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Username Post: ECM 12v relay drive no power from ECM?        (Topic#363623)
Posts: 3

Loc: hawaii
Reg: 12-11-20
12-12-20 09:17 PM - Post#2809874    

1986 GM Iroc 305 TPI swap into my 85 CJ7 with T400, 4WD, HEI, oil pressure sensor, map sensor, o2, tps, iac, temp sensor, obd1 (no 0 EGR) vehicle sat for 9 years. Now the most current issue fuel pump failed to come on/ no start? I tested pump/ to relay no issues. I end up at the {ECM 12v relay drive wire} I removed 12v Batt power removed harness connector, circuit good from this point to pump I used my Power Probe to power circuit from connector going back all checked out. I do not have 12v relay drive power output from ECM. Anyone ever have to go down this path, could I be missing something, before I continue to try to locate a ECM 01227730? If anyone has an idea of possible other ECM options or maybe you can tell I missed something based from this brief statement.


Member #409
Posts: 27800
Loc: Katy, Tx, USA
Reg: 07-06-00
12-13-20 06:13 PM - Post#2809923    
    In response to joop85

Are you sure the ecm is supposed to supply power to the fuel pump? Most systems supply ground to a relay.

Very Senior Member
Posts: 4638

Reg: 12-29-02
12-15-20 09:51 AM - Post#2810041    
    In response to Rick_L

The older GM ECM's usually power the fuel pump relay, not ground it.

The relay will only get power when the key is first turned on and when cranking the engine IF the ignition module is providing the reference pulses to the ECM.

Also, check you have all the constant and ignition power sources connected at the ECM.

Did you put a computer on it to see if the ECM is awake?

"4th Year" Gold Supporting Member
Posts: 135
Loc: Texas
Reg: 11-01-16
12-19-20 07:48 AM - Post#2810334    
    In response to 65_Impala

1986 GM ****865 ECM should be a ground on the fuel pump circuit. I believe from the B connector, also thru the oil pressure switch. I think the relay gets power from the ECM fuse. But this is all 40 year memory, so...


Very Senior Member
Posts: 4638

Reg: 12-29-02
12-19-20 03:35 PM - Post#2810374    
    In response to Texasray

Well, you got the power source right, sort of. Power for the PUMP comes From a fusible link, not a fuse, that also provides constant 12V to the ECM.

The ECM should be a '165. The mask is $32.

The relay is driven by +12V from the ECM, pin A1.

The ground for the relay coil is the same ground as for the ECM, pin A12.

The power comes from the fusible link to the relay then to the pump on the other side of the relay contact.

The oil pressure switch bypasses the relay contact and powers the pump directly.

Here's a schematic. m...

Super Senior Member
Posts: 5733

Loc: paradise
Reg: 09-05-03
12-20-20 10:37 AM - Post#2810428    
    In response to joop85

with the relay removed turn the key on. do you have a 2 second signal to the relay? could be a weak signal.

70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails.


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