What controls power to the fuel pump relay?

[joeinblueout4u]

I've been having an issue with getting power to the fuel pump on my 1995 Mazda B4000 (Ranger clone). I have no CEL or stored codes. I have replaced the fuel pump and ecm relays, but the truck will not activate the fuel pump when the key is turned on. There is no power to the relay. I have bypassed the relay and installed a fused 12v power supply via a toggle switch to the fuel pump and the truck will run fine, but...

Is there an oil pressure switch or some other device that eliminates the 12v power supplied to the relay?

-Joe

 

[805RangerVc]

Have you checked the Inertia shutoff switch? Its on the passenger side next to the transmission hump.

 

[calypso93]

If it's not the inertia switch, there are two sources of 12V to the relay. One to the coil, and one to the switching contacts. The coil side also supplies the injectors with a constant 12V, and gets grounded to operate thru the PCM. The switched side draws from a 20 amp fuse in the engine compartment fuse box, goes to the inertia switch and then the pump. My diagram is for a 93 Ranger with the 4.0.

 

[Earl43P]

Yeah, it's called the PCM Power diode. (EDIT: scratch that idea)

Should be right there, in the UHJB.

Nope, that controls the PCM Power relay. If it's pulling in, the only thing left is the 20A fuse or another bad relay. PCM provides the ground for the FP relay (using the LB/O wire, which also goes to the data Link connector - which I assume is on the kick panel).

So, you could try grounding that wire and see if the FP runs. If it does, it kinda sounds like the PCM isn't telling the FP to run.

I ahve 93 schematics and stuff if you think it'd help. PM me your e-mail address and I'll send it.

 

[AllanD]

Using a Test light with the clip attached to ground (-neg terminal of the battery)
Test ALL the terminals in the relay

There should actually be two that are "Hot" because the EEC switches
the Ground to turn on the relay coil

And there should be 12v supply to the switch terminals of the relay.

There IS a fuse in the '93-04 and 95-on underhood load centers (Fuse & relay boxes)
It's usually a 20amp mini-fuse...

If your fuel pump is starting to die of old age a common symptom is
that it starts eating fuel pump relays.

AD

 

[joeinblueout4u]

Using a Test light with the clip attached to ground (-neg terminal of the battery)
Test ALL the terminals in the relay

There should actually be two that are "Hot" because the EEC switches
the Ground to turn on the relay coil

And there should be 12v supply to the switch terminals of the relay.

There IS a fuse in the '93-04 and 95-on underhood load centers (Fuse & relay boxes)
It's usually a 20amp mini-fuse...

If your fuel pump is starting to die of old age a common symptom is
that it starts eating fuel pump relays.

AD

Thanks AD. I'll get a check on that tomorrow. I replaced the fuel pump last Spring when this issue first showed up. When the new FP didn't fix the problem, I ran a 12v supply to the FP via a toggle switch. The problem fixed itself for a while and then came back permanently a few months ago. I've visually checked all of the fuses, but that still doesn't exclude a bad one. I did check the fuel pump safety switch and it appears to be operational/not tripped. I am not getting any power to it.

-Joe

 

[AllanD]

That should have read "Starts to eat relays and fuses"

My front tank pump is getting iffy (old age)

Fortunatly on my truck a single pump failure doesn't leave me "stuck"
out on the road.

That is a result of the amazing beauty of redundancy.

AD