Rev counter problems

Technical Area Engine, Transmission, Exhaust etc
2

OK for a start I am gong to apologise for the length of this post but thought without the background good advices would be impossible.

Mazda MX5 Eunos J2 1.8 1994

Following a full service the car ran fine for quite a while and then one day stoped and would not restart. Coil Pack changed for a new one and car was fine except for the rev counter occasionally going “wild” when the car was made to work a bit (overtaking etc…) but the engine seemed fine.

On way to Silverstone classic the rev counter again started to behave wildly but shortly afterwards engine started to misfire and would not pull. Once stopped would fire but not run. Enter the RAC (and there is another long story!) who eventually recovered the car having said it was too old to diagnose by the roadside.

After trying a number of options including the Cam Angle Sensor decided that all pointed to ECU. Managed to get one with same numbers from eBay. Fitted and engine now runs fine.

Problem is that the rev counter shows 0 when at idle and only goes up to about 3000 revs when car pushed. It also goes erratic at times.

Now at a loss for what this might be and open to constructive  thoughts and suggestions!

 

Thanks in advance 

Jim & Liz

3

Hi

The problem will be associated with the 4 pin coilpack.

On the early 1.8 cars the fourth wire on the low tension circuit plugs controls the tacho. 

I see  you have changed the coilpack and as the tacho has worked albeit not efficiently must be a 4 pin replacement.

I would check the continuity of the tacho wire on the low tension plugs from connector in plug to a position up each wire beyond where it typically gets stressed.

Should be a BPF3 on this car - no connection to the tacho. Ecu control started with the introduction of the three pin coilpacks around 1995.

Do you still have the original ecu?   

 

4

Hi Rhino

Thanks for that. Looks like need to do some wiring checks then :frowning:

Yes we still have the original ECU

Also excuse my ignorance but what is a BPF3?

Jim

5

No ignorance I just forgot to add ecu:-)

I would get this issue sorted and then retest the orginal ecu - it may well be fine.

Either way I would like it as have several of these ecus that require spares for repair.

The tacho wire is next to the thick blue wire - I’m colourblind but looks like black with a white stripe.