97 850 NA trip computer callibration

[dairyqueen814]

Hey guys
In a previous post I had ordered a trip computer to replace the dash clock and I love it so far. However, I just noticed that the odometer on the trip computer and the odometer in the actual cluster are different. I checked and it appears the one in the cluster is accurate so that means that the one in the trip computer is wrong. Its reading about 10 percent high. Do they read from the same place? Are they callibrated for different tire sizes? I know that the size of my tires now is slightly smaller that Stock (something I'd eventually like to fix but they are great tires)
Is there a way to fix this? Because I feel like it's throwing off the trip computer's MPG calculations as well
Thanks
Nick

[dairyqueen814]

Now I don't know if my tire size is small. Mine are currently 195 60 R15s

[esl_97_850_T5]

Have you figured out what causes the 10% discrepancy?
Is it really near a 10% discrepancy, rather than closer to a 5% or less?
What # of miles did you use in your initial check?
And what did you use as your measurement baseline -- freeway mile markers, Google map distances, etc?

The COMBI in the '96-'98 850/S70/V70/C70/XC70 has a slot for Tire Size -- at least according to the Brick-Diag Kombi-Parameter screen's Reifen parameter it seems to.

Can you read your COMBI (ECU 51) B9xx data using the ELM327 commands to read ECU 51 DTCs and B9nn data and the instructions listed elsewhere on that page, making sure you read B90E successfully?

I'm especially curious about your values for B901, B90A, B90B, B90C, B90D, and B90E, since those values aren't well understood yet (by me).

Does your B90E value (after translating from hex to decimal) seem to relate to some element of the tire size? For example, does B90E have a value of CD (ie, 205) or D7 (ie, 215) instead of the C3 (ie, 195) that might match your tire size?

I'd suspect -- based on my work with modifying the Service Interval Reminder limits using B8xx requests -- that the tire size could also be changed with B8xx requests, once the Tire Size parameter is definitely located. The most likely candidate for the Tire Size parameter, I think, is B90E. If it's true that B90E is the tire size, then I would expect that B80Eyy requests could change it and would eventually change the rate of incrementing the Trip Computer's odometer (and maybe even change the rate of incrementing the mechanical odometer in the '96-'97 850).

I haven't yet experimented with B80Eyy, since I'm not yet certain that B90E is the Tire Size parameter and I don't yet have a clear understanding of the various values that I've seen in it on some other owners' cars. Some owners don't have values that are equivalent to C3, CD, or D7, but have much lower numbers that seem to relate to wheel size in inches or other elements of the tire size. It's a bit confusing to me, at present.

You could always go to the dealer and have them change the Tire Size parameter with their equipment, then report back to us the resultant changes. But that seems to be a more costly approach.

If you use the jonesrh.info/volvo850/elm327_reads_volvo_850_svc70_mileage.html page's DIY, ELM327 based approach, you'll likely be out less money than going to the dealer. And you might be able to change the Tire Size parameter yourself (after a bit of B8xxyy experimentation) so that your new Trip Computer portrays an accurate odometer mileage (and accurate Avg MPG).

If you want to pursue this ELM327 ECU 51 B9xx / B8xxyy / B9xx route, post your B9xx results back here and we'll go from there.

esl_97_850_T5

[precopster]

195 60 15s are the standard tire size.

A discrepancy between the trip and the odometer seems like a rare but however possible fault. The P2 series was designed to have software changes to let the DIM know that tire changes are made. There's a software table in Vida for this purpose, however both odometer and mileage counters would be equally affected.

I doubt if Volvo could program tire circumference changes into earlier designs.

[dairyqueen814]

Wow alright I had this written off as a dead post but here goes
I used google maps as my distance standard and my dash odometer was right on the mark 100%
The ten percent was a pretty accurate ten percent over a whole tank of gas but I will certainly report back with concrete numbers on my next fill up. I have read that some of them were calibrated for the later model Rs which I believe are on bigger wheels. But a discrepancy that big seems like a different problem. Now that I know there are people interested in this I'll be sure to keep it updated.

[precopster]

A 16 inch Volvo tire uses a 205/50 series tire. That's only a 0.6% difference in circumference.

A 17 inch Volvo tire uses a 205/45 series tire. That's only a 0.2% difference in circumference.

Volvo had no need to adjust speedo calibration because they couldn't do that at the time.

If they could all the poor R series drivers running on bone jarring tires would have been very greatful to upsize their tire profiles and have accurate speedos. There is no TCM software for 850s or 98 model V70/S70s for tire profile changes.

Turbos ran a different differential ratio (taller) however this was probably accounted for by decreasing sensor spacing on the output drum of the transmission.

[dairyqueen814]

Update time!
Alright guys
Just filled my tank and here are the numbers
On my regular dash odometer it read 381.6 miles for the trip
I pumped in 14.6 gallons of gas
that averages out to 26.14 mpg
Trip computer data
The trip computer says I went 421.4 miles
and averaged 28.7mpg
If you use the 14.6 gallons of gas I pumped in that calculation is pretty much right for the info it is displayin +/- .1mpg
so the computer is reading the fuel consumption correctly (for the most part)
Blah blah blah math math math
thats a 10.4% difference
for every 10 miles I travel the trip computer thinks I travel 11.04 miles
Thats a pretty big discrepency if you ask me.
Does this new information help at all? Where is the sensor that tells the car how far it is traveling?
Thanks for anything you guys can give me.
Also I'm not on a P2 chassis. My car is an 850 so it's a P80

Nick

[rspi]

Nick, one of the setting on the trip computer is trip mileage, at about 2 o'clock, one tick from the temp. Is that what you are using to read the mileage?

[dairyqueen814]

Nick, one of the setting on the trip computer is trip mileage, at about 2 o'clock, one tick from the temp. Is that what you are using to read the mileage?

That is where I am getting the mileage that the trip computer is giving me. It is reading ~10% more than the mechanical odometer in the dash. I know that the odometer in the dash is right on the money because I have used a GPS as well as google maps to figure out which one is wrong. I'm trying to figure out why the trip computer is reading higher than the mechanical odometer.
I think I read somewhere that the trip computer gets its data from the ABS module and the regular dash odometer reads from the speed sensor on the wheel. I could be wrong though.

[esl_97_850_T5]

My understanding is that the wheel sensor feeds the ABS [near certain],
the ABS speed signal feeds the COMBI microprocessor [near certain], and
the COMBI microprocessor feeds the Trip Computer [near certain].

But I'm uncertain of...

- Does the ABS speed signal feeds the mechanical odo directly, or does the COMBI microprocessor feeds the mechanical odo (after the COMBI has processed the ABS speed signal)? I couldn't seem to infer this from the wiring diagrams. I had previously assumed that the mechanical odo and the Trip Computer were supplied by the exact same signal -- from the COMBI after it has processed the speed signal from the ABS. But this assumption might not be correct.

- Assuming that the COMBI microprocessor feeds both the mechanical odo and the Trip Computer after having processed the speed signal from the ABS, does the COMBI microprocessor feed the exact same signal to both the mechanical odo and the Trip Computer? I couldn't seem to infer this from the wiring diagrams.

- If the mechanical odo and Trip Computer are supplied by the exact same signal from the COMBI, is there some component which can fail in the Trip Computer and cause dairyqueen814's 10% discrepancy between an accurate mechanical odo and a Trip Computer? I couldn't seem to infer this from the wiring diagrams. I presume that there could be some electronic component in the Trip Computer that could age, continue to operate, but skew results in one direction. The way to test that idea would be to swap in a known good Trip Computer.

- Is precopster accurate in his assumption that the P80 cars do not have the ability to handle differing tire sizes? My nature is to doubt this assumption unless it is proven to be definitely accurate. I'm not concerned with whether the Transmission Control Module is cognizant of tire size changes. I'm only concerned with whether the COMBI is able to respond to user-enterable tire size changes with predictable changes in its updating of the Trip Computer (and/or mechanical odometer).

- Are any of the "not yet fully understood" COMBI parameters used to somehow infer an approximation of the Tire Circumference, ie, ECU 51 B901, B90A, B90B, B90C, B90D, B90E? The best candidate seems to me (at present) to be B90E.

- Does the COMBI actually use the ECU 51 B90E data for some Tire Size purpose? I now know that you can program the COMBI's B90E value with an ELM327. I'm changed my value from C3 (ie, 195) to CD (ie, 205) to reflect the actual Tire Width in mm on my car. But I've yet to draw any conclusions, because I do very little driving, I don't have a GPS, and I don't have a functioning mechanical odometer. It will take me several weeks to draw conclusions from this ECU 51 B90E change.

Source of the "near certain" conclusions is various posts at MVS, but most importantly:

- 1997 Volvo 850 Wiring Diagrams -- see its clickable link "Combined instrument panel VDO".

- Volvo 850 Service Manual (primarily 1995) -- see its file "instrument panel.pdf", then see "Fig. 14: Instrument Panel Wiring Diagram (1996 - With VDO - 1 Of 2)" and "Fig. 15: Instrument Panel Wiring Diagram (1996 - With VDO - 2 Of 2)". Alternatively, look for the VDO wiring diagrams in the "system wiring diagrams.pdf" file.

- The "Wiring Diagrams component list" in a Haynes Volvo 850 1992-1996 manual. I needed this to understand more of the connections on the wiring diagrams just mentioned for 1995-1997, since the Component list in 1997_850_wiring_diagram.pdf is incomplete and I couldn't find any wiring diagram component list in the (1995 & 1996) Volvo_850.zip.

I'm indebted to precopster for his 2014-05-14 post in which he included examples of two tire sizes that showed only 0.2% and 0.6% differences in circumference compared to the 195/60R15 standard tire size. That triggered me to reevaluate my ECU 51 B90E description, to discover and correct some errors in my ECU 51 B90E description, to calculate that my 205/55R16 tires have a noticeably larger 2.75% difference in circumference compared to 195/60R15 tires, and it triggered me to do some research and see that the COMBI B90E parameter can be reprogrammed.

For those that want to experiment with reading and modifying their '96-'98 850/S70/V70/C70/XC70 COMBI Tire Size parameter using an ELM327 device, check out this B90E - Tire Size section. Also, you might follow its two links for some more details, especially the one relating to "Tire Size Comparison...".

My 205/55R16 tires might show a 1 mile difference every 37 miles. That should be easier to notice than the two tire sizes that precopster mentioned. We'll see how it goes.

esl_97_850_T5