Why I Use an ELM327 on Volvo 850 OBDII Port

I ventured into the OBDII world in order to solve several problems with my 1997 Volvo 850 T5:

• Speedometer failure,

• Unknown vehicle mileage, due to mechanical odometer failing,

• SRS light on, and

• SERVICE light on.

According to a slew of forum posts on the Matthew's Volvo site Volvo 850, S70, V70, C70 & V70-XC Forum (1992-2000) forum, I discovered:

• The most common reason for the 850's speedometer failure is faulty solders in the ABS module.

  Contrary to how I thought the 850's speedometer would work -- a signal from the transmission feeds to the instrument panel's speedometer -- I discovered that in the 1996-1997 Volvo 850s and the 1998-1999 S70 & V70 cars, the vehicle speed signal is fed to the instrument panel speedometer from the ABS controller module and that the ABS controller module almost always fails in those cars due to bad solder joints. An unreliable, inaccurate, or missing vehicle speed signal from the ABS will: a) cause the speedometer needle to swing unpredictably or to cease operating entirely, and b) fail to properly increment the COMBI instrument panel's mileage -- both the mileage visible in the Trip Computer's trip odometer, and the invisible mileage that can be read by a Volvo Scan Tool (VST) under: 800 SERIES / ELECTRICAL / COMBI / PROGRAM COMBI / READ COMBI (see jimmy57's VST instructions to read mileage on Diagnose2 850s contribution to the "Re: How do I retrieve the correct mileage on 1996 850" thread).

  I concluded that with the car being 15-16 years old (at the time), the cheapest, most effective solution -- if the ABS did have faulty solders -- was to do a DIY removal of the ABS module, then send it in so a professional could do the resoldering. Many people had done that successfully, and I had almost no trust in my soldering skills.

  But before doing that, I wanted to rule out other possible problem sources: faulty ignition switch, faulty wheel sensor, brittle wiring, etc. After ruling out the ignition switch possibility, I wanted to explore which ABS Diagnostic Trouble Codes (DTCs) existed and see if I could hone down the analysis using that info.

  Hint:
  Hindsight has shown me that if the ABS module has lots of DTCs, then it's useless to try and debug them. That's especially the case if, after clearing the ABS DTCs (with the ELM327 cable), many of them return.  I eventually determined that my ABS module had 15 DTCs, even more than Vol-FCR v1.7.3 for FTDI Demo reported.  After clearing the ABS DTCs, 10 of them returned within 12 days, so I sent the module to Midwest ABS for repairs.
  
  For a '96-'97 Volvo 850 (or a '98 S70/V70/XC70/C70), when you have lots of ABS DTCs that reappear after you clear them, and you're sure you don't have power, fuse, or wiring problems to the ABS control module, or from the ABS control module to the Instrument Panel, then just assume the solder joints need repairing by someone who knows what they are doing.  Then after that is done, you can deal with other possible problems: wheel sensors, brittle wiring, etc, if necessary.

• The most common reason for the physical odometer failing is the little odometer gear(s) breaking. But fixing that wouldn't have solved my problem of knowing how many miles had been put on the car after the gear was broken.  My CarFax report suggested that the odometer gear was broken years ago, sometime between early 2003 and late 2005.

• The 1997 Volvo 850 Wiring Diagrams indicated that the ABS module, and, in fact, all the computers which the Vol-FCR Demo purportedly talked to, were connected thru the OBDII port's pin 7. So it was likely that Vol-FCR was using OBDII port's pin 7 to communicate with the computers, rather than pin 3, pin 6, or pin 13. I could see that pins 4 & 5 were ground related, and pin 16 was power supply related.

• After reading that a VST could read the vehicle mileage from the COMBI Instrument Panel -- see jimmy57's post here and my additional thoughts here -- and after reading that the Vol-FCR Demo also talked to the COMBI Instrument panel to read its DTCs, I concluded that the mileage might very well be accessible via the OBDII port's pin 7. Since I suspected that Vol-FCR was developed after monitoring VST communication, I thought the principle of engineering design simplicity would tend to suggest both tools used the same protocol that communicated via the OBDII port's pin 7.

• Since the demo version of Vol-FCR had the capability to read the SRS DTC, and I had read that the Vol-FCR Full version could clear the SRS DTCs, I assumed that if the SRS DTC reading is done via OBDII port's pin 7, then most probably the SRS DTC clearing is done via that same pin 7. That way I might be able to turn off the SRS light without having to visit the dealer.

• Since the full version of Vol-FCR had the capability to reset the SERVICE light, I assumed it likely is done via the same OBDII port's pin 7.

Getting the speedometer operational again was the #1 reason why I began to explore the OBDII world. That and the fact that:

• I didn't have the money to pay $40, $50, or $80 for code readings by dealers and independents that had a VST or its equivalent, then to cough up even more money to do any actual repairs, then to cough up more money yet again every other time in the future that I would need to read the DTCs for any computers that only a VST (or the Vol-FCR software) could talk to.

• And I certainly didn't have the ~$150 it cost to buy Vol-FCR, the Full version (and I refused, for moral reasons, to acquire a free, pirated Vol-FCR Full version).

• Even if I did have enough money for Vol-FCR initially, it still didn't read the vehicle mileage.

There's several other important things that prompted me to explore the possibility of talking to the 1997 Volvo 850 T5 computers using an ELM327 cable:

• discovered (when researching OBDII communication protocols) that the ELM327 initializes a ISO9141-2 connection, then automatically sends the watchdog periodically to maintain the connection, thereby relieving the user of having to write or buy software to do that, yet provides an easy-to-understand interface for manually sending commands to the car's computers, and the ELM327 can be controlled with free software to effectively mimic (or improve upon) a cheap handheld OBDII scanner (like the Autel MaxiScan 300) in order to: read/clear the emissions diagnostics DTCs, and view the emissions diagnostics data in an easy-to-understand display;

• discovered that the ELM327 (via its ATMA command) can also be used to passively monitor communications when it is used with one of the splits from a Y-cable whose base is connected to the car's OBDII port and whose other split is connected to another program which is actively communicating with the car's computers;

• bought an ELM327 cable since it could be used to replace my Autel MaxiScan MS300 as well as passively monitor OBDII communication;

• bought an OBDII Y cable;

• monitored the ilexa Vol-FCR v1.7.3 for FTDI Demo with an ELM327 cable -- when evaluating a VAG/COM KKL FTDI USB cable for possible use with Vol-FCR Full (if I ever got enough money to pay for it) -- and discovered that all the Volvo 850 computers (except the Motronic 4.3 engine ECU) communicated with Vol-FCR Demo using the same protocol;

• researched ways to talk to the Motronic 4.3 that Vol-FCR Demo (and Full) do not talk to, and in that search discovered Dilemma's "MotronicSuite" thread (at the now defunct forum.ecuproject.com) which: a) mentioned that KWP71 is the normal, post-initialization protocol for Motronic 4.3, and b) provided the link to the rkam post mentioned below -- a veritable Volvo 850 communication gold mine;

• while researching what the KWP71 / KW71 protocol is, discovered from the fiatforum.com's Fiat Cars > Stilo > "FiatEcuScan diagnostic software" thread's 2 posts by Greggers on 2010-05-17 and elsewhere in that thread that the ELM327 can be pushed to establish communication with, then read and clear DTCs on, several of the Fiat's various computers, not just to do emissions diagnostics DTC reading/clearing;

• discovered from the extremely informative, highly enlightening ECUPROJECT.COM Tuning Forums > Volvo > General Topics > "Diagnose and communication" first post by rkam on 2009-05-19 of much detail on the Motronic 4.4 communication protocol, which looked surprisingly similarly to the Vol-FCR Demo communication;

• speculated that the ELM327 approach which had been used successfully for Fiat (and other brands of) cars might be fused with rkam's examples of the Motronic 4.4 protocol so that an ELM327 could be used to read and clear DTCs of my 1997 Volvo 850 T5 -- not to just only read (and not clear) the DTCs like the Vol-FCR v1.7.3 for FTDI Demo did. ...and even then, only a small number of DTCs were listed and explained by the Vol-FCR Demo after they were read. Most of the DTCs indicated you had to buy the Full version to see their meaning.

So, since I never had enough money (during the first 2.4 years of my exploration) to buy a solution outright -- but I did have a great deal of time -- I experimented with the ELM327, using the rkam notes as a guide to the possible command set, and eventually discovered:

• how to clear all DTCs for all my car's computers that communicate via the OBDII port's pin 7,

• a different "00-padding" way to read the '97 850 DTCs (compared to how the Vol-FCR Demo does it) that initially worked better with the ELM327 cable when communicating with 4 of the 6 computers that could speak with the ELM327,

  Update:
  However, that "00-padding" method became moot in late 2013-10 after discovering that the use of the ELM327 command ATRA 13 allowed all 6 of those 6 ELM327-connected computers to work with the exact same (slightly shorter) requests that Vol-FCR Demo and Brick-Diag Free use when my Volvo 850 is connected via a VAG/COM KKL cable.

• how to read the vehicle mileage and Service Reminder parameters and counters in the COMBI Instrument Panel (though I still don't fully understand all the COMBI's other parameters completely),

• how to read most of the COMBI live data,

• how to monitor the ELM327 communication using Realterm with a VAG/COM KKL FTDI USB2 cable to see car responses that both the ELM327 and Vol-FCR don't display, and

• some limits/disadvantages of using an ELM327 cable vs. using a VAG/COM KKL FTDI USB2 cable with Vol-FCR.

To continue following my foray into the world of the Volvo 850 OBDII port, click on one of the links in the Volvo 850 OBDII pulldown menus at the top or bottom of this page.