1995 ZJ 5.2 – used SBEC PCM 56026974 from VTA vehicle in non-VTA vehicle: EEPROM read / VTA byte?

[Kiaro93]

Hi, I'm restoring a 1995 Jeep Grand Cherokee ZJ 5.2L (46RH, non-VTSS, Italy). The original PCM 56041878 died (CPU failed). I installed a used 56026974 from eBay: engine cranks, reaches ~1000 rpm and dies immediately. Spark, injector pulse, fuel pressure, CMP/CKP all verified OK; only DTC 12 and 55 stored.

The 1995 FSM (group 8Q-2) states: "A PCM from a vehicle equipped with VTSS cannot be used in a vehicle that is not equipped with VTSS." So I suspect the donor car had VTA.

Questions:

1. Can the ChryslerScanner read the EEPROM of a 1995 SBEC (not SBEC3) via SCI (DLC C140)?
2. Does anyone know which EEPROM byte holds the VTSS configuration, or have an EEPROM dump from a non-VTSS 56026974 / 1995 5.2 PCM to compare?
3. Is writing possible (security seed) on this PCM generation?

My old 56041878 (non-VTSS car) is dead but its EEPROM might still be readable off-board, if that helps as a reference.

Thanks!

[laszlodaniel]

SCI-Bus @ 7812.5 baud; half-duplex (wait for the echo before sending the next byte).

Arduino setup with UART logic inverter

VTSS/SKIM off

EEPROM:
0x12: immobilizer status/info
0x13: key-on cycle counter
0x16: status hb
0x17: status lb
[WRITE PROTECTED IN NORMAL MODE] !!! SKIM !!!
0xE0: key-on cycle counter
0xE1: status hb
0xE2: status lb
[WRITE PROTECTED IN NORMAL MODE]

off: set all bytes to 0xFF; remove SKIM connector

Read EEPROM:

TX: 15 B6 12
RX: 15 B6 12 WW

TX: 15 B6 13
RX: 15 B6 13 XX

TX: 15 B6 16
RX: 15 B6 16 YY

TX: 15 B6 17
RX: 15 B6 17 ZZ

TX: 15 B6 E0
RX: 15 B6 E0 XX

TX: 15 B6 E1
RX: 15 B6 E1 YY

TX: 15 B6 E2
RX: 15 B6 E2 ZZ


Write EEPROM:

TX: 1C 12 FF
RX: 1C 12 FF E2

TX: 1C 13 FF
RX: 1C 13 FF E2

TX: 1C 16 FF
RX: 1C 16 FF E2

TX: 1C 17 FF
RX: 1C 17 FF E2

TX: 1C E0 FF
RX: 1C E0 FF E2

TX: 1C E1 FF
RX: 1C E1 FF E2

TX: 1C E2 FF
RX: 1C E2 FF E2

E2 = write ok
00 = write error

[Kiaro93]

Hi László,

Huge thank you — with your VTSS-off answer I managed to sort everything out on
my 1995 Grand Cherokee ZJ 5.2L. The replacement PCM I bought came from a
VTA-equipped car and wouldn't let my non-VTA vehicle run; your instructions
were exactly what I needed.

If a copy of this PCM's EEPROM dump would be useful to you — for reference or
for others with the same problem — just ask and I'll send it.

And if you have a PayPal or any other way to receive a donation, please let me
know: I'd like to buy you a coffee as a thank you. Thanks again!

[laszlodaniel]

Hi, cheers! I'm glad it worked.

These old PCMs are relatively easy to work with.

Check out my Sponsor repository here; it has a PayPal-connected Ko-fi link for donation.

Thank you for your support!