Hello everyone !
I've a 86 Corvette, 700r4.
I've got a problem with my TCC.

From 42mph, lockup is activated (in 2nd, 3rd or 4th). So far so good ! Even if I prefer the 4th only...

But, when really accelerating, TCC is always ON, instantly from 42mph, whatever the gear.
Under 42mph, it's desactivated. When braking, it's desactivated until reaccelerate.

For exemple, when tranny shifts 2nd to 3rd at WOT, TCC is activated instantly from 42mph.

A specialist (here in France) looked at its workshop manual for 86 Corvette, in his view, 86 Corvette doesn't take into account the TPS for desactivating the lock-up (probably only latest C4).
So lock-up should be permanently activated from 42mph, no matters TPS ? I'm afraid destroying TCC...

Thanks for your knowledge
Is anyone here have a 86 ? And tell me its experience ?

 

This is just a question, TCC is subtle, what indication are you using that TCC is not unlocking ? ALDL pin F ? Those in the know are inevitably going to ask you about the TV cable.
Thanks
Donny

 

Thanks Donny,
No checked at ALDL.

When TTC is unlocked, gears are normally slipping, like every TH350 or others.
When TCC is locked, gear are exactly like manual transmission. No slip in rpm.

I feel it well at the behavior of the transmission, and engine speed.

 

It will take a few others here in the forums to help you, I am not the one. Do you have the FSM and the Drivers manual ?

 

Thanks Donny,
I've got FSM in PDF, but for 1987 models... No found any useful information yet.

 

I've checked a few 1986 auto trans bin files (AANM, AANL, BUA, BUC). The tcc forced upshift/lockup speed is set to 60 mph in all of the 1986 bins that I have checked, so 42 mph is a bit low. Which bin/bcc is in the car? Do you know the prom id? Gear ratio?

LC295: FCB 60 ; MIN VSS, (60 MPH) FOR FORCE UPSHIFT

 

Thank you Tequilaboy,
I've already seen this TCC speed (42mph) in another Corvette, I think. There are no many C4 here that I know... So I ask to you all.
I don't know what do you mean by bin/bcc and prom ID... sorry.
The gear ratio is 2.59

Other question : Is there any vacuum to inform ECM ?

 

Once the vehicle speed is greater than the forced upshift speed, it will not unlock. The tps% (load axis) is considered in order to lock/unlock at lower speeds.

Useful link: https://www.corvettesalvage.com/c3-c...des-1980-1995/

 

Ok, i didn't know that. Thanks.
I'll test again to see.

I'd love to drive another 86 to feel the difference !
In 87 FSM, i can see that :


So vacuum is taken into account by ECM ?
In 1986 too ?
Where is this vacuum part in the engine ?

 

There is no vacuum sensor. It is a Maf car. Load is calculated from (mass airflow/rpm)*factor . The tps load axis is a function of the tps voltage. The tps load axis along with vehicle speed is used for locking/unlocking the tcc. Tps load axis example data:

https://datazap.me/u/tequilaboy/aldl...ark=21-175-117

At 44 mph (for example) it should unlock with tps load axis > 65% (low gear) or with tps load axis > 78% (high gear). It is possible that your tps voltage/tps load axis is too low to unlock the tcc. Try it in OD as well as D. Check the tps voltage.

 

have found this in 86 FSM.
.

.

 

1986 does have a 26 mph bias to the lock enable threshold.

The normal lock enable threshold (for both low and high gear) is 15 mph. 15 + 26 = 41 mph (with offset applied). It appears that this is the mechanism for providing different lock enable speeds as a function of the third gear switch state.

LC297: FCB 26 ; BIAS TO LOCK ENAB THRESH, (26 MPH)

 

Thanks for your help, guys ! For all your informations

So i thought TCC would disengage at about 30% of TPS.
But I think TCC on mine is too present, and is activated instantly when shifting...
I wonder if I shouldn’t wire in manual, just to allow automatic mode by ECM, just when cruising.

 

It is worth noting that the vehicle speed parameter to enable forced tcc lockup was increased in the later model years (87, 88, 89) to 76 mph (3.07 gear) and 85 mph (2.59 gear).

Custom tuning is always an option, but it may also be worth upgrading to a newer eprom/memcal. Do you know if it has iron or aluminum heads? I assume that the car is a coupe, not a roadster.

 

I've iron heads, it's a coupe.
I don't have the intention of upgrading ECM, the best easy way is an additionnal wiring for me
Thank you Tequilaboy !!

 

Here's an example log from an 88 auto trans car that I happened to have on hand. This car likes to lock the tcc even under heavy throttle around 34 mph (and 3200 rpm interestingly). I would not have expected this behavior based upon simple inspection of the tuning, but the control is somewhat complicated with all of the settings involved.

https://datazap.me/u/tequilaboy/ailv...ark=67-363-594

Here are some of the relevant parameters for ACKY:

SCALAR: MPH LOW SPEED LIMIT FOR LOW GEAR LOCK TO UNLOCK TRANSITION 27.00 MPH
SCALAR: MPH LOW SPEED LIMIT FOR LOW GEAR UNLOCK TO LOCK TRANSITION 28.00 MPH
SCALAR: MPH LOW SPEED LIMIT FOR HIGH GEAR LOCK TO UNLOCK TRANSITION 33.00 MPH
SCALAR: MPH LOW SPEED LIMIT FOR HIGH GEAR UNLOCK TO LOCK TRANSITION 35.00 MPH

SCALAR: Bias to Lock Enable Threshold 11.00 mph
SCALAR: Coolant temp threshold for lock/upshift enable 50.00 degrees C
SCALAR: Min 4th gear N/V ratio 30.00 rpm/mph
SCALAR: Max 4th gear N/V ratio 35.00 rpm/mph
SCALAR: 4th gear N/V timer 0.80 seconds
SCALAR: Kickdown prevent rpm (Not 1st Gear) 3200.00 rpm
SCALAR: Kickdown prevent rpm (1st Gear) 3200.00 rpm
SCALAR: Min VSS for Force Upshift 85.00 mph


TABLE: TCC Unlock Speed Vs. %TPS (Not In 4th)

MPH % TPS

100 92.19
92 92.19
84 92.19
76 92.19
68 92.19
60 92.19
52 85.16
44 64.84
36 50.00
28 35.94
20 28.13
12 23.05


TABLE: TCC Lock Speed Vs. %TPS (Not In 4th)

MPH % TPS

100 82.03
92 82.03
84 82.03
76 82.03
68 82.03
60 82.03
52 64.84
44 44.92
36 33.98
28 26.17
20 23.05
12 19.92


TABLE: TCC Unlock Speed Vs. %TPS (In 4th)

MPH % TPS

100 99.61
92 99.61
84 99.61
76 99.61
68 99.61
60 99.61
52 87.89
44 78.13
36 67.19
28 55.08
20 44.14
12 99.61


TABLE: TCC Lock Speed Vs. %TPS (In 4th)

MPH % TPS

100 99.61
92 99.61
84 99.61
76 99.61
68 99.61
60 99.61
52 83.98
44 67.97
36 53.13
28 41.02
20 30.08
12 0.00