ben73

I am in early stages of tuning a '8625 / $E6 PCM to suit my application (383 Crossfire/TBI).
Previous ECM was a 7747.
I've got it to the stage where it idles well and runs well enough to drive. But I have struck a problem. There seems to be something cutting the fuel off at 4400rpm. This PCM does have a rev limiter, but it doesn't seem to be related to my problem. I have had the limit set at 5800 and 6375 (max) and still it cuts fuel right at 4400rpm. I am using tunercat, and my own definition i created for tunerpro. Both address the same byte when i make a rpm limit change. I'm going to have another look thru the hac to see what else may be causing this, but in the meantime, does anyone have any ideas??
Here is a datamaster scan, done in neutral. See the fuel get cut off at 4400rpm!! Grrr!

ben73

There's a few things here that might do it. Its a bit late at night now to be burning chips and seeing if it will rev cleanly to 5500rpm ;-)

;----------------------------------------------
; FUEL CUT OFF AT HI RPM and/or MPH
;
;----------------------------------------------
818A: L818A LDX #$4956 ; 4800 RPM LIMIT
818D: LDY #$495A ; 99 MPH LIMIT
8191: BRCLR L0046,#$10,L8198 ; BR IF NOT b4, 1 = FUEL SHUT OFF

8195: INX ; POINT TO LWR RPM LMT
8196: INY ; POINT TO LWR MPH LMT
8198: L8198 BRSET L0016,#$10,L81A0 ; CURRENT ERR WD1
;... else
819C: BRCLR L0093,#$03,L81A2 ;
;... else
81A0: L81A0 INX
81A1: INX
81A2: L81A2 LDAB L0062 ; RPM
81A4: CMPB 0,X ; COMPARE 4300 RPM LMT
81A6: BHI L81C5

81A8: LDAB L02A5 ; MPH
81AB: CMPB 0,Y ; COMPARE 96 MPH LMT
81AE: BLS L81E9 ; EXIT

81B0: LDAB L0062 ; RPM/25
81B2: CMPB L4955 ; FUEL TURN OF IF 1000 RPM
; DURRING C/O
81B5: BLS L81E9 ; EXIT

81B7: LDAB L028B
81BA: CMPB L4954 ; MPH HI FOR 3 Sec
81BD: BCC L81C5

81BF: INCB
81C0: STAB L028B
81C3: BRA L81EC

********************
* RPM LIM FUNCTION *
********************
81C5: L81C5 BSET L0046,#$10 ; SET b4, 1 = FUEL SHUT OFF

81C8: LDD #0 ; ZERO SYNC FUEL <<<<<<
81CB: STD L026F ; SYNC BPW

81CE: JMP L8300 ;

RednGold86Z

iTrader: (1)

Many cars also have a lower rev limit if it's in neutral - as detected by the park/neutral switch. I've never used that ECU so don't know if it's in it, but it's worth a quick check - it may be in the ecu, but not in the .ecu file, so maybe some experimentation with grounding the park neutral wire, or whatever to fool the ecu into thinking it's in gear is needed.

ben73

I thought that may have been a possiblity also, but it happens while the car is moving, and I do have the pk/n switch wired ok..
At WOT, its brutal when it cuts the fuel, then re-introduces it!! ouch!

Grumpy

RPM Fuel Cut Off
L4954 FCB 120 ; 1.5 sec QUAL TIME
L4955 FCB 40 ; 1000 RPM FUEL ON
L4956 FCB 192 ; 4800 RPM FUEL OFF
L4957 FCB 164 ; 4100 FUEL ON
L4958 FCB 172 ; 4300 FUEL OFF, (XMISH IN DEFAULT)
L4959 FCB 154 ; 3850 FUEL ON, (XMISH IN DEFAULT)
L495A FCB 98 ; 98 MPH FUEL OFF
L495B FCB 95 ; 95 MPH FUEL ON

ben73

Thanks Grumpy,
I'm no expert at reading these hac's, but i did see that, and its the first thing i plan to try at a reasonable hour. Gotta keep the neighbours happy ;-)

Grumpy

;--------------------------------------------------
; Transmission Abuse Test
;--------------------------------------------------
L5143 FCB 40 ; 500 msec transmission abuse Check Period
L5144 FCB 3 ; 3 MPH, MIN Vss For Abuse Test
************
L5145 FCB 255 ; If abuse test not enabled, enable above 4775 RPM
************
L5146 FCB 255 ; 4775 RPM MAX Vss for abuse test
************
L5147 FCB 51 ; 20% TPS MIN for abuse test