Sorry this is a bit long but i'm trying to put as much info in as i can. Basically our rev counter is an old type that took it's reading from a single coil and shouldn't read correctly with just a reading from one side of the coilpack but it does!?
My dad and I are installing a MJLJ on a MK2 fiesta XR2. Its the early mark 2 which ran an igntion system with used a coil, hall effect distributer (same as the MFi RS Turbo) and a basic ECU that used a map sensor that took its reading from an outlet on the carb not the black top dizzy system with the small black module on the dizzy and vacuum advance unit fitted to the later XR2. The original system and it's wiring (which fitted into a small 3 pin multiplug in the loom) was completely removed before we started the installation. Of the 3 pins on the multi plug. One of the pins is the send to the rev counter, the other the 12v feed for the system and the other is the oil pressure switch feed to the dash. So only one of the wires was relevant to the actual runninf of the original system.
The problem is that because the rev counter took it's reading from the coil as most older rev counters do we assumed we would need the diode circuit in the instructions to get the rev counter to work with the coilpack. We bought a ready made module from trigger wheels for this purpose and it didn't work. The rev counter would read 800rpm at idle fine but as soon as the engine was revved the needle would return to 0. Which was a bit weird. Not really understanding how the diode circuit worked or being able to take it apart and check it was as the instructions said it should be we had to give up with it.
We decided to try a few other ideas and decided to connect one of the coilpack wires to the rev counter. We assumed the rev counter would work but would read half the actual revs. It doesn't it reads as it should and agrees with what we are seeing on our readout in the program in our laptop.
It's great that we have a working rev counter but we are just worried that something isn't right. We followed the instructions exactly and have a working engine first just running on the edis and then 2d with the megajolt (have DCNF carbs which have very little spindle sticking out so 3d will have to wait) and it runs fine. We haven't done any setting up yet but the engines idles smoothly and revs ok with the figures we have taken from a rough graph of the 3d map in the haynes manual from the module that was fitted to the car.
Is it possible that the coilpacks are firing more often than they should? we thought about taking the spark plugs out to check this but wanted to know the likelyhood first.
Did ford do rev counters that only needed half the signal anyway? The original rev counter wire was wired to the coil and not the ecu module thing so we don't thats anything to do with it.
We're stumped and really don't like the fact that we can't understand why it's working!!
Any suggestions would be greatly appreciated!!!!
Why does our rev counter work
Moderators: JeffC, rdoherty, stieg, brentp
I just read this in the megjolt manual on here.
Using the Tach Out on the MJLJ
Many tachometers can accept the signal emitted from the Tach Out on the MJLJ module. This Tach Out signal provides a 12V pulse for every ignition event. You may use this to provide the signal to compatible tachometers, or to provide a signal to a separate Engine Control Unit, data logger, or other device which requires this signal.
In the manual we have i'm sure it says that it's a 5v signal designed to work with modern rev counters!?
Which is it? As a 12v signal would work with our rev counter.
Please still read the original problem as i really would like answer!!
Using the Tach Out on the MJLJ
Many tachometers can accept the signal emitted from the Tach Out on the MJLJ module. This Tach Out signal provides a 12V pulse for every ignition event. You may use this to provide the signal to compatible tachometers, or to provide a signal to a separate Engine Control Unit, data logger, or other device which requires this signal.
In the manual we have i'm sure it says that it's a 5v signal designed to work with modern rev counters!?
Which is it? As a 12v signal would work with our rev counter.
Please still read the original problem as i really would like answer!!