4age Incorrect RPM signal

[slowrolla4age]

Hello I have been trouble shooting my early bigport 4age that is unable to idle, I have been unable to locate the exact problem but I have found a couple incorrect readings.

I have 3 million ohms reading on the ecu between terminals ne signal for rpm and g- (I believe is ground). The book spec is around 160ohm.

The engine sounds like it is at 800 rpm when the tach says 2k

I checked resistance from the ne wire from ecu to the 5 pin connector that goes to the igniter module with the ignition module connector and distributor disconnected from the harness, I got around 5 million ohms with the ecu plugged in and .7 ohm with the ecu unplugged

Next I tried to replace the ecu with another one and I got the same reading now I am debating if I should try another ecu incase both the ones I had are bad or if I need to be looking at something completely different

Where might a reading of 3 million ohms in that circuit come from?
What other wires should I check or component?
Has anyone ever had this problem?

[jondee86]

Once you get the engine running, does it function normally apart from not
being able to hold an idle ?? Can you drive the car, and if so, does perform
as usual... no loss of power, missing, jerking etc ???

The tacho signal comes from the igniter and is derived from the back EMF
spike generated when the coil fires. The same signal is conditioned by the
igniter and sent to the ECU as the IGf input to tell the ECU that an ignition
event has occurred.

The Ne signal is a small voltage generated by a reluctor inside the distributor
when a toothed wheel passes it's fixed position. The Ne signal is processed
by the ECU to calculate the engine rpm.

There is no connection between the tacho signal and the Ne signal. So if the
tacho reads high when the engine is running slow, one of them is lying !!!!
I'd start by swapping out the igniter (or coil plus igniter) for a known good
one and see if anything changes.

Cheers... jondee86

[slowrolla4age]

The engine sounds smooth once I step on the pedal maybe because it is using the info from the tps. No jerking. SO since I get a bad reading at the ecu between ne and g- but I get 160 ohms from the g to g- at the ecu and I get 160 ohms from the two wires that go to the ignition module ups at the dIzzy I can eliminate the distributor as a problem causer what I would like to know is how to test the ignition module I’d like to see where I am losing those 160 ohm but see I only found one wire(green) on that five pin connector coming from the ignition module that I believe is ne after it passes through the ignition module where is the other wire which I believe is a g- ground so I can test it to see if it loses the 160 ohms after the ignition module it almost that 2 wires turn into one wire after the ignition module? And the ground for that turns into the body?
Srry if this is confusing
I will try to replace the ignition module, I heard you can use one from a 22r as long as it has to pickups at on the dizzzy so I should just have to figure out how to change the connector pieces and make sure to match up the wires correctly though that may take some deciphering lol or I can go pay an outrageous price for a early bigport one will any mr2 ignition modules work? Or do they have the signal sent straight to the ecu on that loom?

[totta crolla]

The TPS input is a correction to the main fuel map.
Given that your reason to suspect an issue is it sounding like it is idling at 800rpm and the tacho showing 2000rpm it might be best to confirm this. Some timing lights will give a true rpm readout.

[jondee86]

Difficult to be specific about this without a bit more information... like, what igniter you
actually have on the car. There a couple of possibilities depending on the year and market.
Some have one plug and some have two plugs... a picture of yours would be helpful

That aside, if the engine runs normally apart from not wanting to idle, the chances are that
the igniter is at least providing the correct Ne signal to the ECU. It would also seem that
the ECU is working normally as far as providing an IGt signal to the igniter. But the reason
for the tacho appearing to read high is remains unknown. Further investigation required.

There are a number of factors that can affect the ability of the engine to idle. Does your
engine use AFM or MAP for load sensing ?? AFM engines have an idle switch inside he AFM
that needs to be triggered by (cranking) airflow to allow the engine to run. As previously
mentioned, ignition timing needs to be in the correct range (not over-retarded) to maintain
a steady idle. Fuel mixture not over-rich, airflow not restricted, idle air bypass screw open
(if MAP), clutch switch (if fitted) operating correctly, adequate fuel pressure, plugs not
fouled by excessive cranking, no splits in the intake ducting (if AFM), ash tray emptied.

Around 800rpm is normal idle speed for a stock bigport when it is warmed up. Do you have
to keep the throttle open a little to get it to continue to run at that speed ?? As suggested
above, it would help to define the problem if you could actually confirm the engine speed
with a strobe or timing light with rpm function.

Some info here... https://www.mr2oc.com/threads/4age-igni ... ut.686091/ but not
sure if it will be any help.

Cheers... jondee86