Ta­chome­ter trou­ble

Car Mechanics (UK) - - Help! -

The in­stru­ment clus­ter ta­chome­ter doesn’t func­tion at all in my 54-plate smart road­ster with 107,000 miles. The car is fit­ted with the op­tional ex­tra trip com­puter which still shows the cor­rect RPM, so I’m as­sum­ing the RPM sig­nal/ sen­sor is func­tion­ing prop­erly.

Un­for­tu­nately, I caused this prob­lem by swap­ping the in­stru­ment clus­ter dial face from white to black, al­though the pro­ce­dure went ex­actly as per the Evi­lu­tion web­site in­struc­tions. The prob­lem arose when I reat­tached the in­stru­ment clus­ter con­nec­tion plug, as I un­know­ingly failed to seat and con­nect it prop­erly. I then con­nected the bat­tery and turned on the ig­ni­tion, but noth­ing lit up on the clus­ter. I dis­con­nected the bat­tery and re­moved the in­stru­ment clus­ter, which was when the ca­ble con­nec­tion dropped out of its seat­ing po­si­tion. I re­fit­ted the clus­ter, mak­ing sure it was cor­rectly con­nected, reat­tached the bat­tery and turned on ig­ni­tion. This time ev­ery­thing lit up, but the ta­chome­ter wasn’t work­ing.

Is there any way of check­ing the clus­ter with­out cre­at­ing more prob­lems? I have in­spected the ca­bles lead­ing to the clus­ter and noted some fusible links on some of the wires, but I’m not sure how to test them with­out dis­turb­ing the SAM unit be­low. I’m not sure how to check the clus­ter cir­cuit for the tacho step­per mo­tor, or if the tacho step­per mo­tor can be re­placed eas­ily and what prob­lems would arise if I tried to re­place it.

I have man­aged to get hold of a sec­ond­hand in­stru­ment clus­ter – I’m un­sure of its mileage and other set­tings stored in it – but I’m not too con­fi­dent about fit­ting it as I have read that the mileage stored in the clus­ter EEPROM could over­write the ECU/SAM mileage. I have also got the cor­rect wiring di­a­gram for the car, but I’m un­sure what to look for as ev­ery­thing else is work­ing fine on the clus­ter. I can only find one fuse in the SAM for the clus­ter and that is ob­vi­ously OK.

Si­mon Wright

Pro­vided the plug to the ta­chome­ter wasn’t mis­aligned or in­ad­ver­tently placed in the wrong po­si­tion, it is un­likely that any dam­age should have oc­curred to the unit, given that you took ad­e­quate pre­cau­tions by dis­con­nect­ing the bat­tery. How­ever, it is pos­si­ble that even though you be­lieve the plug is now cor­rectly lo­cated, a poor con­nec­tion is still present. With this in mind, my first move would be to re­move the plug again and spray the plug and con­nec­tion with an elec­tri­cal switch cleaner spray, then firmly re­con­nect the plug in the socket and check the op­er­a­tion of the ta­chome­ter.

As the se­cond rev counter func­tion is op­er­at­ing, you can dis­count the pos­si­bil­ity that the ta­chome­ter cir­cuit is at fault.

As you men­tioned the web­site, I pre­sume that you are a mem­ber of the site, in which case ex­cel­lent in­struc­tions are avail­able for cloning the ECU. From the de­tails I have, the ECU will over­ride the clus­ter and not the other way round.

Other prob­lems that can oc­cur in­clude wa­ter dam­age to the SAM unit, but as ev­ery­thing was work­ing be­fore the dis­con­nec­tion, this is un­likely. One op­tion may be to let a com­pany such as this one check and, if re­quired, re­pair the unit. Prices start at around £40, but as I've not per­son­ally used this com­pany this is only a sug­ges­tions and not a rec­om­men­da­tion.

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