Problem adding MAT to $0D TBI 7427, Not pulling timing.

Most of you guys have seen that I am successfully running boost on a 7427 with a 2 BAR map. The truck is running great. I added a MAT sensor to the setup so that when the air coming in gets hot, I can rely on the sensor to pull some timing. For some reason that function is not working. The MAT itself is working, because I can log the temp. This "MAT & CTS Blend Multiplier vs. Airflow" is working as well. I have the MAT sensor flag set.

I have the MAT Bias set to 9.8

As a test I populated the entire "MAT Spark Advance Compensation vs. MAP vs. Temperature" table with -10 degrees, and drove the truck. My timing showed the same as what is in my Main Spark tables. the ECM never pulled any timing base on MAT. I am trying to figure out whats going on. I did find a flag that the CPI bins have set that the TBI does not, that has to do with the MAT, I tried both ways, didn't make a difference. Its at 442F "LD SELECT, ( 0 = VACUUM 1 = RPM) MAT". Could it be something with the Negative or Positive multipliers?

I just noticed the CPI bins do not have anything in the Main spark bias? They are zeroed out.

Cool build...wish i could help..
I did notice you had a malf code 25 Mat HI set in the log, I don't know if this would cause the mat compensation not to run..or when it was set.
as for the biases and how they all play together :mad1:

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Originally Posted by CFI Z51

Cool build...wish i could help..
I did notice you had a malf code 25 Mat HI set in the log, I don't know if this would cause the mat compensation not to run..or when it was set.
as for the biases and how they all play together :mad1:

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Huh....I didn't even notice that. I don't have a SES light. I will take a look at it. I zerod out the MAin Spark Bias, which I knew would screw up my timing tables. So I copied them first, zeroed the bias, saved, then copied the table back in. Set it up like the CPI bin. I don't think it did anything.

I don't know where everyone is on this forum lately, I am hearing a lot of crickets chirping. Sometimes I wonder if I am one of the only ones left messing with these old PCM's. Probably should have upgraded the 411 LS1 computer and use my HP Tuners.

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Originally Posted by 96lt4c4

I don't know where everyone is on this forum lately, I am hearing a lot of crickets chirping. Sometimes I wonder if I am one of the only ones left messing with these old PCM's. Probably should have upgraded the 411 LS1 computer and use my HP Tuners.

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I don't think its lack of people on the site, its a shortage of people who know enough to help on the more advanced topics. I have been watching some of these threads as I am interested in using the MAT also when I go to MPFI.

Every time the MAT subject comes up on here it goes about 4-5 post then dies. I'd really like to get in working in my truck but the first time I tried it my BLMs went nuts. Wish I had time to figure it out...

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Originally Posted by PJG1173

I don't think its lack of people on the site, its a shortage of people who know enough to help on the more advanced topics. I have been watching some of these threads as I am interested in using the MAT also when I go to MPFI.

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Yeah, I thought about that too....I guess I am in uncharted territory here...LOL. Might take me a while to figure it out. Not sure why I am throwing a code and the light is not coming on though. The sensor seems to be working and reading air temp. Maybe that code 25 is part of my problem.

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Originally Posted by brian617

Every time the MAT subject comes up on here it goes about 4-5 post then dies. I'd really like to get in working in my truck but the first time I tried it my BLMs went nuts. Wish I had time to figure it out...

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I have not seen that. Do you have a MAT or IAT? I have a MAT in the intake.

I know when i switched last fall from a 7747 to the 7427 on my tbi vette i had to 0 the altitude bias in order to get the timing table anywhere near reasonable ,(missing 10* degrees of timing.. ) 30*in the bin(and dash) only read 20ish* on the crank, the timing light doesn't lie.

Also i dont see pe timing added to tunerpro dash or history tables in my logs.? pretty sure its working , pings if i add more then 4*, no knock sensor..

All im saying... you wont know for sure untill you put a light on it.

FYI Im a noob when it comes to EFI ...no experience with tuning mat...but been turning wrenches for 30 years.


(funny you say old PCM its worlds ahead of the 7747 that ive tuned in the past)

Using the diesel IAT in the #8 runner wet manifold TBI.

I added a MAT to mine (7427, mpfi) but I could never really get it to do anything worthwhile. I copied in the CPI parameters and fiddled around with them some, but most of the time, it never really did anything. It seems like for some reason I was having some type of issue that went away when I disabled it. So I just left it disabled and said heck with it. Its still installed in my intake and hooked up, can even still see the temp in the data stream, I just have it disabled in the bin.

Like said above, your probably in uncharted territory, as I do not think many people have fooled around with adding a MAT to the 7427 non cpi bins.

Well, I've kinda given up on the "old" PCMs myself because of shit like this. They are supposed to do something and don't, or the results are the opposite of expected. I know if this gen VI timing cover fits on my gen V like I believe it will, I'm going to retrofit cam and crank sensors in and run a full sequential coil on plug setup.

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Originally Posted by 1BadAction

Well, I've kinda given up on the "old" PCMs myself because of shit like this. They are supposed to do something and don't, or the results are the opposite of expected. I know if this gen VI timing cover fits on my gen V like I believe it will, I'm going to retrofit cam and crank sensors in and run a full sequential coil on plug setup.

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Did you ever figure out while your 80E was shifting soft? I built a 4L80E for a buddy of mine. It was a 91 model tranny, and he was going to run it on a LS1 PCM in his 68 Camaro, LS2 402 Stroker. Installed the tranny and shifted soft, checked line pressure and it would not go over 150 psi. The problem is in 1994 they changed the Force Motor Solenoid and valve body, it is not compatible with the older electronics, vice versa, new electronic not compatible with old solenoid, which would be the case in your setup. We changed the valve body and Force Motor to a 94 version. Fixed it, firm shifts, line pressure shot up to over 250-275 psi.

Off topic but thought I would mention that, so if you go to the LS1 style PCM, on your 91 4L80E, it will still shift soft, until you change the valve body. You can not just change the solenoid either. They operate at different frequencies.

Part of the reason you get unexpected results is because the code in the 7427 is more complex than the simpler machines like the 747. This complexity increases on an order of magnitude when you get to OBDII and I can promise there are plenty of cases floating around where a newer pcm doesn't do as expected, either.

Sorry for the crickets. You're not the only one left playing with the old pcm's. I just don't have the time that I used to.

CPI trucks use air temp as a compensation. IAT or MAT, it's really no different from the ecm perspective. Sensor generates data, code responds (or doesn't). First, ensure the air temp sensor code is not a current code. ECM will fall back to default temp if there's an error. Then look for minimum temps to enable / disable compensation, or timers that have to expire before compensation begins. Are you certain that the definition file you're using is configured to display timing correctly? Did you notice any performance changes?

Quick edit: It looks like there may be two tables for MAT spark correction. One is a $44A4 and one is at $4493. Not sure how they work... hac says one is negative advance and one is positive. Also a table ID'ed as "base MAT SPK ADV CORR" at $442F. Data obtained at $0041, bit 1 will indicate if MAT spark correction is negative or positive... can be viewed in data stream by altering definition file.

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Originally Posted by 1BadAction

Well, I've kinda given up on the "old" PCMs myself because of shit like this. They are supposed to do something and don't, or the results are the opposite of expected. I know if this gen VI timing cover fits on my gen V like I believe it will, I'm going to retrofit cam and crank sensors in and run a full sequential coil on plug setup.

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If I could afford it I would be right there with you, but until then I gots what I got....