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Old 06-26-2020, 09:23 AM
tloftis tloftis is offline
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Default Loss of power to fuel pump relay intermittent

my 1997 Mazda b4000 4.0 will not start sometimes. Lately it has gotten worse and won't idle at redlights.
I can keep turning the key off and on a few times and I can see on the multimeter it flash 12 volts and then I know it will start. This is when the leads are connected to the outside terminals which is the main power going into the relay. 85 and 86. when I don't get power at 85 (I test this by hooking up the meter between 85 and 86) I know it won't start. something is not sending power to my fuel pump relay. ECM? It just about has every new sensor on it. Idles great when it will start.
In the last month I thought it was the fuel pump so I replaced it. My truck would die at a red light or not start. I was only getting 30 lbs of pressure after new fuel pump so I put new fuel filter on. have not checked it yet to see what pressure. It wouldn't start and I got sidetracked looking for electrical problems instead. I have never reset inertia switch. I am going to put my smoke machine on it when I get off from work. runs great when it runs.
nothing to do with the temp of the engine. will not start sometimes when cold... or hot.
This is what I found last night if I can post pics.
The igntion does NOT have to be turned completely to start it. Today it stalled at a light and I just started it again without turning the igntion back off and then starting over. I just kept cranking it... stopping for a second and cranking again until it started. I had been turning the ignition completely off to get the relay to power back up. At least that was what I thought I was doing. When I test it... i turn the key on but do not start it and I can watch the fuel pump get 12 volts for a brief moment. It flashes for a second then goes away. It will start if I see that. If I do NOT see the 12 volts blip on the meter for a second I begin to attempt to start it... it won't start.
testing the relay hooked up and trying to start... it would not... this is the readings I got:

relay reading when it wont start:
86 0 volts I am hooking my ground (-) lead to this connection
30 0 volts
87 12 volts
85 12 volts
87a 0 volts

When running:
86 ground
30 volts
87 0 volts
85 14.3 volts
87a 1.4 volts

I have a rely tester that I can hook up under the relay and then hook up to all the leads. When the truck would not start 86 and 85 had 0 volts when I hooked the leads up to them.
In other words, something is cutting power to my relay and I cannot find it.
When it would start... it had over 12 volts.

Nothing else that I ever saw went out when the truck wouldn't start or died. All power to lights and dash stayed on... air conditioner stayed on... radio.. everthing that I could tell. I would just loose power to the fuel pump relay
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  #2  
Old 06-26-2020, 11:57 AM
Alan_nc Alan_nc is offline
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Join Date: May 2020
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Default Re: Loss of power to fuel pump relay intermittent

Probably your computer. Had the same problem with my 94 Ranger. Pull the fuel pump relay next time it does it and jumper across (mine were listed as 87 and 30). Bet it starts right up and runs fine.
I now have a permanent jumper with a switch on the dash.
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dies, fuel pump, relay, stall, stalls

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