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Old 03-01-2020, 11:44 AM   #1
ODDmanout
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Thanks for looking.

I have fuel pump issues. My 86 740 N/A died on the road. So, I swapped out the relay with the one in the glove box and it fired right up. It happen the next day and i swapped it out again and it fired right up. I am using two different pump relays, that i swap around, when they get hot. So, what is the issue? Do i need a new main pump? The one in the tank works.
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Old 03-01-2020, 08:14 PM   #2
dl242gt
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I recently helped a friend with his 740. His car needed the main pressure pump when it had the same symptoms. We tried a couple of different relays that we knew were good and it would randomly stop running and strand you. The main pump was bad.
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Old 03-01-2020, 10:25 PM   #3
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check the plug for corrosion, and half pulled out connecrtorts
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Old 03-02-2020, 08:21 PM   #4
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check the plug for corrosion, and half pulled out connecrtorts
did you do this?
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Old 03-02-2020, 08:24 PM   #5
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did you do this?
He probably doesn't know what a connecrtorts is.
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Old 03-02-2020, 10:54 PM   #6
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He probably doesn't know what a connecrtorts is.
OK that was funny...
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Old 03-02-2020, 10:59 AM   #7
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I replaced the main pump with a good used one and the wiring is a new harness from Dave Barton. Still no start. Replaced computer, still nothing. So, now I am looking at the hall sensor, since with no signal it won't turn on fuel pumps, I think...... Anyway, I pulled the harness from the dizzy and checked voltages, Red had 11 volts, blue had 5 volts and black had around 380 Ohms. These numbers seem right except the last one, should be around zero. Is that true?

In case it is the sensor itself....has anyone tried one of those $50 dollar dizzy units from e-bay/rock auto?

Thank you any comment.
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Old 03-02-2020, 11:57 AM   #8
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Apply 12v directly to the pump and see if it starts.

If it still doesn't (and you presumably do have fuel pressure), I would think Hall effect sensor or even more commonly a failed radio suppression relay.

A failed RSR will not allow the injectors to fire, so no matter how much fuel pressure you have it won't start. A lot of times it's not even the relay itself that fails, but rather the wiring harness.
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Old 03-02-2020, 05:34 PM   #9
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To check for the initial switching for spark. Put a test light on terminal 1 of the coil. It should flash very quickly if the hall sensor is triggering the EZK unit. If it doesn't flash then check the hall sensor and it's wiring. If it does flash then move onto checking the radio suppression relay and the fuel pump relay.
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Old 03-02-2020, 08:09 PM   #10
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the latest: Did the jumper in fuse box and pumps ran. Checked for spark. No spark. One side of coil is hot when key is on. The other side of the coil the test light glowed dimly when it was cranked over, nothing changed with test light. Thinking the hall sensor was bad i replace the dizzy with one that came off a running car years ago. Nothing changed, no start.

Any ideas? How do i test the hall sensor?

Thanks again!!
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Old 03-02-2020, 08:34 PM   #11
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Look out for the pins getting pushed out of the connector right at the distributor as mentioned above. If the hall sensor is getting power and ground and nothing switches on the signal wire. It's bad. If this is a head mount distributor. You may have oil damaging the hall sensor from the seals in the distributor leaking oil into it.
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Old 03-02-2020, 10:18 PM   #12
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Looks like there is no radio suppression relay on this car. My 90 740 GLE had one that failed due to contorted connecrtorts. Yes it is the head mounted kind of dizzy. Oil in the dizzy? Of coarse, it is a 740. I will take a more careful look at the connections on the dizzy.

Thanks for the time!!
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Old 03-02-2020, 10:38 PM   #13
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When you checked for spark, did you check near the plugs? You could potentially have a healthy spark getting to the distributor but not at the plugs. Could be due to oil or water (humidity). I've seen it a lot of times on Volvo's, especially the head mount distributor cars.

I like to remove the coil wire and check there first while cranking.
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Old 03-02-2020, 10:41 PM   #14
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I pulled the connectorts at the dizzy and plugged it into a spare dizzy i had. When i spun the shaft the injectors clicked and the fuel pump came on. So it has to be the wiring or the connector.

That makes me happy!! At least I know what the issue is!
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Old 03-03-2020, 09:20 PM   #15
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apachchef is the winner!! He don't no much spellen, but he got it! I cut off the rubber boot and the insulation on the wiring had turned into stone from heat and bare wires was the result.
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