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A relay is an electro mechanical switch. There is a coil which gets energized by placing 12v across pins 86 and 85 which then closes the switch by moving the contacts to connect pins 30 and 87 together. In a 5 pin relay when the relay is off then pins 30 and 87a are connected. So this 4 pin relay is a single pole single throw (SPST), normally open switch.
So you should go from no voltage reading to 12v on either socket pin 85 or 86 (try both) for 2 seconds at key on (negative meter lead on the negative battery post).
Hello, good morning, I put a multimeter on the gray and black wire going to the pump, then had someone put a jumper wire in the ports to bypass the fuelpump relay, it's sending 12 volts to pump and I can hear pump activate, but still no luck with the pump coming on with the relay in and key on, so what would be the fix? Is it a bad relay or a bad pump?
You are asking if the relay is bad. I thought that you bought a new relay, did you replace the existing one with the new one yet? If so then its likely not the relay. That's why we are exploring the control signal to the relay on pins 85 and 86.
Your asking if the pump is bad. I am trying to get us to proper power at key on and while the truck is running because we know that this needs to be fixed regardless but if you have a need to answer that question before we fix the power to the pump then jumper power to the pump with a fuse power source for 2 seconds and do the fuel pressure/leakdown tests in the sticky?
If you don't want to do the diagnostics that I am suggesting and you prefer the parts cannon and trynostics then replace the relay and the pump but I caution you that there are many other possibilities and all that work and expense may not fix your problem. next up would be the ignition switch, oil pressure switch, engine computer and then wiring. I am trying to get us to a targeted fix. Old vehicles have problems with corrosion, damaged wiring, shorts, engine computers, ignition switches, oil pressure switches, relays, fuse centers, fusible links, .... The parts cannon can be very expensive and time consuming as I have seen so many times here.
I have replaced most everything new, the computer, all sensors,distributor, wires plugs, the blazer will start with a shot of starting fluid or if I use the prime fuel port like you said, I also replaced the relays with new ones, I ded the test you asked for, on pin 85 and 86, when I turn the key on, it gets 1.5 volts just for a few seconds on those ports, so I don't know where to go from here, I just ordered another pump from Amazon, the pump that's in the tank was also from Amazon, but it's a cheap pump, after reviewing reviews I've come to the conclusion that the pump as failed, it only lasted a week or so, from cranking the blazer over and over, key on key of, when it worked you could here the pump cycle on for a few seconds with key on, now it don't do it anymore, so I also was told to buy a oem pump or ac Delco, or Delphi brand pump, as they last longer and actually work! Uike these cheap pumps on amazon that fail often and do t work.
Patience grasshopper, I am trying to get you there. You keep giving up before we finish finding your problem.
Yes we strongly recommend a Delphi or AC Delco pump so if you will feel better then you can replace your cheap Amazon pump at some point
In the meantime it looks like your relay is not getting energized to power the pump at key on by putting power from pin 87 on to pin 30 then on to the pump. To be clear: negative meter cable on battery negative, relay removed, positive meter lead in the pin 85 socket, key on, and you do not get 2 seconds of 12ish volts? Same test on pin 86? You also do not get 12ish volts for 2 seconds? If the answer to both of these questions correct, no 12v power for 2 seconds at either pin then:
We need to test the output signal from the VCM on circuit 465, dk grn/wht wire on pin 1 of connector c4 (24 pin) at key on. Connector in place and back probed with the relay removed, meter negative lead to battery negative.
Not sure what the vcm is? And the circuit 465 with 24 pin? Are you referring to the main computer on the passenger side fender well? It's got 4 large plugs going into it.
The VCM is the engine computer. Some years call it a PCM or ECM. There are 4 large connectors to this module, Three of them are 32 pin and the one we need is a 24 pin.
09-17-2023 | 01:41 PM
