I have an 85 camaro with a bad maf, so I read that in 85 they used an analog signal, but they switched to fm in 86-90, but in 94 they switched back to the analog. So my question is that could I use a 94 maf on an 85?

 

You have the years correctly listed, but the applications backward. The 1985 TPI used a Delco/Bosch FM (digital) MAF operating between 30-150 Hz. From 1986-1989 TPIs used a Bosch analog voltage (0-5VDC) output MAF, while the V-6 MPFIs retained the digital MAF with resolution from 30-150 Hz. Later V-6 MPFIs used a MAF with a 0.1-5 KHz output. In 1993 the V-8s reverted to a digital (FM) MAF but at an entirely different frequency range than the earlier units, operating between 0-12 KHz.

To use a later digital MAF on an '85 TPI you would have to condition the signal via a decade counter and separate modulator to convert the signal to something the ECM could use. Frankly, the digital concept is better since it is more tolerant of voltage fluctuations, poor connections, and can allow for a noisy signal circuit without false readings. However, constructing the D-D converter would require a bit of engineering and tuning.

 

For fun, I recently modified an 85 870 ecm to use a modern analog MAF (from another platform's 5th gen). It sure looks like the tpi 870 ecm maf input (pin B11) is an analog circuit to me.



http://www.exatorq.com/ludis_obd1/1227170sheet3.gif

http://www.exatorq.com/ludis_obd1/1227170sheet2.gif

And the trace leads to an A/D converter like this one:

http://pdf1.alldatasheet.com/datashe...C/ADC0829.html

So my conclusion is that the 85 tpi MAF is analog contrary to what has been posted in the past.

Interestingly enough Ch7 is really a dual purpose pin, so it could serve as a digital input, but I don't think that is the case in this application.

It is possible that f-bodies used another pin and/or different hardware, but I cannot verify.

FWIW: My modified 870 ecm with an analog maf and programming changes has been tested successfully on an 85 Corvette in California, since I didn't have access to a local car for testing.

 

Cuervo Kid - What model is using an analog MAF these days? I would have guessed that most manufacturers would be using digital or an analog current loop by now just for the signal integrity. An analog voltage MAF with a bigger body might be an interesting experiment.

It seems that the diagrams you linked are for a 1986-89 '170 ECM used on the V-6 MPFI cars, and those may have variations from the '870 ECM. We know the '870 ECM was altered for TPI use by the addition of the MAF module (16050021) strapped to the back of the ECM.



I've read previously that some people have installed a later analog voltage MAF in the FM application and made it run, and I have wondered if the front end input filtering somehow. The output of the Bosch FM MAF used a common emitter pull-down to switch the 5VDC into an FM stream. If the circuit sees the varying analog voltage as some sort of RMS value that it can interpret it might work but probably not very accurately or reliably. These early 160 baud ECMs were not very good at reporting errors and codes, and certainly weren't quick about it.

The schematics you linked show the '870s B11 input employing a voltage divider pulling up the circuit to a predetermined level rather than having a raw input (not what I would expect in a 0-5V analog voltage application). The FM MAF unit output is arranged like this:




When I checked into this about 15 years ago I was able to measure the input from the MAF as a relatively square wave instead of an analog voltage value. Excuse the quality of the photo, but it was from about 2001. Channel A was the 8MHZ on board clock and B was the J211 input at transition of about a 5V P-P:




If you really want to verify whether the MAF is analog or digital, just look at Bit2 of Word $B4 (LC014: FCB $B4) in the ROM mask to see if it is on for analog or off for FM.

 

Interesting stuff, thanks.

The bin I used for this project was based on HLH. LC014 is 0x39 or 100111 and was unchanged. It is clear that the hardware can support both Vmaf and Fmaf inputs since there are separate circuits for both of them.

The B11 circuit in the 870 is just like B12 Vmaf input on the later 165 Ecm with a pullup resistor intended to pull the signal high in the event of an open circuit.

The photo is from my 870 ecm. The 870's actual B11 circuit also matches the schematic that was shown.

It is possible that different bins were used in production in 85 which may have supported either Vmaf or Fmaf, but I would expect the input to change from B11 (Vmaf) to the Fmaf (pin number uknown).

The dual purpose nature of the A/D converter pin, also opens up some possibilities to handle either input differently in the software.

My experience strongly supports the analog premise, but there may be more to the story.

 

Also Interesting to note the use of F or H in the bcc naming from 1985 bins for both y and f cars. Perhaps there is a pattern here...

 

Supporting data log from the subject test vehicle provided. Fuel trims running slightly lean, but pretty good for the first calibration.

For 85 tpi cars hanging on with the 870 ecm and wishing to upgrade to a modern sensor, this approach may be a good option for you.

No more burn-off, potential to support significant horsepower increases due to additional sensor range with advanced tuning, potential HP gain due to increased flow with larger housings, just to name a few advantages.

 

Vader, Tequila boy = Keepers of the flame (thirdgen style)