Bad Ecu Relay

Buy a new relay for 10 bucks, plug it in in place of the old one, and find out. Not rocket science. imp

The ecu grounds the relay, so if checking to ground you will get no reading. I chased my tail around for a bit untill I figured this one out..... Lol.

Ecu does not supply 12v. The relay gets 12v from a fuse then the ecu supplies ground to one side of the coil to trigger the relay.

billison said:

What cha mean?

Like if the ecu is out you won’t find ground?

Click to expand...

No computer plugged in and functioning properly, the fuel pump relay doesn't work...



Automobile computers use current sink technology. They do not source power to any relay, solenoid or actuator like the IAC, fuel pump relay, or fuel injectors. Instead the computer provides a ground path for the positive battery voltage to get back to the battery negative terminal. That flow of power from positive to negative is what provides the energy to make the IAC, fuel pump relay, or fuel injectors work. No ground provided by the computer, then the actuators and relays don't operate.

One side of the any relay/actuator/solenoid in the engine compartment will be connected to a red wire that has 12-14 volts anytime the ignition switch is in the run position. The other side will have 12-14 volts when the relay/actuator/solenoid isn't turned on. Once the computer turns on the clamp side, the voltage on the computer side of the wire will drop down to 1 volt or less.


The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972


Correct negative battery ground cable.


The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally: it comes off the ground pigtail on the battery ground wire.
This is a must have ground for the computer to operate properly.
Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
You'll find it up next to the starter solenoid where the wire goes into the wiring harness.

I am glad to hear that you got it working good.

It seems like $20 is a lot to pay when the innards are the same as a $5 relay. The main difference is the plastic case that the actual working parts of the relay fits in. It is just like everything else: if you want to play, you have to pay first.