This article was first published in 2004.
As we’ve covered previously, on cars with On-Board Diagnostics sockets
there’s a wealth of high quality information that can be accessed. Things like
airflow, intake air temps, long and short term fuel learning trims, and stuff
like that. But what if your car doesn’t have an OBD socket? In that case, you
might like to do what we’ve done in this story and fit a custom diagnostics
socket. It won’t give you read-outs in engineering units (like airflow in grams
per second) but it will allow you to easily access a heap of interesting
So what is this DIY diagnostic socket, then? Well, put simply it’s just a
multi-pin socket that’s wired to certain pins of the Electronic Control Unit. By
connecting matching plug to either specialised electronics (eg a LED Mixture
Meter or a digital oscilloscope) or to a general purpose electronics measuring
tool like a multimeter, you can then quickly and easily access ECU sensor
information - just by plugging into the socket. That’s so much easier than having to pull off
trim panels, locate the ECU, find the right pins, connect the wires... and then do
the measurements. Only, when you have finished, to be required to undo the whole
While installation of the DIY diagnostics socket is straightforward, it’s the
sort of job which is best done when you are already accessing the ECU and making
some connections to it. So for example, if you are fitting an interceptor (for
speed, fuel or ignition timing), you’re connecting an aftermarket tacho, or
you’re connecting a voltage switch circuit that clicks over when certain sensor
conditions occur, then's the time to also fit the DIY diagnostic socket. The
extra work isn’t all that great – and the convenience and ease of use sure make
The Plug and Socket
You’ll need to source a multi-pin, polarised plug and socket combination.
(Polarised simply means that the plug fits into the socket with only the one
orientation.) In our case we used a 6-pin DIN socket (available from electronics
stores) because we figured that would give about the right number of outputs and
we had a matching plug and chassis mount socket already on hand.
If you want to be able to easily measure nearly every ECU input, you’ll need
to have a correspondingly greater number of pins. We suggest that the following
sensor (if multiple units, the one
located before the cats)
meter (or MAP sensor)
- Cam angle sensor
intake manifold changeover
Now if you decide to do all of these you’ll need to have a plug/socket
combination with at least 10 pins. In our application we decided to initially do
only +12V, earth, front oxygen sensor and airflow meter. This approach used four
of the six pins – at a later date another pin will probably be wired for
injector output. Note that pretty well all sensors are referenced to chassis
ground – so only the one earth connection is needed.
Having +12V and earth connections on the DIY diagnostics plug makes the use
of devices like a LED Mixture Meter really easy – it can get power, earth and
signal with a simple plug-in, making it a doddle to temporarily fit when
checking mixtures or looking at the health of the oxygen sensor. Incidentally,
the reason that you would normally connect to the oxy sensor that’s located in
front of the cat is this sensor has a more meaningful output than the post-cat
Nearly all ECU’s use regulated 5-volt outputs to at least some sensors. It’s
possible to draw a small amount of extra current from this supply without
causing dramas, so if you’re connecting a custom measuring instrument that
requires a regulated 5V supply, you can save on some components by using the
5-volt supply that’s already available at the ECU.
A chassis mount socket is likely to give the neatest job. This DIN socket was
mounted on a bracket made from 3mm thick aluminium bar. Why so thick? Well, you
don’t want the bracket flexing when you’re pushing in and pulling out the plug.
For the same reason, the bracket should be held in place with a strong bolt.
Depending on the design of socket, the wires will be either soldered or
crimped into place. It’s very easy
to make mistakes when wiring sockets and plugs, so do as we’ve done here and use
the same colour wires to the same pins of both the plug and the socket. That
way, it’s both easy to make sure that you have continuity across the plug/socket
(just use your multimeter to make sure that each colour wire is continuous
across the plug/socket) and it also makes it much easier to wire the system into
place in the car.
Place the new socket where it can be easily accessed but without being in your
face. Also note that the exposed pins will be carrying voltages – so either
locate it where that’s not a problem or put a second, unwired plug into it
whenever it’s not being used for diagnostics. In this car, the glovebox lid can
be pivoted down further than normal in order that the cabin air filter can be
changed. That movement also reveals a good location for the socket.
Most ECU wiring – especially when something else is being installed – looks
like spaghetti junction. In addition to the DIY diagnostics socket, here a
Simple Voltage Switch kit is being installed on the airflow meter output, and a
Digital Fuel Adjuster kit is also being installed – again using the airflow
meter output and corresponding ECU input. For the DIY diagnostic socket, use the
workshop manual and a multimeter to find the signals that you’d like to be able
to monitor. Then connect one of the colour-coded wires from the diagnostic
socket to each of these ECU pins, connecting to the ECU wire back a little from
its plug. Use solder or high quality crimps to make these connections and don’t
forget to insulate them well.
Here’s the view looking up under the dash. (1) access door to the cabin
airfilter, (2) new DIY diagnostics socket, (3) new bracket to hold the Simple
Voltage Switch box in place, (4) the ECU.
The green arrow shows the diagnostic wiring connections to the ECU and the
red arrow shows the diagnostic plug connections which are at this stage are
unused. The latter are cable-tied out of the way for easy access when the time
comes to connect them.
Here’s the view of the completed install. (1) new DIY diagnostics socket, (2)
Simple Voltage Switch kit, (3) Digital Fuel Adjuster kit. Note the
yellow-shrouded cable, which is for the airbag. Be careful of yellow cables!
Accessing the DIY diagnostics socket is as easy as pivoting down the glovebox
lid and inserting the plug.
OK, so what use can be made of the information now available?
Obviously, what you can do with the information depends on what ECU pins
you’ve accessed and what measuring instruments you have. However, here’s a
- Oxygen Sensor Output: measure with
multimeter (millivolts) or LED Mixture Meter to assess oxygen sensor health,
when the ECU is in closed and open loop, and approximate mixture
- Airflow Meter Output: measure with a
multimeter (volts or Hertz) to make sure that meter isn’t maxing out (ie hasn’t
reached its ceiling output through increased airflow throughput caused by mods);
assess tuning changes to see if peak airflow reading alters (more air usually means more
- MAP sensor: on turbo or supercharged
cars a logging multimeter can be used to very accurately track boost levels,
allowing (for example) the picking of overshoots. On naturally aspirated cars the manifold pressure at full load can be measured - if it's below atmospheric, an intake restriction is indicated.
- Injector Duty Cycle ECU Output: measure
with a multimeter (duty cycle) to make sure that injectors aren’t maxing out at
100 per cent duty cycle, or see how much more fuel flow capability is left after
- Intake Air/ Coolant Temp Sensor
Outputs: Use a multimeter (volts) to watch trends in these temps (volts will
go down as temp goes up).
- Crank or cam angle sensors: Use a dual
trace oscilloscope to watch real time cam timing variations
- Variable Intake System Changeover: Use a
multimeter (volts) to watch when dual-length resonant intake system changes from
one mode to another.
Even if you take the short time needed to install a socket with just power,
earth, oxygen sensor and airflow meter outputs, you’ll certainly not regret it
when you’re modifying and tuning your car. More pins? Well, that’d be even more