inertia switch

[K4_Khalid]

Car cranks but no start, when i turn ignition on there is no fuel pump prime noise. I checked fuel pump relay and other fuses it all works fine. No power coming to inertia switch thats why no power to fuel pump. I checked fuel pump relay pin which gives signal to inertia switch, it works fine. What it could be,, Any thoughts?

 

[J_C]

If you have 12V (relative to chassis ground) at the output contact on the relay, but not at the input on the inertia switch, then you must have a bad connector or damaged wire between the two.

How did you check the fuel pump relay? What it gives to the inertia switch is the power for the fuel pump, not just a signal AFAIK.

 

[K4_Khalid]

I checked fuel pump relay by tester, one pin which I believe is 30 positive and another pin energise by ignition, ground also working fine 3rd pin I could test by pulling relay slightly but still attached so I could touch the relay pin and confirmed that relay passing power to inertia line. But nothing when I come to check inertia wire. Strange thing is that after sometime power is back into inertia wire without doing anything. It happens almost once a month and it's daily driven car. Once power is back to inertia switch I tried to wiggle the wires around inertia switch to find out the broken circuit but nothing happens power is there. And if there is no power wiggling wire does nothing. I test it by plugging tester into inertia connector and keep ignition on.

 

[J_C]

When it does not work, you could measure resistance between the relay socket output pin, and the connector to the inertia switch. Should be well under 5 ohms, maybe the wire has an intermittent fault in it.

 

[K4_Khalid]

When it does not work, you could measure resistance between the relay socket output pin, and the connector to the inertia switch. Should be well under 5 ohms, maybe the wire has an intermittent fault in it.

I will do that and update here, thank you

 

[K4_Khalid]

Can someone tell me If engine is running and connection disconnected from inertia switch, engine should stop. Am I right? I'm curious because it never happened. It only happens when I park car and turn off engine, after sometime or few hours I come back and try to start and it keep cranking no signal in inertia switch. Why would it only happen after parking and shutting off engine?

 

[J_C]

Yes, no power to inertia switch would cause the pump to stop.

If you hadn't checked anything yet, my first guess why it would not work after parking with engine off is that the fuel pump itself is failing.

 

[K4_Khalid]

Yes, no power to inertia switch would cause the pump to stop.

If you hadn't checked anything yet, my first guess why it would not work after parking with engine off is that the fuel pump itself is failing.

When it doesn’t start I check inertia switch wires and it has no power and all of sudden after few hours power is back.. it happens intermittently. Besides motorcraft fuel pump recently replaced hardly 20k miles on it

 

[J_C]

If all else fails and you're getting power at the relay but not at the inertia switch, then I'd just replace the wire running between the two.

 

[K4_Khalid]

If all else fails and you're getting power at the relay but not at the inertia switch, then I'd just replace the wire running between the two.

I didn’t know it’s only the wire between relay and inertia switch. For now it’s running fine. If engine not starting again I will replace the wire. Thank you very much

 

[K4_Khalid]

I just looked around fuse box, the wire which connect from relay to inertia switch it doesn’t look easy job to replace it. Can I just run another wire from relay pin directly to inertia switch?

 

[J_C]

Yes, as long as you secure it away from sharp edges, and hot engine parts. I mean if the relay is in the engine compartment, I don't recall its location.

 

[K4_Khalid]

Update: Again it just crank and no start. Fuel pump relay output working but no input to inertia switch. So I just ran new wire from fuel pump relay output to inertia switch input power and kept original wire connected as it is. Car started and it ran fine. Than I thought to keep new wire connected and disconnect original wire. And there is power input at inertia switch through direct new wire but again it only cranks and no start. I must keep original wire coming to inertia to keep car running. It makes me think that there is some connection between fuel pump relay to inertia switch. It doesn't seem direct wire.