Electronics gurus, Im going to be putting together an inteface box for my MAF. Basically, all it will do is provide clean +5 volt power so the MAF can generate its pulsetrain to the PCM. So far, I have a 10 ohm resistor and a 1000 uF cap acting as a low pass filter, a 30V, 1.5 kVa TVS, diode for reverse input protection, and a HD 5V regulator with some damping caps recommended by the manufacturer. The low pass filter provides power to the regulator while the 1.5 kVa and diode supply the regulator section and the MAF with cleaned power.

Is there anything else I should add? Ill see if I can post some schematics later. Right now it only exists as a cad drawing.

 

I'm no EE, but if was looking to design something like that, I'd look at Ludis's site, and the DIY-WB.

 

It occured to me that Im probably making this harder then it needs to be. I can probably just backfeed one of the 5V references to the MAF switched output. Theres probably only going to be around .5 mA draw, so I couldnt see it being a problem. I recall doing this on the bench and having no issues.

 

It doesn't have to be so complicated. A twisted pair wire from one of the sensor reference supplies should work fine. Since the load won't change, there is no need for the capacitor. All you want to do is cancel any noise and the twisted pair will do that. HTH

 

Is noise on the 5v power known to be a problem for those MAFs? I would imagine the sensor itself has some built in suppression, as typically the power wires don't have any shielding (not even twisted pairs) in most automotive applications that I've seen, just the signal lines (especially for crank signals with VRs and lots of teeth, or very high impedance sensors).

 

Dimented, in your original post I would change the TVS to a lower voltage, say 24 volts. The reason is that most of the 3 terminal regulators are limited to 36 volts on the input. The reverse input protection diode needs to be before the 1000 uf electrolytic (polarity on the electrolytic).

As for the other power ideas, it would depend upon what the +5 volts is for. And, if the MAF is analog or frequency. If an analog MAF then the +5 V reference would be OK. If a freq MAF I wouldn't use it. No need to put noise onto a reference voltage to be used for analog devices.

Two ideas:

If the +5 volts is not critical as far as the actual voltage, then use a diode/resistor shunt arraignment. The resistor feeds in the +12 v ignition power, with a 4.8 or 5.1 V diode shunted to gound.


If the voltage is a reference and needs to be 5.0 volts. Can use an LM431 shunt regulator. Same idea as the zener above, but has a bandgap reference for voltage stability.

RBob.

 

I am trying to understand what you are doing.

Are you attaching an LT1 style frequency MAF to an P4 ECM? If so, you don't need any interface box. Those MAFs are designed to run off 12 volts. Just hook it to an +12V ignition source, a good ECM ground, and hook the MAF output to the desired ECM input.

Scot

 

The only reason I really need the 5 volts is to comply with whats used by GM in the later LT1/LS1 PCMs. +5 volts with a 10 kOhm resistor. My guess is that the only requirement is that the PCM and MAF share the same ground so there arnent any ofsets that will cause the counters not to trigger.

Haulin', are there any unused 5 volt references? The only ones that will be in use are the TPS and MAP. Does the linear EGR have a 5 volt ref? It seems to have some form of a position sensor in it from the code.

 

DP...

 

Originally the output to the ECM was analog with code similar to what was used in the $6E's, with a converter for the LS1 MAF. Clean power was there for the converter, so I fed the MAF from there as well. Now that I'll just use one of the ref's, this will no longer be needed.

Does it really trigger like that?

 

Mine does.

 

Are you using an LT1 PCM?

 

DOH! The TBI truck PCM has a 5 volt ref on the freq input line as well. No need for any of this. Thanks Scott...

 

I wish everything was this easy. It really can just plug into the TBI PCM. Works great on the bench.

 

Glad ya got it figured out.