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Low B+ voltage according to the ECU


NAXJA Forum User
Peabody, MA
88 cherokee ecu is showing low voltage on my REM with a/c on and blower on high its reading 12.0-12.6 this should be a reading at the b+ latch voltage at the ecu. at the diagnostic port D2-12 im showing ~13.28 and at the battery 13.58. so charge voltage is good.

i have not checked the actual voltage at the ecu yet.

i also have not checked the ground for the ecu. pin D1-3 has no connection to battery- i will try and check at an injector.

also according to the manuals i have D2-12 shouldn't be populated. and latch should be on D2-5?

i do have a 90 ECU in this, i suppose i could check it with the 88 ECU and see if it reads right. it may be a faulty ecu.
My in dash voltmeter drops to about that (as best as I can read it) with AC, Full fan. Drops a bit more with the headlights on. Once I turn everything off, it goes back to nearly 14v. Checking at the battery reveals only about a 1/2v drop.

On the cluster, A10 (BLK) is the ground for the vm, oil pressure, fuel gauge, tach and water temp. A6 (WHT-T) is the B+.

According to my 88 fsm, Diag connector D1 and D2 are the Tx and Rx serial data in/out that I guess the REM uses. D3 (BLK-T) is B Latch gnd. D4 (Yel) comes off the ign sw pin #11. D5 (PNK) is the B Latch pos and goes to Pin B10 on the ECU and the latch relay.

Here's a pic of my diag ports. You can see which ones are vacant. Since mine is an auto, it doesn't have an upshift indicator (D11). Mine all match the diagrams.

this is not a voltage gauge issue. the REM is a renix engine monitor. and its reading its data live from the ecu. so the ecu itself is reading low voltage.

also i may be wrong it might be pulling the voltage reading from the O2 relay or the fuel pump relay. i need to do some more searching. i am also having o2 issues too. it sometimes(most of the time) wont go into closed loop. unless i restart it after its warmed up.

i think someone may have moved pin 5 over to pin 12 at some point. pin 5 is unpopulated on mine.
i ran an additional 12g wire to my ignition switch. according to diagrams the fuel pump relay is fed from the yellow wire from the ignition switch. it had a pretty low voltage at this point anyway and i joined it just before the switch. and it did nothing for this issue. it did however help with low voltage at the fuse box.

it also bumped the voltage to my ignition coil from 12v to 13v so thats a plus. next im going to need to check at the relays. the ecu is reading 12.6 and so does the diagnostic pin.
i swapped the ecu out and the reading is stable and higher. it was swinging pretty rapidly before. the ecu detects and reads voltage on pin c11 or d10. testing at these pins did show 13.4v and it seemed stable however I'm not sure how fast my meter updates. one thing I'm not sure of yet is if the ecu is only grounded from pins b11 and b12 or is it bonded to the ecu case also. so I may beef up the grounds to the ecu.