AUM to ACXT Problems with 4th gear switch hooked up - Third Generation F-Body Message Boards

elcam84 · 10-28-2009, 03:01 PM

Ok I haven't burned chips for anything in quite a while and I'm getting back into it.

I have a goofy problem I haven't figured out yet. Engine is an 86 305 TPI.

I have been running the aum chip for quite a while and it has some obvious drivability and starting issues so I popped in my ACXT (87 305). Now it drives pretty good for the most part though has some hot start issues. The problem came when I hooked up the 4th gear switch(like it should be for the 87).

With the switch disconnected the ECM thinks 4th is engaged all the time. It actually runs well this way except that the tcc engagement is way too early as in just as you start moving unless you are in the throttle enough to delay it.

With the switch connected the engine is lower in power and the tcc is erratic. The tcc kicks in and out constantly and only engages at very light throttle.

I'm not sure what's going on. Both chips are Gm chips and the AUM is the latest one GM made. Any thoughts.

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RBob · 10-28-2009, 03:54 PM

AUM is an early release for the '86 MY. It was released in mid '85.

AXCT is a later release for the '86 MY, sometime in '86.

Note that early on GM had the TCC locking up way too soon. There are several PROM releases to correct this issue. Much had to do with federal drive-by noise tests. Since you are chip burning set the TCC lock ups to the way you drive.

There is an option flag to set whether the 4th gear switch is active high or active low. GM used both types of switches. So get that correct first. Then attack the TCC lock up tables. Although for some reason I don't believe that the $32 mask has that. I know that the $6E mask does.

See the Tuning Guide Book sticky, chapter 5, '8746/'7747/'8063 section for a write up on how the TCC tables work. It makes a world of difference in how a car drives with a decent set of lock/unlock tables and minimum MPH for lockup values.

RBob.

elcam84 · 10-28-2009, 04:19 PM

I have a few $6e bins but I'll have to figure out which one I can use for my app but for now I'm temporarily sticking with what I have.

What's really wierd is with the switch hooked up and cruising in 4th the ecm shows that it's in 4th but it will not lockup.
For some reason I'm only getting steady predictable lockup (albeit very early) with the switch disconnected.(I unplug the wire from the tcc connector). I have seen a couple of phantom blips of the flag flipping on and off even when the switch is totally disconnected. Beginning to wonder if the problem is the ECM... It has always done strange things with the switch hooked up. Hmm

elcam84 · 10-29-2009, 02:20 PM

Figured it out after getting my osilliscope out. It's the usual prob I see with GM cars... The ECM is bad... I usually check that first as it's real common until the PCM era.

RBob · 10-29-2009, 05:37 PM

Quote:

Originally Posted by elcam84

Figured it out after getting my osilliscope out. It's the usual prob I see with GM cars... The ECM is bad... I usually check that first as it's real common until the PCM era.

I was wondering what you found bad in the ECM. For some reason the '7165s seem to have issues with reliability. Not sure why.

One thing I see on the C3 ECMs is the 16-pin DIP quad driver chip. They can go in a spectacular fashion. Can't miss 'em.

RBob.

elcam84 · 11-15-2009, 05:06 PM

Back when I still had a CCC Q-jet I replaced a couple of chips when they went. You could hear the chip pop and break into pieces with the engine running. I really hated those Q-jets...

Anyway when I started poking around at the 4th gear switch feed I was getting really goofy things. Voltage all over the place which would explain why the data stream showed the switch intermittently connected and disconnected.

I did a quick test with the terminal out of the trans connector and the scan tool hooked up. Watching the 4th gear switch flag I manually grounded the wire. It flipped but then started flipping back and forth. Also the terminal got hot in my fingers after only a few seconds so it was running way too many amps through it...

I have been running with the wire disconnected and finally got around to ordering an ecm yesterday... I have had allot of trouble getting the scantool to access the data stream and have had some other goofy things happen like stalling at idle which were never an issue before. It's acting like the ecm is failing at an amazing rate now which is common when they finally go.

I did however figure out what caused it... Way back when I put the new trans in I couldn't start the engine cause a fuse kept blowing. That fuse also feeds the tcc so I unplugged it and all started and ran fine.
When I investigated the tcc prob I found that the shop (monster motorsports in FL)had put the internal connector on 90* to where it should have been. Once put on correctly all was fine.
My 86 chip didn't use the 4th gear switch so I pulled the wire out of the connector. Now that I'm using a chip that needs that switch I hook it up and nothing works right. It's only a problem now cause I started using that part of the driver and well it isn't right cause it was fed 12v during that trans issue and fried.

With the old chip that part went unused and didn't receive a ground but as soon as I start using it again it has really crapped out.

So I should have an ECM sometime this week and everything should be running allot better. I will do some more probing of the ECM once the new one is in there and take pics if there is anything that spectacularly failed.