Hi. I am hoping someone can help me. My 85 threw a code 24 the other night. That is the Vehicle Speed Sensor. The speedo and all dash functions appear to work fine. The only thing that doesn't work is my lock up converter. I locks up about once every 10 drives. It has a newer soleniod (also ohms out ok), and a new fourth gear switch. I followed the flow chart in the fsm to fix the lockup, but it always points to the fourth gear switch (and I don't understand it at all).

I guess my question is, could the vss circuit be bad and have the speedo still work, yet effect the tcc? I'm sick of trying to fix this lock up, and am starting to wonder if it is an internal problem in the trans.

Shouldn't I be able to ground one wire at the trans to manually engage the tcc? Two of the wires have 12 volts (fourth gear switch wire, and tcc circuit). Shouldn't the one at the brake pedal switch be a ground wire interrupt? That one has 12V too. I have all the schematics, but am having a lot of trouble. This has been going on for 2 years. At first I was really happy to have the vss code, b/c I thought it would lead me to the problem. Now, after searching, it looks like the vss is only bad if the speedo stops working.

Please help. If nothing else, should I take it to the stealer?

 

Without looking at the Manual (which is at Home right now...) - It would seem to me that if the Speedo is working fine and the ECM isn't getting (or doesn't Know it's getting - which is Another problem altogether) a speed signal, then the problem lies within the dash cluster electronics.

I would Start with driving the car with a scan tool hooked up to verify (or not) that the ECM is seeing a speed signal.

If it's Not, then check the wiring and connections for that input between the ECM and the dash cluster. If That is OK then condem the cluster.

This does Not have to mean a big $$$ bill to repair or replace the cluster - I would probably bypass the factory cluster electronics with an aftermarket DakotaDigital speed sensor box - get the input for the ECM that way and worry about fixing the cluster another day.

BTW - does your Cruise control work? That would also depend on the speed signal electronics within the dash cluster.

 

Thank you very much for the good info. I know the tcc circuit goes through the digi dash too. Maybe I should take it out and check all the connections to see if that does anything.

I am going to try winaldl, and see if I can see anything. Also, I will try the cruise contol.

This is the part right?
http://www.dakotadigital.com/index.c...prd_zm_126.jpg

 

ok. I ran winaldl (thanks to Joby) and found that the speedo works there. I am almost sure the means the ecm is receiving the seed sensor signal. I only tested in the driveway up to 8 mph. I doubt it is different up at 50 mph.

So that means the signal is going throught the digi dash and computer ok. Now what? I don't get the part in the FSM that says to disregard code 24 if it occurred while the vehicle is stopped. That seems a little weird to me. I never just had check engine lights for no reason.

I guess I will just follow the fsm and check the ohms for the sensor and then to the trans as ground. I don't have a reliable way to check the sine waves as they suggest.

Dave

edit: The stealer option is getting tempting, but I know I will have to bend over.

 

I'd say, yes, the ECM is getting, and Knows it's getting, a speed signal from the cluster (actually it's not that the ECM is Getting a signal, it's that the cluster is providing a pulsed Ground on that circuit, but you know what I mean..).

WinALDL should show you weather or not the ECM is commanding a TCC lockup. Well, I was going to check my copy and be sure, but I guess I must not have it installed on this machine... My Snap-on Scanner (MT-2500, aka the Brick) shows that flag, Tunerpro shows it, I assume WinALDL will. That would tell you if the ECM was Trying to lock the converter or not, at least.

Failing that you Can "manually" lock it up by providing a ground path on circuit 422, which is all the ECM does. That's the tan/black wire at the TCC transmission connector. While you are there verify that the Purple wire (circuit 420) in that connector has power (key on, brake pedal Untouched). Look in the FSM on page 130-131 for the schematic.

Actually - if you ground pin F at the ALCL you ground circuit 422, or you can Check to see if that circuit is already Grounded there.. Grounding that pin Should make the converter lock up - Any time.

Don't forget that this is a 700R4 - these were Known for converter lockup problems

BTW - Yes, that's the Dakota Digital part. I recently put one of them into my old (1978) truck (long story, but it has an '85 vintage TPI in it, to be the Same as the 'vette.) I needed the ECM speed input but the thing doesn't have a digital (or electronic) speedo. I do have a speed sensor added in to the speedo cable, the DD box did the rest. Neat piece.

 

So you can ground terminal F? It has voltage there sometimes. I found that out using a test light to check the brake switch and tcc circuit. When you are stopped in closed loop and put a test light between terminal F and ground, it lights the light. Is this from voltage coming through the other side of the solenoid? It just needs a ground signal?

I just found the code 24 chart in the fsm. I guess I will jack up the back of the car and run it in idle while I check ecm A-10. It seems like there is conflicting information on the subject in the fsm.

Things like:

"...The above conditions (to throw the code) will only be met during a road load deceleration.

"Notice: To prevent misdiagnosis, disregard code 24 if set when drive wheels are not turning." That kind of conflicts with statement 1.

and "Clear codes...transmission in low range....increase vehicle speed to a steady 32km/h....note check engine light" This also conflicts with the first statement.

 

Pin F at the ALCL (Or ALDL, which is what I have always called it..) will be hot whenever the TCC has power supplied, which is at all times with the key on and the brake pedal Up, and the ECM is Not commanding a lockup and grounding the circuit.

I think that's right - working from memory of the schematic in the FSM - which I was just looking at last night..

If pin F is showing +12v then the ECM is Not commanding a lockup (not grounding, but the Power supply to the TCC is good). If Pin F checks as a Ground then the ECM Is commanding a lockup and the converter Should be locked. If it Isn't locked at that point then the problem is "downstream" - bad wire or connector or a bad lockup solenoid or converter itself. Or the TCC isn't getting +12v on the Other side of the circuit.

Manually grounding pin F ought to complete the TCC circuit and enable the lockup - assuming that the solenoid has Power and that the solenoid is Functioning.

If you are using a test light to monitor the status of pin F - checking for ground while driving - Keep in mind the logic of the PROM program when it comes to the lockup function: The Program Wants to lock the converter, essentially all of the time, Unless it finds a reason Not to.

It'll be locked (ECM providing a ground to the circuit) in 3rd or 4th gears as long as the PROM program is NOT seeing the conditions needed to Unlock it. Here's a list of what the PROM program is using for TCC stuff for the '85:

Set values (constants):

TCC lock speed (mid gear)
TCC Unlock speed (mid gear)
TCC lock speed (high gear)
TCC Unlock speed (high gear)
TCC Enable Min TPS threshold (mid gear)
TCC Disable TPS threshold (mid gear)
TCC Enable threshold (High gear)
TCC Disable threshold (High gear)
TCC force lock speed
TCC Unlock prevent RPM

Then there are four Tables that come into play:

TCC Lock % TPS vs. speed (two tables - mid gear and high gear)
TCC Unlock %TPS vs. speed (two tables - mid gear and high gear)

Lots of stuff to play with in there, easy to mess up how it works. Tougher to make slight changes to adjust the lock/unlock speeds and throttle positions to Just what you want, but it's do-able.