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 information.
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 process!
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 it worthwhile.
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 are possibilities:
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 one.
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 smorgasbord:
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 useful....