: 4T80E Troubles?

11-17-06, 01:31 PM
A friend of mine with a 96 Deville has some interesting issues in the 4T80E that we are trying to figure out.

1. Starts in second, no way to trigger first gear.
2. Intermittant Internal Speed Sensor
3. Traction Control fault (is this ISS related?)
4. Misses when starting off the line (2nd gear starting), runs generally rough under power. New plugs, gonna check wires and coils today.
5. No evidense of coolant on plugs that were recently removed, but 140K miles on the stockers.
6. Trans fluid has been changed, and LubeGard put in. No changes yet.
7. MPG is a sorry 12.4 mpg.... My LT1 Fleetwood gets 19-19.5! Power is that of a Yugo with a turbo....
8. A couple codes, TC0073, and 717. I suspect the 73 is the key, the 717 is a dubious code from the 73 problem. Where is the trans wiring connector. I can't see it. I want to check and clean it up and see if that is the issue.

Any thoughts on what else to look for and to perk this N* up? It is a sorry driving car right now....

11-17-06, 03:01 PM
I cannot find TC0073 in the OBDII code list, but it does sound like a T/C problem disabling first gear. Check for continuity at the wheel speed sensors.

11-17-06, 03:06 PM
My 1994 OBD1 book said TC0073 is PCM/TCS Interlink Fault (NorthStar Only). Same code on OBD2?

11-17-06, 03:38 PM

11-17-06, 04:54 PM
The PC0717 is not a dubious code associated with the traction control code. It is the code for the input speed sensor...$25 part $2000 deep in the tranny. I sold my '96 STS when I started getting the code. Read the link below...


11-17-06, 11:49 PM
A friend of mine has a mechanic that said he will do the sensor swap for $100. Good price?

Will the ISS cause TC issues?

11-18-06, 01:45 AM
I don't know if the two are related. If the mechanic will do the swap for $100 jump on that deal!

11-18-06, 11:17 AM
He said he would drop the engine/trans for him for $100 if needed. But he thinks that on the Deville you can change the ISS with the trans still in the car. Can you? It is hard to tell.

11-18-06, 02:30 PM
The cradle has to be lowered far enough to gain access to the side cover. Here are a few words and pictures of the same operation in a different chassis.


11-20-06, 11:25 PM
I doubt he'll stay at 100$ after all the work he's gonna have to do. But if he does, then dammit you got a good deal.

11-21-06, 12:44 AM
He went and talked to him more about it, and he said $300 now, he realized what was involved... He was thinking VSS, then realized it was the ISS.

So, what is the purpose of the ISS? If bad, what things don't work properly?

11-21-06, 12:00 PM
Theres a HUGE thread over in the Cadillac Tech Tips forum that JUST might solve the problem.

What happens is that the ISS start to short out and when that happens, it doesn't know at what speeds to shift, so it ends up shifting late (in my case) and its a BIG loss of power. Well in my Cadddy it is, it feels all sluggish because it takes forever to shift if you WOT it.

Possible causes for ISS messing up:

1.Signal weak or sensor is shot
2.SOMETIMES a wiring harness can hit the sensor and trigger a false alarm (regardless, you still lose the power because the car senses that the code is on and therefore deems it a "true" current problem)
3. Somehow transmission fluid gets into the connector and shorts out the connection


1. If sensor is shot, the sorry, you got the 20$ part 2000$ inside the tranny scenario (where in your friends case its only 300$, which is why we said its a DAMN good deal)

2. If signal is weak and you think its a false code, a simple 20$ amplifier circuit can boost the voltage of the signal and "fix" it. Magnum51 did this and he said he had no problems with it for over 1000 miles. Which is pretty damn good. It's battery operated (2 or 4 AA can remember, so all you gotta do is switch them everyonce in a while)

Ill get tha link for ya...

11-21-06, 12:14 PM
Found it...

not so huge as I thought but still a nice size, 7 pages


11-21-06, 08:30 PM
But what goes wrong with the transmission when the sensor fails? It almost doesn't sounds like anything happens other than a code, right? No shifting issues?

The only major trans issue other than the ISS is it refuses to start off in first gear. Anyone with a FSM can run through the troubleshooting chart for no first gear start? All other shifting seems normal.

11-21-06, 08:40 PM
With the ISS effed up, nothing really goes wrong. I have been told that you can drive it all you want, you won't break anything.

The only bad thing is the late, HARD shifts. So silky smooth Cadillac-like shifts anymore.

11-21-06, 09:54 PM
If that is the case, he'll probably ignore it, he got the car for $1400, and doesn't want to put much into it, just drive it as a secondary car if needed. It hasn't shown any ill effects except for the lack of first gear (that sucks too, the N* needs help down low rpm, at least this one, it seems to have an ignition miss too, which we haven't pinned down yet).