After my semi-classic 21 year old GTE failing the MOT on a missing ABS bulb, it was time to find out why the bulb had been previously removed.
My immediate thoughts were of nightmare hassle and vast expenses… I’ve written this to help others to understand the system, and have some confidence in trying to repair their ABS issues for free, themselves.
(Image taken at the excellent Midlands based Lutterworth MOT Centre: lutterworthmotcentre.co.uk)
With a bit of help from friends on the astra-mk2.com forum, and the good old Haynes Manual, I managed to find the fault and repair it for free!
Is there a fault or not?:
After fitting the bulb in the dashboard, the ABS light was permanently ON. This spells trouble, and is why the the bulb had previously been removed, to mask the fault.
However; your car will fail it’s MOT if the bulb doesn’t light. It should illuminate for a couple if seconds when the ignition is turned on, this proves that the ABS is gong through a self-test, if it can’t be seen to self test, then no MOT.
This is done for FREE, using a humble paper clip, or short piece of thin single core wire. No need to fork out for expensive diagnostics from an auto electrician.
With the ignition off, locate the diagnostic connector in the engine bay, near the LH strut top (the only connector that’s blue). Disconnect and remove the plug side, then short out pins A and K in the socket with your paperclip. It doesn’t matter if the paperclip touches chassis, as it’s at Ground potential anyway.
What should happen:
Switch on the ignition, and watch the ABS light. It should flash out a series of fault codes, note the codes down and look up on: topbuzz.co.uk you ideally want to see lots of 12’s being flashed out, that means all is well. Each code is flashed 3 times, and real fault codes are seperated by 3 x 12’s.
Mine didn’t do that… the light just stayed on… grrrrr…..
The ABS hydraulic pump unit contains two relays, one for the hydraulic Pump and one for the hydraulic Solenoid valves. There is a third relay, external to the pump unit, for ABS surge protection.
Using the Haynes manual wiring diagrams; I bench tested all the relays, by applying 12Vdc to the coils and checking the contacts open / close with my multimeter… This is how I found the fault!
The Fault was found:
The external ABS surge control relay (K47) didn’t want to click when I applied 12Vdc to the coil. (There is a diode in series with the coil, so you have to have the power on the right way round, otherwise it won’t click). I took the plastic cover off the relay, checked through the circuit with the multimeter, and found an open circuit between the coil and pin 31 on the relay base.
The Exact Cause:
Looking closer, I found a Dry Joint where the relay coil -ve side was soldered to the PCB!!! It mush have shaken loose with years of motoring.
So a quick re-solder, and re-assemble, and hey-presto – she worked! :o) yay!!