8l90
 

Have an Gen V LT4 8L90 auto in my 66 C10. Have put 75 test miles so far. Noticed when in full automatic at city speeds (30mph) the engine rpm is about 1000 rpm. My rear tires are 31" tall and rear gear is 3:73. The engine does it but rpm seems low?

Do you have it in overdrive?
What gear is it in driving in the city?

Who did the tune? Did you give them the tire diameter and rear gear?

Don

The 8L90 has drive (full auto) and manual (I use paddle shifter). Near as I can tell at 30 mph it is in 5th. I seems to like to be around 1000 rpm at pretty much any speed until your at highway speeds in 8th and rpm goes up with speed. This is a crate package and all "tuning" was done by GM.

Right, but the ECU doesn't know what tire diameter or gear ratio you have. Consequently the shift strategy is based false information. It's close for a lot of applications but with a 31" tire it won't shift the way it should.

I was thinking the same thing. The ZL1 uses 2.85 gears with a 27” tall tire so it’s similar to 3.73 gears with 31” tall tires if I did the math right. Got me wondering what the Connect and Cruise tune assumes.

Don

id have to agree , not that it matters but my 17 silverado with 3:42 gears isnt even that low lol

The install docs for the LT4 8L90 specify 3:08 to 4:11 gear and 26" to 30" tall tire. Yes I went with a 31" tire but assumed the transmission only sees the driveshaft rotation as an end result? Funny my speed to the Dakota speedo was within 5 mph first time out.

Ya I think the rear gear ratio sort of cancels out the tall tires.

Don

Do you have HP Tuners? You can change it that way in the gear/tire wizard in the tune. You are using the Gen V connect and cruise pcm correct?

I am using the GM connect and cruise pcm. Is HP tuner the way to go? Are there others? What other stuff will it do?

Get with a really good tuner and they can make that trans work awesome with HPT. Just make sure they have experience with tranny tuning.

HP Tuners is your only option with stock ecu. Where in BC are you again?

On the island, Port Alberni

Im not super familiar that far west, You could try calling the guys at Gen III Speed in Kelowna and see if they can help you out or recommend a shop closer your way.

Spent the day researching HP tuners. I might pop for it as it kind of matches my back ground and we are a little remote here from professional tuners. From what I can see I would need to buy a MPVI2 and two credits. HP tuners don't make it straight forward on their site list showing what is supported as far as crate engines go. Mine is a LT4 8L90 which I purchased new in Sept. 2019. (not on their list) but they support the new LT5 so I assume they do the LT4 8L90?

So I popped for the HP tuners and the first thing I was going to do was adjust the tire/ gear ratio to bring up my cruise rpm (even though I am in the middle of GM's recommendations). Pretty disappointed to find the TCM is locked. Yes I understand I could send it to HPT, WTF! Anyone else running the 8L90 transmission have feedback on their cruise rpm's?

So truck has been sleeping all winter and took it out a couple of days ago. Drove fine until I gave it significant gas and it rev'd then not several times. Came home and has check engine light on PO700 which is a transmission code and P18C2 pending. Also am told shifter positions are PRNDM. Mine and at least 2 others have one more detent, PRNDM? Heard it could be PRNDML?

Have you called GM or talked with a GM performance dealer about it?

I chatted with a GM performance tech today. Says delete codes and see what happens. I asked about the extra shifter detent (2 of my buddies have same) and all I could get from him was don't use it.

So I removed the battery power and then powered backup. Problem remains. Works fine in drive but when I shift to manual the shifter indication does not change, still shows its in drive. It does go down a gear and stays there but cannot manually shift.

Have an appointment with a GM dealer. Called GM tech line again to ask for a answer on the extra shift detent, no answer yet. Also asked for warranty as I have just 600 miles on it but just over the 2 years. It is always tough building a vehicle under 2 years as you generally buy your drivetrain to fit first and then build the rest. My buddies that bought the same engine and trans packages about the same time I did do not have their projects completed yet. But they both have the extra shifter position that I have.

Had my appointment with a Pacific GM dealer today. They treated me very well. Spent a couple hours testing to checkout the P18C2 code which is "internal mode switch". Next step is taking the pan off so we have ordered a mode switch in case that ends up being the problem. Still have not heard back from GM on warranty or extra shift detent.

I finally got a email from GM telling me there are 5 detents used and 6th is not. Not really any help as I and 2 others I know of have 6 detents. When shifted from M to the 6th detent I get "error" displayed.

Any improvements in drivability with the things you tried?

Is the extra detent the only issue you still have?


Jeff-

Don't know if the extra detent is an issue or not. The issue I do have is no manual operation but hopefully get that fixed tomorrow with "internal mode switch". Other than that transmission is fine.

Took to the GM dealer and they changed the internal mode switch. During final testing the same code came back. It appears that when we plug the Dakota Digital BIM module into the OBD2 port it causes the code. Dakota's engineers on it now and I have no doubt they will come up with a fix. The high tech modern age we live in. Another thing to complicate things is the code P18C2 is now a "permanent" code. Yes its a thing. Google it.

I have had similar issues related to the BIM. You can go into the BIM settings and change the bus address of the BIM. I’ve solved a couple of strange issues changing the address. There is four address options if I remember right.

Having the BIM and a scanner connected at the same time can also cause some strange things to happen depending on the style of Y connector you use on the port.

Don

Thx for that. I went to GM dealer today and had them reset the code several times. Funny thing is the GM scanner says the code is gone but my HP tuners says its still there?? Questioning if its the BIM as I had GM reset and then drove around without the BIM plugged in and Code came back. Being a "permanent" code not sure which scanner to believe. All the GM tech had heard of are soft and hard codes. Just looked at my BIM-01-02 manual says normally it does not use Bus Id's as other BIMs do.
So this code P18C2 is the code that had us change the Internal mode switch. GM tech noticed manual didn't work when all back together after he reset, and I guess assumed the BIM cause it was the last thing plugged in. My HP tuners scanner always says the code is there even right after GM tech reset. Now I was assuming the GM scan tool was super duper and could reset permanent codes. Never assume. If you look up Permanent OBD2 codes they tell you they are relatively new and after the repair the vehicle ecu will keep the code active but not light the check engine lamp until it decides the repair is good after a certain number of cycles.

I am not talking about codes. I am talking about the BIM bus address. The instructions to change it are in the BIM instructions. There are four options. Try all four and see if the issue goes away. The permanent code will probably not clear, that is something different.

Don

Again, thz for that. So far I have not had a problem with the display. Once I get this permanent code gone hopefully I'll be good.

Again, it has nothing to do with the display. The issue is a bus conflict which jacks up the ECM and generates codes etc…. Flaky stuff, just like you are describing.

Don

Gotcha. My original code I had was P18C2 for internal mode switch. I reset several times. Now that the switch has been replaced I get P18C2 "permanent".
Cannot reset. This is suppose to go away after several "cycles" and the ECU decides the repair is good. Nice to have a backup just in case. Wouldn't that be sweet if all I needed to do is change bus setting. Lol

So my P18C2 "permanent" code is gone. As I had researched on the internet the error code on newer OBD2 goes "permanent" after you repair. Once the ecu sees the repair and monitors 3 fault free cycles or 1 fault free cycle after a scan tool clear DTC request, the permanent DTC will be erased. I am still amazed my GM dealer didn't know this. Caused me a lot of grief. Now back to our regular schedule of cruising and car shows.

The following are a couple of email contents I received from a Techy at Dakota Digital.

The forums are probably refering to V1.8 on BIM-01-2 for OBDII, that version polled for information that the J1850 ECU’s didn’t support/have and it would cause the ECU to go stupid and hang, then the watchdog timer would reset the ecu and it would spring back to life. This would cause fuel pump to cycle on/off and make the engine stumble or stall.
This problem does not apply to a newer CAN bus ECU.

The BIM ID’s are only used for J-1850 (late 90’s – 2006ish) ECUs. Any thing newer than 2008 is usually just CAN bus and the ID settings should have no effect (61/62 or F1/F2) (Gauge or scanner emulation) you can always try changing them but I doubt they would make any difference if you have a newer ecu.

Were you able to get the manual shift to work like you wanted?



Jeff-

Yes. Manual shift (paddles) works like it did before my internal mode switch failure.

I have a Ididit column to shift my 8 speed. My LT4 8L90 came as a crate "connect and cruise" package. Going from park to reverse takes what seems to be excessive effort. In between the other R,N,D and M positions it takes minimal effort. Anyone else have this issue? Wondering if in production cars if they have a solenoid that is energized when key is on to make easier and the crate harness doesn't have it?

You didn't say cable or linkage rod from the column to the trans.

Either way, it is not easy to get the geometry right. Cant see from here if yours is wrong but reclocking the arm on the column can give you more leverage when coming out of park. The further away the lever on the column is from perpendicular to the rod/cable the harder it is to move.

Thx for that. It is cable. Whatcha think?
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