RPM signal to the ECU

[ulysess1966]

Are there any ECU experts out there? Just a quick question. Is the RPM/engine speed signal into the ECU a voltage or a stream of pulses? I'm building some (non-invasive) electronics for my car and want to intercept this signal.

All the best, Glenn.

[cartledge_uk]

I'm pretty sure tacho is a pulse. I'm sure someone else will confirm

edit: sorry Tacho is a pulse JIMI will answer the ECU one in a min

[jimi]

It depends which signal your looking for. If it's the SPD signal input to the ECU then that's actually the car speed and is derived from the speedo.
The rpm inputs to the ECU are from 2 signal generators in the the distributor, NE and G, on terminals NE, G+ and G- (G- being common to both)
If it's just a engine rpm signal (same as the tacho) you want, then it comes direct from the ignitor and is also available in the engine bay at the check connector pin, IG-

[jimi]

Not quite sure what you mean by

a voltage or a stream of pulses

It's going to be both, probably a square wave, but that's a guess.

[greglebon]

The actual signal to the ECU is an AC signal from a VR sensor: it resembles a dirty sine wave. The actual RPM signal is "Ne": the "G" is the engine position signal.

The voltage of this AC "Ne" signal varies a lot: it's voltage increases greatly as RPM increases: IIRC, it can reach 50v-ish (dont quote me on that, though!)

As Jimi says, the RPM signal to the dash is from the igniter: a dirty great spike, not a nice clean square wave, though......!

What are you working on? Anything exciting..!?

[jimi]

The actual signal to the ECU is an AC signal from a VR sensor: it resembles a dirty sine wave. The actual RPM signal is "Ne": the "G" is the engine position signal.

The voltage of this AC "Ne" signal varies a lot: it's voltage increases greatly as RPM increases: IIRC, it can reach 50v-ish (dont quote me on that, though!)

As Jimi says, the RPM signal to the dash is from the igniter: a dirty great spike, not a nice clean square wave, though......!

I knew that NE was rpm and G position, didn't know it was AC (makes sense though) or about the voltage varying so much as for the rpm signal, well I suppose a square wave was wishful thinking
Handy to know though, good info

[JMR_AW11]

About 9 years ago I reverse engineered my mk1b ECU but I've not looked at this ECU in detail for ages.

So I've forgotten a lot of the detail...

But I think this ECU has a 24 tooth Ne wheel in the dizzy so you will get 12 Ne pulses per single engine rpm. Ne is the signal the ECU uses to work out the engine rpm accurately. These numbers might be wrong because I looked at this so long ago...

As Greg says the signal looks a bit like a train of sine wave shaped pulses and you get 24 of them for each camshaft rotation. The signal is a few volts peak to peak.

However, I don't recommend tapping into the Ne or G wires as your tap can act like a noise gateway if it is ground referenced to the chassis or battery etc. You might get away with it if your tap is really high impedance and you connect close to the ECU but I wouldn't do it as a long term feature.
I also recall that there is a tiny voltage offset on the common G- wire and you don't want to attach anything to Ne or G that might pull this offset slightly and/or leak noise into G G- or Ne.


Your other alternatives are the Ig- connector in the service connector or the IGT or IGF ECU wires.

I'd leave IGT alone as this is the pulse that defines the ignition timing.

IGF is a spark confirmation signal from the Ignitor and this is a pulse at normal logic levels. IIRC you will get two of these pulses per engine rpm assuming you have a healthy (sparky!) ignition. BTW If the ECU fails to see anything on IGF, i.e. it misses several spark confirmations then it assumes there is a big problem and it cuts off the fuel injectors.

You can safely tap into IGF but the IG- in the service connector might be your easiest route. I don't know how dirty IG- looks like on a scope though..

[ulysess1966]

That's interesting stuff, there must be some signal processing/cleaning in the ECU then on the sine waves. It wouldn't be too difficult to feed them into a high impedence voltage follower/conditioner and pass them through a frequency-to-voltage converter to get a proportional voltage. The voltage signal would be taken and then into a microcontroller. Purists would write some code to count the pulses, but it's often easier to just read a voltage when you need it.

I have an Apexi S-AFC/S-ITC combination hooked up on my car, but got to thinking that I could do the same job as these and more by building my own unit, but at the minute I'm just thinking through some ideas.

[JMR_AW11]

That's interesting stuff, there must be some signal processing/cleaning in the ECU then on the sine waves. It wouldn't be too difficult to feed them into a high impedence voltage follower/conditioner and pass them through a frequency-to-voltage converter to get a proportional voltage. The voltage signal would be taken and then into a microcontroller. Purists would write some code to count the pulses, but it's often easier to just read a voltage when you need it.

I have an Apexi S-AFC/S-ITC combination hooked up on my car, but got to thinking that I could do the same job as these and more by building my own unit, but at the minute I'm just thinking through some ideas.

I think it detects the 'zero' crossing point on the waveform. This point has a slight offset from 0V but the ECU has to find this quite cleanly in order to determine the exact engine angle and also to get steady rpm readings.

If the waveform gets any noise or jitter then the ECU will not be able to set the ignition timing as accurately.

I have an Apexi S-AFC/S-ITC combination hooked up on my car, but got to thinking that I could do the same job as these and more by building my own unit, but at the minute I'm just thinking through some ideas

I'm not convinced you will gain much unless you do other mods to the car.

I remapped my stock ECU fuel and ignition maps and it did run a bit better and it held onto power longer up the rev range but I think you would notice a bigger difference comparing a cool day vs a muggy day on the stock ECU. The difference really is only marginal.

[ulysess1966]

The car is an SC and has every other mod known to man on it - big pulley, induction air filter and a mental Janspeed exhaust. Trimming the ECU was the only thing left apart from rebuilding the whole car in carbon fibre. I got the 2 units for about £150 for the pair some years ago direct from Japan.

Now any gain does remain to be seen because I've only fiddled with them, but once I have the car back on the road I'll have a chance to tinker properly.

My thinking was to build a combined AFC/ITC unit using a microcontroller with a more detailed map in it. I know that re-mapping the ECU would be better, but I'm not taking the ECU apart no matter what. Piggy-backing is okay though.

[JMR_AW11]

The car is an SC and has every other mod known to man on it - big pulley, induction air filter and a mental Janspeed exhaust. Trimming the ECU was the only thing left apart from rebuilding the whole car in carbon fibre. I got the 2 units for about £150 for the pair some years ago direct from Japan.

Now any gain does remain to be seen because I've only fiddled with them, but once I have the car back on the road I'll have a chance to tinker properly.

My thinking was to build a combined AFC/ITC unit using a microcontroller with a more detailed map in it. I know that re-mapping the ECU would be better, but I'm not taking the ECU apart no matter what. Piggy-backing is okay though.

Didn't realise it was an SC, sorry.

The info above was for the UK NA mk1.

Is your SC the AFM version or MAP?

[ulysess1966]

Sorry for the delay, haven't been online.

The engine is the AFM type with the 10/18/24 pin ECU (I think, something like that anyway).

I downloaded some Toyota training materials and had a good study at them. The IgT (IGT) signal from the ECU looks like a clean TTL signal straight out of the computer. Connected to an LM2917 that should generate a voltage proportional to engine speed.

[JMR_AW11]

Sorry for the delay, haven't been online.

The engine is the AFM type with the 10/18/24 pin ECU (I think, something like that anyway).

I downloaded some Toyota training materials and had a good study at them. The IgT (IGT) signal from the ECU looks like a clean TTL signal straight out of the computer. Connected to an LM2917 that should generate a voltage proportional to engine speed.

Hi the IGT signal sets the ignition timing on the edge of the IGT pulse. I don't recall it being TTL level but it is a few volts.

I don't wish to appear negative but this ECU has a few issues that make it less than ideal for piggy adaptors.

First up is the AFM signal. This works backwards and a lower voltage is more airflow. Also the sensitivity goes UP as the voltage goes down.

i.e. near full flow it only takes about 50mV to hop across several mapping points in the ECU mapping. So you have to be ultra careful how you tap into the AFM wire and also how you ground your new wiring.

The loom is designed such that the AFM ground (E2) return feeds direct back to the ECU and this is done to reduce noise on the AFM measurement.

If the E2 ground wire were to touch the engine or chassis ground at its far end then it would make the ECU measurement of the AFM very noisy. So you have to make your connections close to the ECU, not close to the AFM.

Hope I've explained this OK and apologies if you knew it already




Also the AFM ECU has limits programmed into the code for the AFM measurement. Above a certain AFM reading the ECU ignores the AFM and puts in a fixed ceiling value. Not good.

It does the same thing for computed engine load. i.e. it uses rpm and AFM to compute load. If the SC is ON it consults a lookup table of ceiling values for computed load.

if the computed load is above the value in the table it uses the ceiling value in the table. Again this means it is effectively ignoring the AFM.

The lookup table is rpm dependent and the ceiling value goes down with reducing rpm in the table. This is OK on a stock car as the car probably never hits the ceiling limit.

However, this table causes the lean condition on some cars wilth oversize pulleys running more boost across the rpm range.

I don't see what you can do with a piggy adaptor to overcome this unless you spoofed the air temperature to get more fuel?



Sorry that the graph isn't scaled or anything but the graph below is the LOAD ceiling vs rpm table taken from a JDM SC ECU. The map is the same in all the SC ECUs I looked at.You can see the trend towards leanoff at low rpm.

Usually, the LOAD ceiling value is a fixed (high) value independent of rpm. This has been the case on all other AFM ECUs I looked at. But the SC is different. I don't know why they did this. Maybe it was to prevent easy overboosting via a pulley that would generate levels of torque that make the engine/drivetrain unreliable?

So they put the spoiler map in? That's just a guess...

[ulysess1966]

This was the thinking behind the famous Grunt Box, which uses the cold start injector to overcome the problem. I fancied building my own version with a couple of extras thrown in.

All the elctronics would be tapped right at the ECU and signals run through high impedence buffer stages. I was taking some readings tonight as I finally went and got some petrol for the car (in a jerry can!).

[russbost]

Apologies for reviving an old thread & also for being a little off topic, but some of you guys obviously know the distributor/ecu very well. First post so please bear with me!

I have a F reg MR2 4AGE engine in a kitcar - at some time in its life the plugs that connect the loom to the distributor have been discarded & the wiring soldered together, I have strong suspicions that they may be incorrectly connected. Sometimes when you start the car it runs completely normally, absolutely fine, but sometimes it's as tho' it has started in a different "ignition mode" - it sounds as tho' timing is really retarded & if you try to rev, it won't rev out, just sounds really flat, turn it off & restart & everything is fine again, it rarely takes more than a couple of starts b4 everything returns to normal. All other wiring & sensors are standard, hence my suspicions of this distributor wiring. The odd thing is that the wiring colours don't match - from the loom I have 3 wires a black, a white & a red, the black & white go directly to the ECU, (but don't match the pins on a wiring diagram I found!)
unsure at present where the red goes. At the distributor the wiring colours are red, blue & white, the way it's currently connected (dist to loom) is red to black, white to white & blue to black.
Any advice most welcome