700r4 w/tbi: What tells the ECM to lock up the converter?

[eagle mark]

You've really covered this. From my end I would have had laptop or scantool to tell actual speed and TCC was locked, there both covered. If showed locked? I would have gone through what you did, then put another ECM in.

Have you asked the tranny shop if it is possible to have a TCC wiring etc.. issue when ALDL will lock up TCC? They will probably say scantool to see if ECM is commanding lock up, if so replace ECM...

 

[dyeager535]

Well with what Scott said, and the OP being on the second rebuild in this truck, is it possible the internal wiring is different enough that he can force it to lock up, but that it's not wired internally as it should be for his year?

Been awhile since I've dug into an automatic, but on these early ones, isn't all the wiring (TCC and switches) visible once the pan is dropped? If you can get the wiring diagrams for the '89, and someone else concurs with my idea, I think I'd drop the pan and verify that the internal wiring is correct.

Maybe this is a common problem on the '747 ECM, but Mark I think your experience is the same as mine, the ECM is rarely the problem. And I can't recall ever hearing an ECM that the TCC control failed on. Not saying it can't happen, just saying ECM is normally at the bottom of my list.

 

[eagle mark]

That's why I suggested he ask the tranny shop.

Without seeing data if TCC is happening, I'm at a loss... he can get TCC lockup from ALDL port...

At this age ECMs rarely fail without water or human intervention...

 

[dyeager535]

OP, go buy an ALDL cable and download tunerpro 5!

As to chain auto stores, I don't believe they have the scan tools, only code readers. Could be wrong, but had a buddy take his vehicle there and that's all they could do for him.

Can always check ebay for an autoxray XP240 if you like, but nowhere near as functional as Tunerpro.

 

[eagle mark]

He's got a Mac, will TunerPro run on a Mac? Some Macs will run Windows programs under a compatbility type program...

 

[nutt7]

Hey guys,
Yes, a scan tool would be nice right now! Ive got my eye on a few. You might be on to something with the wiring...I looked at countless diagrams. Most were identical...The first was my 89 wiring diagram (thanks for offering the blazer diagrams).

In one of my previous posts (after dyeager mentioned it), I thought all wiring was good besides the possibility of the high gear switch being n/c and not n/o like it is currently. Should've investigated further. Referring back to my 89 diagrams, the switch shows n/c! I swear it said n/o!...must have been another diagram.

I can wire a relay to reverse the effect, although I think I will rig up a test to be sure. Best case would be the tranny shop replacing it.

 

[dyeager535]

I'm sure you will let us know how that works. Be real nice if it's a pretty simple fix. I mean realistically it has to be, everything else works right!

 

[nutt7]

Still nothing. I even pulled the high gear wire pin from the ecm to make it look like I was in high gear. I have a feeling its the VSS, but who knows. I couldn't find a definite way to test it, but I know it is doing SOMETHING. My auto xray will be here in a week or less, we will see then...I will post back (hopefully with a fix). Thank you for helping so far, I really do appreciate it

 

[dyeager535]

VSS luckily is an easy test with a scanner etc., if the ECM is reporting speed correctly, good to go!

Just a random thought that popped into my head, is the Park/neutral switch working properly? I don't have a P/N switch, but I'm kind of surprised that isn't mentioned/listed as a criteria for TCC lockup. Although pressure switches probably do the same thing, I would imagine electrically locking TCC out of P/N (through the ECM P/N input) to be a good idea.

 

[4X4HIGH]

Still nothing. I even pulled the high gear wire pin from the ecm to make it look like I was in high gear. I have a feeling its the VSS, but who knows. I couldn't find a definite way to test it, but I know it is doing SOMETHING. My auto xray will be here in a week or less, we will see then...I will post back (hopefully with a fix). Thank you for helping so far, I really do appreciate it

1989 doesn't use a VSS but rather an optic eye that attaches to the back of the speedo head. One of the wires is for cruise control and another is the speed signal to the ECM which if you had a scanner you would know if it was working as you could see the speed increase as you drive and compare it to your speedo reading. Those optic eye boxes were used until 1989 on the TBI trucks then starting in 1990 the DRAC module was used instead. You should be able to find an optic eye box at the local wrecking yard pretty easily.

 

[nutt7]

So I got my scanner. A quick drive indicated that my VSS is fine. My ECM is commanding TCC lock-up, which means my high-gear switch is fine too. It commands lock up at all the right times. This must mean that the issue is either in the wiring to the ALDL/solenoid or a bad ECM, right? Am I missing something else?

Remember that I can get lock up when I groung the F port, and it lets go when I hit the brake. So all is good from the ALDL down to the tranny.

 

[dyeager535]

ALWAYS doubt yourself when replacing the ECM is thought of as the solution. Everyone has been there, it is so rarely the issue though, it should be the last thing you even contemplate replacing to try and fix a problem.

You could most definitely blame the ECM if you KNOW that signal (ground or 12V?) is not making it out of the aluminum ECM case on the proper pin...but it could be something as dumb as a corroded terminal or bad locking connector just as easily, so you have to test the pin itself.

And yes, the wiring could be an issue too. Just need a bit more diagnosing.

"I have a feeling its the VSS, but who knows." It's a good thing you have the proper tools now and didn't just replace parts like many do, but don't talk yourself into thinking you are close enough to guess what the problem is. This isn't condescending at all, I've replaced a MAF and swapped ECM's before (but to my credit, they were spares lol) only to find the problem was something else. Even following the flow charts won't work all the time, it's a very good learning process.

 

[nutt7]

Yeah, like the time I thought I had a short in my wiring. My DMM said 12v to the main power wire into the cab, but when I turned anything on (lights, even turning the key), it went to a couple volts...turns out my fusible link was allowing just enough voltage in to show 12v under no load (it was bad from heat damage and it had been cut and reattached with a butt connector. it was the one by the starter). THAT taught me some electrical troubleshooting. When I found the problem I was both happy and pissed.

Thanks for the advice. I will continue my hunt with perseverance

 

[eagle mark]

Yeah, like the time I thought I had a short in my wiring. My DMM said 12v to the main power wire into the cab, but when I turned anything on (lights, even turning the key), it went to a couple volts...turns out my fusible link was allowing just enough voltage in to show 12v under no load (it was bad from heat damage and it had been cut and reattached with a butt connector. it was the one by the starter). THAT taught me some electrical troubleshooting. When I found the problem I was both happy and pissed.

Thanks for the advice. I will continue my hunt with perseverance

I thought I was the only one that could happen to? Another is bad grounds will show as fine when using DVM, but fail with a test light because of laod as little as a light bulb is...

 

[Blue85]

Is the ALDL pin the same node as the ECM pin? Can you just connnect a voltmeter to the ALDL and compare with what the scanner is saying? The problem could be the wiring between those two points, a bent pin on the ECM connector or similar. It's also possible that output transistor in the ECM is fried.

 

[black dawg]

ALWAYS doubt yourself when replacing the ECM is thought of as the solution. Everyone has been there, it is so rarely the issue though, it should be the last thing you even contemplate replacing to try and fix a problem.

I dont think I would go so far as to say always doubt yourself when you think the computer might be bad.

As long as the vehicle is screwing up, the facts are right there in front of you, just gotta find them. Its all just parts, and they all can fail. I guess what I am saying is, dont go out of your way avoiding replacing the ecm, go right to that pin, and see if the ecm is actually doing what it says it is doing.

I have seen a lockup sol. fail and burn up that circuit in the computer.

Also, I would use a test light (with a big bulb), to actually load that circuit right at the ecm. If the ecm is grounding that pin, then it is easy, just gotta find out where it is going away.

 

[dyeager535]

I dont think I would go so far as to say always doubt yourself when you think the computer might be bad.

What I probably should have said is, resist the urge to replace the ECM UNTIL you can pinpoint the problem to it. It is far too easy to blame the box of mystery than it is to actually check wiring continuity, connections, etc.

They do fail, it does happen, but there are far more swapped/replaced than are actually bad. If you have a spare ECM (and I think that is a reasonable precaution since they are fairly easy and cheap to get assuming you plan to keep the truck) it does make diagnosis easier. But IMO it's just ridiculous to pay full price for a reman unit as a diagnostic tool.

Also, impossible to know if a used ECM is bad or not if you don't know it was working right initially.

 

[eagle mark]

I think the reason people want to replace the ECM is 3 fold. One they can't find the issue, two if they have read factory test procedures they end up short of full diagnostics and say "Replace ECM" Which does not always work. Three is a lot of them did fail when new do to bad solder joints. They have been weeded out and the ones that are this old are pretty solid without human intervention like shorting, welding and water.

Have you tried the GM factory service procedure during TCC Lock Up? Tap the ECM... this is a factory procedure and often found the loose solder joints.

That said I always have a few or at least my one tuning ECM that has a Moates G2 adapter to use my emulator. I can't count how many time I have used it as a replacement ECM when I can't diagnose an issue following the GM service manual procedure and it still was not the ECM!

The ECM in my 1990 Suburban permanent location for so many years is hanging under dash on floor because I use the vehicle to constantly test spare ECMs before I solder in a G2 adapter or send one out with a chip. So it would be fairly easy to check the pin during TCC lock up. THis would be next step instead of just buying parts and replacing ECM, check that wire coming from ECM and see if the signal is correct during TCC lock up ! If correct then it's wiring somewhere from there down...

 

[nutt7]

Is the ALDL pin the same node as the ECM pin? Can you just connnect a voltmeter to the ALDL and compare with what the scanner is saying? The problem could be the wiring between those two points, a bent pin on the ECM connector or similar. It's also possible that output transistor in the ECM is fried.

The wiring diagram shows that the TCC lock up from the ECM is spliced to the ALDL port and the tranny plug. All the scanner says is "TCC Locked", I have no readings. It might do it, haven't tinkered with it much... I will test the ECM pin first, before I do anything else.

 

[nutt7]

Have you tried the GM factory service procedure during TCC Lock Up? Tap the ECM... this is a factory procedure and often found the loose solder joints.

Dyeager sent some phone pics of the procedure. Some I couldn't read, but I need to repeat the process since I know what the ECM is saying. I can't find it in my FSM.