Off-topic electronics question.

[hoshy]

Hi guys, just wanted to ask here before I sign up to any electronics forums etc... and it's for an e30 project.

How can I measure the input impedence of the air meter input on the ecu?

Can I just throw the meter across it? does the ecu need to be powered up? I assume I would do it without the afm connected?

[E30cabalan]

As far as I know its just a potentiometer, so any decent multimeter should do it.

[hoshy]

I wanna measure the ECU impedance mate, not the afm itself.

[jkarran]

The impedance should be fairly high, perhaps into Mega Ohms, as long as your meter can cope with that it should work if you conect it between the sensor ground and the ecu input pin. I'd do it with the ecu powered and off, see if there's a difference.

An alternative would be to measure using a volt meter, an AA battery and a 1M resistor in series, again connected between the sensor ground and ecu input. From measuring the battery voltage and the voltage at the ecu input fed from the 1M resistor you can calculate the input impedance.

Basically that's all the meter does anyway in resistance mode. I'd try the meter, theres less maths and bits needed.

[hoshy]

Nice one dude. I'll give it a go.

Just need to figureout if I'll need to buffer the output for my maf conversion or not. I understand that if it's a high impedence then I shouldn't need to?

[Ant]

high imp wont require any sort of flyback mods M8, TBH as you're replicating the AFM signal only, I think you'll be fine with the pic output straight in.

word of warning though, not all the AFM equipped cars run a 0-5V output, some are unregulated battery output!

MAF should be an easy swop 12V in from fuel pump feed
0/5V output to AFM input on Motronic
IAT wires duplictaed and NTC sensor added to Airbox or trunking
wire to ground

the pic is the signal converter only, I assume you'll be remapping the pic to achieve the stock AFRs ?

anyone tried the MAF from a 750 ? they have the signal converter built in and can be tweaked for idle CO, same early unit is on M50 non vanos e36/34 2.0litres BTW

Hoshy, have you considered the PIC for an alpha N driver ? same load output range as afm 5V baby !

[hoshy]

high imp wont require any sort of flyback mods M8, TBH as you're replicating the AFM signal only, I think you'll be fine with the pic output straight in.

>>> Cool. I'll try withour first. Hopefully get away with it.

word of warning though, not all the AFM equipped cars run a 0-5V output, some are unregulated battery output!

>>> cheers, could have got meesy

MAF should be an easy swop 12V in from fuel pump feed
0/5V output to AFM input on Motronic
IAT wires duplictaed and NTC sensor added to Airbox or trunking
wire to ground

>>> fuel pump feed you say? cool. I was gonna run an extra fused live across, never liked the look of gettin into the fuse box.

>>> *dummy mode on* NTC ???

the pic is the signal converter only, I assume you'll be remapping the pic to achieve the stock AFRs ?

>>> yep. and hopefully some fancy bits for tuning the AFR and lean cruising.

anyone tried the MAF from a 750 ? they have the signal converter built in and can be tweaked for idle CO, same early unit is on M50 non vanos e36/34 2.0litres BTW

>>> Not me, but as you know mate, this is 50% technical challenge and... 50% designed to work with any 5v maf.

Hoshy, have you considered the PIC for an alpha N driver ? same load output range as afm 5V baby !

>>> Nope... I've heard the name Alpha N around but I don't know what it is.... ?

Answers inline.

[Brianmoooore]

don't worry about opening up the E30 fusebox. Theres loads of wires connected to the top half of course, which makes it stiff to pull up, but they are all strong and well fixed. Never had anything come apart in my hands, and I've pulled hard at the top halves a few times, to try and get that extra little bit of access to work inside.
Three self tappers hold the top on, one outside, two inside, one obvious, one under a relay.

[TheDutch]

Hoshy, have you picked yourself up a donor MAF yet? I'm interested in hearing about your mapping methods, I still like the Heath-Robinson leaf blower technique.

the pic is the signal converter only, I assume you'll be remapping the pic to achieve the stock AFRs ?

>>> yep. and hopefully some fancy bits for tuning the AFR and lean cruising.

How precise does this tuning need be? Is there not a risk of running the engine too lean? I would want to mimic the original AFM as accurately as possible.

Also, if the MAF output is correlated against the AFM output, is this valid at all temperatures?

Lastly, you should be fine without buffering the signal. If you measure the resistance across the inputs with a multimeter you can calculate the current needed and compare this to the PIC data sheet to be certain.

[hoshy]

don't worry about opening up the E30 fusebox...

Cheers Brian.

[hoshy]

Hoshy, have you picked yourself up a donor MAF yet? I'm interested in hearing about your mapping methods, I still like the Heath-Robinson leaf blower technique.

YEs I have two donor MAFs, both 5v Bosch devices, I believe they were used on Vauxhall vehicles.

the pic is the signal converter only, I assume you'll be remapping the pic to achieve the stock AFRs ?

>>> yep. and hopefully some fancy bits for tuning the AFR and lean cruising.

How precise does this tuning need be? Is there not a risk of running the engine too lean? I would want to mimic the original AFM as accurately as possible.

I'll be using 8bit resolution, if this turns out not to be enough I'll try to implement oversampling, failing that I'll upgrade to an external DAC.

The idea is that the PIC will track average air-flow and if it doesn't change, within a tolerance to be decided, start to lean back the AFR. I believe some modern cars do this to help with economy and it works quite well. So I hoped I could do something similar to really help with the motorway eco. Hopefully I can do this without adding a lamda sensor, but obviously I'll have to be very careful in testing, if my code is broken then I could destroy my engine... lol.


Also, if the MAF output is correlated against the AFM output, is this valid at all temperatures?

At the moment I'm not worrying about air temp. I will do of course, but the temp will just trim the map. I know these MAFs do have an on board temp sensor but I've yet to play with it.

Lastly, you should be fine without buffering the signal. If you measure the resistance across the inputs with a multimeter you can calculate the current needed and compare this to the PIC data sheet to be certain

Yeah I'll have to have a look. I know this PIC can drive small motors from the PWM output so hopefully I can get away with it.

.