4L80 acting weird AFTER adding wire for TCC

**CDeeZ** · 02-15-2019

Gentlemen,

The root of the problem has been isolated..... Wait, almost rolled into a Clutch song... OOps... Anyways, looks like I fixed it.

Today I had some free time and the weather was a beautiful 70*.... Took advantage of this to look over everything....

The first thing I did was to remove the 2 wires going to the Input Speed Sensor on the trans and twist them up... They were NOT twisted when I installed them and I wonder if they were picking up interference causing this weird issue. Those wires were actually pretty loose and sloppy in their routing before I twisted them up.

The second thing I did was I looked at the main trans plug that goes into the case with 11 wires, the round plug. I noticed there was a little bit of trans fluid on it. I coated the plug in "motor assemly lube". Says right on the side of the bottle "excellent for electrical connections". And I have used it before on electrical things.

One of these two things must have fixed the problem... Went on a 30+ mile trip and everything is working perfectly, shifts through all the gears, hits lockup and releases lockup when I touch the brake pedal....

I also put in maybe 4 oz of fluid into the trans, no way it being just barely low could have caused all that weirdness though surely......

It was mentioned by myburb that it could be an electrical issue and I think that must have been the case because sometimes I could hit a bump and cause this issue, pretty indicative of an electrical problem.

I also noticed that when I key on, the speedo in the dash jump up to 10 MPH or so..... Not sure what's going on there, but I will look into that in the future. Possibly I need to check and retwist the VSS wires coming off the back of the T case.

Moving forward my plan is to check the wiring harness (again) inside the pan and possibly replace it or the seal around it since it's getting ATF on it from somewhere.. And, I'm going to figure out what is going on with the speedo jumping, or at least try to..

I love it when something finally works out. Perseverance is the way to accomplish anything. It's easy to get overwhelmed on a project of this magnitude when you're doing it all by yourself. I built the engine, the transmission, the T case, fixed up the wiring harness, and have been working on dialing in the tune.... 1 Man stand alone factory??? Okay I'm getting off my soapbox now.

Thanks to everyone for weighing in and BillyGraves who I have had numerous exchanges in private messages regarding all things OBD1 and 4L80.....

I'm amazed at how effortlessly this little 355 will pull away in 3rd gear say at 35 MPH.... No need to donwshift or rev up if you don't wanna, TORQUEEEEEEEEEEEEEE!!!

**brian617** · 02-15-2019

Originally Posted by CDeeZ

The first thing I did was to remove the 2 wires going to the Input Speed Sensor on the trans and twist them up... They were NOT twisted when I installed them and I wonder if they were picking up interference causing this weird issue. Those wires were actually pretty loose and sloppy in their routing before I twisted them up.

I also noticed that when I key on, the speedo in the dash jump up to 10 MPH or so..... Not sure what's going on there, but I will look into that in the future. Possibly I need to check and retwist the VSS wires coming off the back of the T case.

Always twist VSS and Data pairs (later CAN comm) Otherwise they act like a TV antenne and do strange things. I fixed an old first GEN Dodge diesel auto with a wacky speedo by re-twisting the VSS pair someone had pulled apart not to long ago.

**myburb** · 02-16-2019

I have had a goofy speedo for some time. It shifts fine, the speeds show correct in data log. I changed drac with no help, I changed instrument cluster with no help. Next time I have the cluster out I will look at adding a hard ground. In the mean time I am using an old gps for speed.

**CDeeZ** · 02-16-2019

Yeah that makes sense. That almost surely was the cause of my strange issue. I just didn't properly route or twist the wires since I added them in to this truck having never had them, wanted to make sure it worked first, before I twisted and routed them up and away. But as things like this go, you move on to sorting out the next issue and forget about stuff.

On the goofy speedo,
I think part of it is just how notoriously inaccurate the GM gauges are. The oil pressure gauge in this particular truck was reading very low, figured it must be the sending unit so I replaced it. Still reads way low..... I just installed a mechanical oil pressure gauge and ignore the stock one. Stock one tells me 0PSI hot idle and mechanical gauge tells me 37.5 PSI hot idle.