So you verify 12V at the pump for 2 seconds at KOEO? Is that with or without the pump connected? Where are you placing the negative probe?

Have you checked continuity from the pump ground to the frame or battery ground?

The best test is the pump connected and measuring net voltage from pump wire hot to pump wire ground at the pump with power applied. During this period of full load and high current any poor connections will cause a large voltage drop and reduce net voltage. If this is the case then we find the poor connection and correct. This assumes that the ground is active at the pump to start with.

George

 

We have 12 volts for two seconds with KOEO both with and without the pump being connected.

We have placed the negative in the connector when we have measured the current in the connector.

Haven't checked continuity yet, but that's what I'll do next.

Now I stand without a helper to turn on and off the ignition so I have to wait for him to return.
I'll be back with updates as soon as I get help again this weekend

Thanks for the help so far George!

Per

 

So you measure 12V at the pump, using the ground at the pump which verifies an active ground all the way back to the frame and 12V at the pump all the way from the fuse box? Have you done the same measurement with the other pump (at the fuel tank location) that verifies good at the battery?

George

 

Finally we got it to start again.

I do not know what has been the fault, because I installed a new ground wire, but it did not start then and I measured the continuity both on the ground cable between the frame and the pump and from the fuse box and until the pump was both good.

But, today it started suddenly after some attempts

Thank you very much George for all the help

Per

 

Glad you got it running.

George

 

Update on this.
The car went well at first, but the next day it refused to start again.

So today we have installed new wires both to the rear grounding point on the frame and new positive to the fuel pump, both laid from the engine compartment to be safe.

So then it finally started again so now it's just to cross our fingers and hope it will be fine.

Suspects that it might be broken somewhere in the old wiring harness or where they soldered the new type of contact for the gasoline pump.

 

Hi again George.

Since you seemed so well versed in this, I hope I can ask you again and maybe get some new ideas for solution

Back to start again, went well all day, started all the time with no problem, so yesterday when we were going to start it was just dead again.

Checked how many amps it pulls on the course, and it dipped 0.9 when we sat on the ignition and went down to 0.7 after about 2 seconds and stood on this until we turned off the ignition.
Removed the relay and at point 30 we had 12.6 volts and good ground at point 86.

Between points 30 and 85, 0.46 volts and between 30 and 87, 12.6 volts ie we had ground at point 87?

Put the relay back in and measured the voltage at the prime point and had 12.6 volts for about 2 seconds which is normal.

And the cables are new all the way from the engine compartment now so they are fine.

Per

 

The absolute best way to approach this is to first understand how much net voltage the pump is seeing during the initial key on period. Every element in the circuit has resistance and when the pump current passes through this resistance, there is a voltage drop across that resistance (voltage = current x resistance or V=IR). This is true for the portion between the battery and the pump and it is also true for the portion from the pump to ground. Any voltage drop on either side of the pump, reduces the voltage seen by the pump and hence reduces available wattage or power (current x voltage) because the net voltage is lower to the pump. The hardest part to understand is the ground side. If there is excessive resistance between the pump and ground then there is an excessive voltage drop across that resistance and the ground is "lifted" as seen by the pump. Lets say that there is a corroded connector or poor frame contact and the resistance is 4 ohms. If the pump draws 2 amps the the voltage across that resistance on the ground wire is V = IR or 4 ohms x 2 amps = 8 volts. Even if the positive side of the pump sees an ideal 12.6V, the pump only sees 4.6 net volts - the pump is part of a resistor divider network. Normally all of the resistance elements in the circuit are small and the wires are sized properly so that the net voltage to the pump is only slightly lower than battery voltage. The reason why you need to measure this during maximum load is because thats the only time that you will see all of these voltage drops since thats when current is at its peak. You can measure 12.6V with no load and an insufficient net voltage level at load. I use meter probes that are needles on the ends and pierce the wire insulation as close to the pump as I can get on both the positive and negative wires, getting into the wires as they enter the tank if possible to also measure the voltage drops through the connectors. Once we know that answer then we go from there. Since you ran new wires then consider this exercise checking your work.

In addition, if you have a DC clamp meter then also measure the current flowing during key on anywhere on the positive wire from the fuse box to the pump.

George

 

Hello again.

Haven't had a look at it until today because it's been around 95F in the sun where the car is standing.

Beginning to wonder if the new pump is not working properly?

I got minus 12.3 volts in positive wire to the pump and when I unplugged the pump the ground fault disappeared and when I reconnected the pump the pump started right away but then the ground fault came back so the car did not start.

Both positive and negative wires are new all the way. So then only the wiring for the tank gauge that is old remains.

Per

 

What do you mean when you say ground fault?

It is literally not possible to have a negative voltage with reference to the trucks ground. Where are the meter leads (and what color) when you measure -12.3V?

Be careful because bad wiring can burn up a new pump. When the voltage to a new pump is too low, the pump overheats and can fail prematurely. On the flip side, these pumps are notorious for a high failure rate in all but a few brands such as AC Delco.

George