7427 shift light programming?

[CamaroDude85]

Anyone have any experience with the shift light in $0D? It works just fine until I'm cruising in 5th, anything over 2200 rpm or so it lights up the shift light telling me to shift but theres no gear to up shift to. It's kinda annoying because on my car the shift light is right next to the CEL and it always makes me think something done went wrong. In all the other gears it works just fine and at WOT it doesn't come on until 4400 rpm just like I set it to. If someone could explain the logic of how it works or what to adjust it'd be greatly appreciated.

[pmkls1]

It sounds to me like the PCM is not getting a signal that the trans is in high gear. The logic just operates off of the load parameters set in the bin and the PCM doesn't "know" what gear the transmission is in without an input. Any trans used in a vehicle with a shift light is going to have a high gear switch to tell the PCM that it is in high gear. I haven't messed with many T-5 equipped cars so I'm not sure exactly where the switch is located and how it is wired. But, you should be able to obtain that info pretty easily over at TGO. I'll look into where to connect the input at the PCM as I'm unsure about that part as well. BTW, Id like to talk to you about the LS1 brakes and some of your other mods at some point. I've been dying to upgrade my brakes for years now, but have never had the confidence to modify the spindles.

[CamaroDude85]

Thanks I'll look that up, didn't know the trans has a switch for that I thought it just had the one for the reverse lights. The brakes were simple enough to do. The hard part was making the caliper brackets and machining the rotors into hubs. The spindle is easy I did it while they were still on the car, just used a drill and tapped it 7/16-14, then a grinder with a cut off wheel to cut off what isn't needed. Send me a pm and I'll share what I know.

[RobertISaar]

perhaps i'm used to the FWD stuff...... but none of ours use a high gear switch. it determines gear based off of N/V ratio.

[pmkls1]

A lot of RWD stuff had a high gear switch, but I very well may be wrong about this particular application. I haven't found a high gear switch in the wiring schematics for a 746 ecm which is what his car would have been originally equipped with. But, GM sometimes likes to put certain circuits in different schematics. For example; I was having issues verifying the generator turn on signal from the PCM on my wife's '00 Grand Prix and that circuit is not shown in any PCM schematics only in the generator schematics even though it is controlled by the PCM. I also had issues with the brake switch schematic for my car. Mine was originally a 5 speed car but somebody swapped in a 700r4 before I bought it and they didn't wire in anything except for the backup lamps. When I went to wire in the power wire for the TCC function the stop lamp switch schematics shown in the ecm schematics were incorrect. I don't have access to alldata anymore, but I suspect that the correct schematics may be found in the cruise control schematics. This may be the case here as well. I have verified that there is only one switch in the T5 and it is called the back up lamp switch. However, it may also serve as a 5th gear indicator as 5th and reverse are on the same shift rail and one 3 wire switch could be used for both functions. I'm not familiar enough with T5's to know if this is the case or not though. I'll have to do a little more investigating.

[CamaroDude85]

I looked up the switch on Autozone's website and it only has two terminals. That's the only one to as I remember thats the only electrical connector I had to unplug when I dropped the trans out. I have the gear ratios set for the T-5 in the bin I'm using. When I datalog the mph shows up correct but it doesn't stay steady it'll jump around 5mph or so from the actually mph at a steady speed. It didn't do that when the 8746 was in the car. I also have output speed not from DRAC checked, when I left that unchecked it would think there was a trans error and limit the rpms. I'm gonna dig around in the hack and see if I can't figure out how the code works.

[pmkls1]

After looking around, I came to the same conclusion. But it was at least a good theory. Since the 427 PCM was mainly intended for auto trans vehicles I don't see much info about the shift light operation. Sorry that I wasn't very helpful with your problem. I'm still curious to know how it turns out for you. And I was mainly curious about the brackets that bolt the abutment brackets to the spindles and how difficult it was to drill and tap the spindles. I have read the article about adapting C5 z06 front brakes to a 3rd gen f body but the bracket fabrication part looked challenging. I'll PM you later for more details.

[1project2many]

the PCM doesn't "know" what gear the transmission is in without an input.

Actually, it uses two inputs. Engine rpm and vehicle speed or trans output shaft speed. The gear ratio relates the difference between the two so the ecm can divide one by the other to work out the gear.

OD uses Engine / Trans Output RPM ratio rather than N/V (Engine rpm / Vehicle Speed) and they're available to edit in the "Advanced OD V200" definition. In the shift light parameters look at "upper engine/trans ratio vs gear" to define the values.

In case your cal shows all zeros like some of the cals I looked at, here are the factory numbers for a 700R4.

3.7 RPM/RPM FOR 1st GEAR
2.1 RPM/RPM FOR 2nd GEAR
1.2 RPM/RPM FOR 3rd GEAR
0.9 RPM/RPM FOR 4th GEAR

They're wider than the actual gear ratios, probably to allow for torque converter slip. 700R4 ratios are
3.06 1st gear, 1.63 2nd gear, 1.00 3rd gear, .070 4th gear

I'd probably set these to match 1st, 3rd, 4th and 5th gear of the T5. I don't know which T5 you have but here's a set of V8 F car ratios:
1st: 2.95 2nd: 1.94 3rd: 1.34 4th 1.00 5th .74

Next issue is how is the pcm calculating output shaft speed when you don't have an OSS sensor. Without looking through code I'd guess it's through the RPM/VSS ratio parameter. Here's the math to put the correct value in that location:

; 44.70:1 NO/VS RATIO <-------*****
; VAL = RATIO * 1024
;
; Ratio = (Tire Rev's Mile x Gear Ratio)/60
; = (721 x 3.73)/60 = 44.82
; [721 = 235/75R16], 95 TAHOE

Finally, you might have to adjust shift light max load and TPS values as well.

Is gear ratio available in the data stream? If so it should be fairly easy to see if the values you're changing make a difference.

HTH

Please post updates if you work on this. Thanks.

[pmkls1]

Actually, it uses two inputs. Engine rpm and vehicle speed or trans output shaft speed. The gear ratio relates the difference between the two so the ecm can divide one by the other to work out the gear.

Thanks for clearing that up for me as well. I know that newer PCM/ECM's use those calculations but didn't think that the older ones (mainly something like a 746/747) were that "advanced". When I posted my first reply I was stuck thinking about how the 746 would operate the shift light and not thinking about the 427 being a lot more sophisticated. Even still, I didn't find any T5 that has a high gear switch so the older ECM's must use a similar method to calculate what gear the trans is in. I happen to have a '91 S10 with a 2.5 4cyl and a T5 that uses a 165 ECM so I looked at the schematics and such for it when I was doing a little research yesterday.