

I am annoyed by the placement of the cruise on a stalk instead of the right side of the wheel, but have learned to use it the way it is.
 
I can understand the desire to have an accurate MPG readout, even though I didn't by my 4Runner because it had that feature.

I never noticed the MPG reading being significantly higher after I disconnected the battery. The one thing I do know is that I've gone about 10 tanks since the last battery disconnect and the error doesn't seem to change.
 
If it consistently reads one or 2 mpg over the actual, why not just keep that in mind and do some basic subtraction when viewing the guage. This seems like a lot of trouble over nothing IMO.
 
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