rev counter intermittent fault.

[MX5Spares]

Just checking out there in case this happened to anybody before but I have a 93 1.8 here at moment.Originally had broken timing belt but I sorted that incl waterpump,pulleys etc but now ( maybe before I got it also but belt broke so I could not start it but I dont think so) but rev counter seems to have a mind of it,s own in that when it,s NOT working car is going perfect but the rev counter cuts in intermittently and witihin say 3 minutes it begins to go very bad and sort of starts misfiring,backfiring lightly and practicably impossible to drive .If I switch engine off or wait until rev counter stops again car is perfect again.Checked CAS and swopped airflow meter,coil,leads and Lambda sensor .Cat is only 4 months old so no issue there.Doubt it,s the instrument cluster itself as I looked on a few UK websites and they are suggesting wire from igniter??? to dash or ECU black/white wire ??Welcome any ideas .

[Steve]

did you check the grounds?
I'd also check the wires at the CAS in case they've broken internally.

[sidewaysreilly]

there is a wire from the coil pack or possibly CAS that signals the tacho. Dont have wiring diagram to hand but i would suspect this. I used it for the VICS signal input when wiring up the speed activated switch. Think its green or black/green. Chase up a wiring diagram and check this wire for grounding. Would rightly feck up the ignition if it earthed.

[franksm]

Hi lads

Think it's the 1.6 that maybe has a connection from the coil pack and ECU for the tacho, but in the '93 1.8 the tacho is fed by the alternator - white-black wire

In the instrument cluster:

2K blk-yel 12v supply (fuse - shared)
1H wht-blk alternator (same wire goes to the ABS control unit)
2J blk Earth (shared)

The two shared wires would show up as a fault in other things in the dashboard, so if only the tacho is bad then either it's the white-black wire or else the tach guage itself.

Here - sounds like it could be the ECU or ignition switch though. The ECU switching in and out could cause backfire, etc, but it will cause the revcounter to go weird.

Frank

[sidewaysreilly]

You sure Frank? I remember chasing the signal wire to operate the VICS switch and found it either on the coil pack or CAS unit. Definately not alt.

[MX5Spares]

Now I have checked all CAS wiring etc,Checked all fuses in both fuse boxes just in case but the only new problem noticed is that when rev counter is jumping from zero to 15 approx,if I turn on heater fan/wipers etc the rev counter stops jumping at stays at zero .I already changed the alternator ( it says built in I.C. if this helps any)in case it was not charging at full voltage even though battery light was not on when car is running .There is a slight possibility jumper leads shorted off each other the other day as one jumper end fell off battery terminal causing a quick spark .I should have spare ECU if needed ??

[MX5Spares]

Sorry, also changed Instrument cluster and no difference.

[franksm]

You sure Frank? I remember chasing the signal wire to operate the VICS switch and found it either on the coil pack or CAS unit. Definately not alt.

hey, you're right, I found a better diagram that's not a scan from a fuzzy microfiche.

Yellow-blue goes to the ignitor portion of the coil. I'd swap in some known-good coilpacks and see what happens

[MX5Spares]

Changed Cas even though I Changed it before and rev counter seems to have settled ok.Tried to do diagnostic check and showed uo Lambda sensor fault so changed that and done multimeter voltage reading so now it flicks from approx 1volt to 13volts so at least its working anyway.Car still running very rich judging from spark plugs colour and smell from exhaust and car is lumpy/hunting.I Swopped air flow meter but I noticed car runs better when airflow meter is disconnected.???

[sidewaysreilly]

Check all your earth straps all over the car.

[MX5Spares]

Finally beat that mx5 into submission !! Faulty ecu causing tacho to be intermittent also exhaust cam was 1 tooth out resulting in car going great but idling very poorly and cat overheating. Reset cams ang all is well.Thanks for the advice and got good info from miata king on youtube.