Philes’ Forum – Spring 2018


By Vic Lucariello, Sr.

Hello bimmerphiles! This time out I would like to talk a bit about that diagnostic connector under the left side of your dashboard on your 1996-and-newer Bimmer. Actually, it is found on almost any 1996-and-newer passenger car.

Diagnostic connectors on cars were nothing new, even in 1996. Indeed, my 2002 has one under the hood, as shown in Photo #1.

Photo #1 2002 “Diagnostic” Connector

On 2002-era cars, before the advent of computer-controlled everything and serial-bus communication, the diagnostic connector merely provided a convenient test point for a few critical electrical parameters, such as whether or not the starter was being commanded to operate. The parameters available on the diagnostic connector could in most cases be easily accessed at their sources with a voltmeter, but the connector brought the parameters together so that a factory diagnostic tool could be easily and quickly connected.

In the E30s [1984-1991 3-series], earlier model years had a different- style connector [See Photo #2] that essentially provided the same information. Later model years had a 20-pin underhood connector [See Photo #3]. BMW used this connector until about the 2002 model year. [Most 1996 – 2001 Bimmers had both the 20-pin underhood connector and the OBD II connector described below.] Diagnostic devices that plug into car-manufacturer-provided connectors, or ports, came to be known generically as “scanners”, because in the computer-control era these devices can scan and display multiple process parameters, or PIDs, such as coolant temperature, etc.

Photo #2 Early E30 Diagnostic Connector
Photo #3 Later E30 Diagnostic Connector

You will note that on the three BMW models above, there are three very different diagnostic connectors, all located in different areas of the engine compartment. And that is just for Bimmers. Add the other car manufacturers into the mix and one is faced with a veritable cornucopia of different connectors and locations. Indeed, for my old Snap-on scanner, the number of required diagnostic adapters pretty much doubled the size of the carrying case! And of course, I still had to buy the BMW 20-pin adapter separately.

Our federal government and the Society of Automotive Engineers noticed that all these different diagnostic connectors posed a huge problem for non-dealer technicians and for technicians who worked on several different makes of cars. And thus was born the idea of a standardized diagnostic connector and OBD II [On-Board Diagnostics – Level II] regulations. OBD II – compliant vehicles [Pretty much every passenger car sold in the US starting with the 1996 model year.] must all use a standardized connector in a pretty much standardized location inside the vehicle under the dashboard [See Photo #4]. So, one should be able to take any so-called Global-OBD II – capable scanner and plug it into any OBD II – compliant vehicle via the standardized connector and view diagnostic trouble codes [DTCs] and emissions-related engine and transmission parameters.

Photo #4 OBD II Diagnostic Connector

What are DTCs? Glad you asked. DTCs are set, and usually the dreaded “Check Engine” light [AKA: MIL – Malfunction Indicator Light] is turned on, when the engine or transmission computer detects an abnormal condition that can adversely affect vehicle emissions. Another advantage of OBD II is that generic-type DTCs [in the form of P0xxx] are supposed to be the SAME among the different vehicle makes. So, if either a 1996 328i or a 2004 VW Jetta or a 2008 Subaru or a 2010 Corvette or a 2018 M4 records a P0300 DTC, that means the engine computer is detecting random cylinder misfires. Similarly, a P0420 indicates that the catalytic converter is not performing properly in cleaning up exhaust gases.

Another function of the OBD II port is to allow downloading of computer-software updates, from the vehicle manufacturer or from aftermarket-software providers [tuners]. But if you have not fallen asleep by now I would like to talk about an available “function” of the OBD II port that I think you should avoid.

Of the pins on the OBD II connector, one is from the battery bus [always “hot” regardless of whether the ignition is on or off] and one is a vehicle ground. The purpose of this is to power a scanner. To repeat: The purpose of this is to power a scanner or simple DTC reader. The purpose is NOT to connect a battery charger, battery maintainer, or so-called “memory saver”.

Memory savers are used to supply auxiliary power and maintain computer memories when the vehicle battery is disconnected, say to install a new battery. I am a proponent of connecting auxiliary power during battery work, but on a BMW, I use the underhood B+ and ground studs thoughtfully provided by BMW.

I talked to three expert, professional technicians about this: Matt Kimple, Sal Puleio and Chris Roberson. Matt and Sal need no introduction to Philes’ readers. Fellow [as is Sal] iATN member Chris Roberson is a BMW-trained technician from the Left Coast whom I came to know via his many valuable BMW-related iATN contributions. iATN is the International Automobile Technicians’ Network, with something like 70,000 English-speaking techs worldwide.

Both Chris and Matt maintain that there is no need to connect auxiliary power to a recent Bimmer when disconnecting the vehicle battery. Earlier cars could lose radio codes, reset DTCs and monitors, and possibly lose seat and sunroof settings. In rare cases, especially if the battery were left disconnected for a couple of days, the “alignment” of the anti-theft system [EWS or CAS] could be lost, resulting in a no-start situation when the battery got reconnected. “Honey, to save some money I installed a new battery in your X5, but now it won’t start.”

Sal and I tend to connect auxiliary power whether we need to or not; one reason being that it is difficult to know for sure whether a particular Bimmer [Also, Sal and I work on other cars in addition to Bimmers] should have auxiliary power during battery work. Sal says, “It certainly does not hurt anything to connect auxiliary power, so provided it is done correctly, why not?”. One thing that Chris, Matt, Sal and I agree on completely is that, if you are going to use auxiliary power during battery work, PLEASE don’t connect a memory-saver device to the OBD II port! That ain’t what it’s for!

That’s all for now, bimmerphiles. See you next time.

Anyone wishing to contribute to Philes’ Forum can contact me at I’m interested in tech tips, repair / maintenance questions, repair horror stories, emissions-inspection sagas, product evaluations, etc.

© 2018; V.M. Lucariello, P.E.

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