Added A Manual TCC Switch

[bowtiebutler956]

My truck is equipped with 3.42 gears, and sometimes when pulling a heavy trailer, I prefer 3rd, but for some reason on my 93, the TCC only engages in OD, not 3rd. So I decided to install manual TCC switch on my truck.
First I located the ground wire for the TCC solenoid. On my truck, its a Brown wire with a Black stripe. I cut the wire, added some extra wire, and routed both the PCM wire, and the trans wire into the cab of the truck. I the purchased a Cole Hersee 55015 double ON toggle switch. That way, with the switch one direction, the TCC is still being controlled by the PCM, and with it flipped the other way, the TCC solenoid is automatically grounded, which engages the clutch in all gears after 1st.

In one of the picture below, you can see the wires to my toggle. The Brown wire goes to the trans, the Green wire goes to the PCM, and the Blue wire is hooked to a chassis ground.

This switch works fantastic, and now allows me to engage the TCC in 2nd,3rd, and OD, as well as keeping it engaged so that my truck now has some engine braking. However, if you accidentally leave the switch on, the shifts from 1st to 2nd, and 2nd to 3rd are very strange feeling with the TCC locked. I won't use this very often, but it will come in very handy at times.

Matt

 

[3500GMC]

Good modification. PCM switches the negative on/off for lockup so no '+' to that switch needed. Switching grounds is all you do.

My '93 has 4.10's so OD is like 3rd in a 3.42 truck. I notice lockup in manually selected 3rd but not in the OD selection while trans is in 3rd. In OD selection I see lockup in OD only. Maybe it's just because it ratchets through the gears so quickly it doesn't have time to think about it... It's in OD by 27mph, lol.

 

[bowtiebutler956]

Good modification. PCM switches the negative on/off for lockup so no '+' to that switch needed. Switching grounds is all you do.

My '93 has 4.10's so OD is like 3rd in a 3.42 truck. I notice lockup in manually selected 3rd but not in the OD selection while trans is in 3rd. In OD selection I see lockup in OD only. Maybe it's just because it ratchets through the gears so quickly it doesn't have time to think about it... It's in OD by 27mph, lol.

Wow, OD at 27mph. Mine won't go into OD till 45, and at WOT it doesn't go into OD till 75. On mine, the TCC won't lock at all in manual 3rd, or 3rd in the OD position for that matter. Its funny, I can be cruising in OD with the TCC locked, and manually downshift to 3rd, and for a few seconds the TCC remains in lockup, then it disengages, and won't go back into lockup until I put it back in OD. Thats not a problem anymore.

Matt

 

[3500GMC]

Yep OD by 27 light throttle and lockup at 45. Back out of lockup at 42.

 

[JiFaire]

It's great for deceleration (engine braking), and for keeping your engine temps down while pulling. I'd be pretty careful doing WOT shifts, though; it isn't all that hard to split the shafts with everything locked up direct.

 

[WarWagon]

JiFaire, the WEAK point in the 4L80E is the overrun clutch lockup friction used for compression braking. It's a dammed small clutch and will quickly completely burn out with the TCC locked doing compression braking. You won't notice it burned out going forward, but, when you lift the throttle the engine RPM drops back to idle at say 65 MPH. I am not sure what the programming is, but, I doubt the required line pressure is used at 0% throttle further encouraging clutch slip and burn up. It is something else to feel the trans shudder as the overrun lockup clutch burns up while compression braking.

My suggestion is do not do compression braking with the TCC override locked up. Otherwise you will no longer even have that choice when that clutch burns out.

To be clear the transmission shifts with a overrun clutch, consisting of a roller clutch, allowing a shaft to 'overrun'. This has a side effect and allows the driveshaft to literally freewheel and spin faster than the transmission/engine on a downhill run. To stop this a small set of friction clutches lockup after shifting. So the roller overrun clutch holds the power while accelerating and the small clutches hold the power when compression braking. I think these clutches are only used in manual 2nd and 3rd.

Do not run WOT with the TCC switch on. The factory* TCC clutch is the first to give up when "abused" as it isn't designed for this. The ECM will lock up the TCC in 3rd at WOT after a time with shifter OD selected. (Aka enough load towing etc. to stay in 3rd.) I think it's a 30 second or so delay. To keep temperatures down? With the wussy factory turbo this isn't a problem. With a modified turbo the TCC clutch had a 50K life before it won't hold anymore. Life with a good turbo has to be even shorter. *Aftermarket converters can be designed to hold WOT.

The TCC manual override should also have a resistor so the ECM sees a ground at all times to prevent a nuisance code.

 

[JiFaire]

Thanks, WW... Got a question though... I had my TCC lock mod for 3 years and a ton of miles pulling 10k 5er around through those little Rockies. I used it for compression braking through all that, a lot. Never saw a code either.

I was under the impression there was a difference between OBD1 and 2... It worked great on my 94 but Bill Heath told me it wouldn't be good on an OBD2.


he also told me it wouldn't hurt my tranny as long as I disengaged it when really romping on it, or I could crack an input shaft.

He used to recommend the mod on his site, don't know if it's still there.

Question: are the clutches in a 94 HD different?

Question: Why does it only work on OBD1?

100k km on it with that mod, pulling trailer... No issues. Truck is still going. Something has to be different.

 

[WarWagon]

Hmmm... figured something out from that question. First of all I am worst case being the 4L80E is a strong transmission. However when pushed with a low power diesel these are the weak points not counting 1000 HP hot rod power that simply snaps things in half.

A reminder (link below) from the past of some of the issues I had includes going over 250 degrees and more or less high trans temps. Turns out there is a TCC lock mode in 2nd and 3rd if the trans temp goes over 250 degrees. I must have regularly triggered this in 3rd at WOT for extended times. I always thought there was a timer and now the heating makes sense and would act the same as a timer. I fought poor trans cooling and AC issues for years before I figured out the trans shop plumbed the coolers badly putting hot fluid from the trans in front of the condenser and then re-heating it in the radiator - not really heating it but cooling it less. When discovered and plumbed to put hot fluid in the radiator and then to the cooler in front of the condenser dropped trans temps 30+ degrees and colder AC.

The newer 4L80E's use PWM rather than on-off for the TCC and this may start to explain the OBD2 difference. Clutches are more or less the same, but, the control solenoids are different.

Really romping on it will have the ECM lock the TCC regardless per above - the real issue is it slipping under high power and load. Shifting with it locked will stress things and IMO could crack/break things.

Absolutely temps being higher is a big reason for early trans failures and would explain why yours is lasting longer. Again temps are usually over 110 here and the cooling issues didn't help.

http://www.thetruckstop.us/forum/threads/1992-1993-tcc-lockup-mod-4l80e.24741/

http://www.msgpio.com/manuals/mshift/4L80E/4L80e.html

 

[JayTheCPA]

So in a 94+, to get more TCC lock-up, build a transmission fluid temperature fooler and let the ECM take it from there?

 

[WarWagon]

The overheat resistor trick to fool the ECM to lock looks like a old common solution.

Looks like I am lucky not to have snapped the input shaft off because I didn't use any care in slamming the TCC on when needed. But, I did find other weak areas.

More hammered out here.
http://www.thetruckstop.us/forum/threads/4l80e-tcc-solenoid-revisited.6481/

 

[JiFaire]

I dunno ... when I towed without using the TCC lock, I overheated like crazy. Keep in mind that's in the hills, or in big winds, and it gets crazy hot up here, too. As soon as I got the TCC lock mod done, I noticed an instant difference in my towing. I overheated FAR less, and the truck could hold speed in the hills or in big 3/4 frontal wind at literally HALF the throttle I was putting in before.

Slippage with TCC unlocked is a HUGE heat generator with a 4L80e.

That's without doing ANY of the cooling mods I ended up with - by the time Bill and I were done with my truck, I was pushing about 250 ponies, and still towing the same big-ass high, heavy fifth wheel as before ... and passing PowerJokes with smaller loads, uphill, instead of sitting on the side of the road.

I never did bother with the tranny cooler I was going to install - I was going to go with a box-mounted cooler and electric fan, to get the hot transmission fluid right out of my cooling stack. I also never bothered with my intercooler - found that I didn't need it, with everything else working right.

Right timing, right fuel-air balance, right cooling, right power transfer ... once you get things balanced, you're dancing.

Thanks for the thoughts, WW... I wish I knew why the mod worked so well on my OBD1, but I see the potential issues with a newer truck. Here is the article from Bill Heath's Blog:

http://heathdiesel.com/news/technic...lutch-in-1992-1995-trucks-with-the-4l80e.html