Work­shop Ba­sics

How to talk to your mod­ern clas­sic’s On Board Di­ag­nos­tics sys­tem

Practical Classics (UK) - - CONTENTS -

Be­come OBD lit­er­ate.

On Board Di­ag­nos­tics – or OBD – can strike fear into the heart of a clas­sic driver brought up on car­bu­ret­tors and con­tact break­ers. Panic sets in when the en­gine man­age­ment light flashes on. In re­al­ity, though, OBD is our friend. If you can read your car’s OBD sys­tem, fault-find­ing be­comes much eas­ier. Buy a ba­sic fault-code reader and OBD turns into a real bless­ing.

OBD has been around since the Eight­ies, when man­u­fac­tur­ers em­braced ad­vances in elec­tronic sys­tems. Per­for­mance cars adopted dig­i­tal en­gine man­age­ment and elec­tronic fuel in­jec­tors, which con­trolled fu­elling and ig­ni­tion far more ac­cu­rately than pre­vi­ously pos­si­ble. Spark tim­ing and fu­elling con­trolled dy­nam­i­cally by an Elec­tronic Con­trol Unit – or ECU – im­proved both per­for­mance and fuel econ­omy. Crit­i­cally, an ECU and ex­haust cat­a­lyst could also re­duce emis­sions to meet de­mand­ing stan­dards im­posed across the in­dus­try. Soon even the most com­mon­place cars were us­ing the new tech­nol­ogy.

The ECU re­lies on in­puts from sen­sors dot­ted around the en­gine. These in­clude air tem­per­a­ture, coolant tem­per­a­ture, man­i­fold air pres­sure, in­duc­tion air­flow, throt­tle po­si­tion, knock de­tec­tion and ex­haust oxy­gen con­tent. The ECU makes de­ci­sions based on the in­for­ma­tion from these sen­sors us­ing pre-pro­grammed sets of in­struc­tions known as a ‘maps’. This is won­der­ful as long as all the afore­men­tioned com­po­nents work cor­rectly. If a sen­sor de­vel­ops a fault, there’s po­ten­tial for the en­gine to run badly or stop al­to­gether.

If the ECU de­tects a sen­sor in­put out­side its ex­pected range, it’ll store a fault­code and pos­si­bly il­lu­mi­nate a warn­ing light on the dash­board. It may choose to ignore the sen­sor and switch to ‘limp home’ mode.

A cheap fault­code reader – or ‘scan­ner’ – is all you need to find out what’s wrong. This guide cov­ers the ba­sic prin­ci­ples. We’re start­ing with OBD1, which was spe­cific to each man­u­fac­turer and re­quired unique scan­ners and pro­to­cols. We’ll then look at OBD2, a so­phis­ti­cated uni­ver­sal sys­tem that took over in the mid-nineties.

You will need Scan­ner ca­pa­bil­ity Short cuts? Con­nec­tions OBD scan­ner and a lap­top, mo­bile phone or tablet with suit­able soft­ware; mul­ti­me­ter; elec­tri­cal con­tact spray. Many OBD1 cars will need an adap­tor to con­nect to the scan­ner. Cheap scan­ners should di­ag­nose and clear en­gine faults, but are un­likely to deal with se­cu­rity sys­tem, airbag or ABS prob­lems. Ad­vice on the in­ter­net may sug­gest short­ing out pins in the OBD socket to re­set the ser­vice in­di­ca­tor. This is best avoided, as mis­takes can dam­age the ECU.

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