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Discussion Starter · #1 ·
Hi Folks,

After a lot of research here, I got my 2004 Odyssey working like a dream with a new Fuel Relay switch. It's been running great until last week, where I started it up, ran it for a bit and let it sit at idle for a bit. When I came back out to shut it off, it had died and wouldn't start again. A new fuel pump later with no luck, I'm struggling to diagnose the issue. Here's what I've done so far:

  • Immobilizer is working correctly (green light is behaving as expected after new Relay Switch)
  • Ignition switch looks ok to my untrained eye after pulling the steering column apart
  • Fuel pump is only getting 5V power on both sets of leads
  • Relay is getting 12V
  • Fuel pump makes no noise when key switched to on

I hotwired the fuel pump to the driver side set of leads and it spun up, supplying fuel to the system and the van would start. Hotwiring to the top set of leads did nothing as far as I could tell.

Before I start replacing the ignition switch and considering a new relay, what other diagnostics should I run to attempt to find the root cause for replacement?

Thanks!
 

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Discussion Starter · #2 ·
Forgot to mention I've gone through this entire post for help:


It's the closest I could find on the forums to my issue.

The van isn't worth enough to take to a mechanic, but I'm fairly handy, just at a loss what to look at next.
 

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Check the fuel pump output pin voltage on the main relay. It's possible the voltage drop at the FP is due to a bad wire from the main relay.
 

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Discussion Starter · #5 · (Edited)
I went ahead and checked all 7 pins on the Main relay with the ground on an exposed bolt on the chassis with the key in the "On" position. The configuration is as follows:

P1 NA P2 P3
X O X X
X X X X
P5 P6 P7 P8

P1 - black - 0V
P3 - Blue/yellow - 0V
P4 - Green/yellow - 12V
P5 - White/blue - 12V (seems like it might vary, but it could be my shaky hands)
P6 - Yellow/black - 12V
P7 - Red/white - 12V
P8 - Black/yellow - 0V
 

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Which pin is the output?
Oh man, I'm travelling for a few more days and I didn't bring my wiring diagrams.

Keeping the 2-second FP prime cycle in mind, look for a pin with voltage (should be 12V but don't want to presume that) from key ON then lasting for 2 seconds.
 

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check the voltage at the pump connection between "On" and 2 seconds?
Yep, good idea!

Make sure you have a solid ground - it can be elusive back there.

If you don't get voltage, you can follow the circuit upstream to determine where it's breaking up.
 

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Discussion Starter · #10 ·
I was able to test the fuel pump lead. There are 2 sets of leads. On the passenger side set or "bottom" of the trapezoid plug, there is a consistent 5V.

On the driver side set or "top" of the trapezoid plug, there's clearly an issue. I get about 3V on turning the key and it quickly drops off to 0.

Since I'm not super knowledgeable about electrical, I'm not sure where to go from here, but that's clearly the problem. It isn't getting enough voltage to even begin pressurizing the line. I'll do a bit of research to see if I can get to the bottom of the electrical issues, but any guidance would be much appreciated. Faulty ground, perhaps? I don't think there is much in the way of wiring between the main relay and the fuel pump. Any way to bypass the current wiring simply?

Seems strange that it just started happening while the van was running.
 

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2 sets of leads
Guessing there's one pair for the motor and the other for the fuel level sensor.
where to go from here
Here's another test on that same wire now giving 3V then 0V in ON: turn the ignition key to START. Test the voltage on that wire. (There's no timeout like in ON.) If it jumps to 12V, the main relay is bad. If it's still 3V, more upstream testing is needed.
 

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Here's the next place to test when you are ready: the FP power wire passes through a connector under the van. If that connector is corroded, it could cause the voltage drop to 3V that you found above.

The connector is grey; it has 4 wires, arranged in a square. It's located beside the driver's side parking brake cable bracket, just below a cluster of rubber hoses (likely part of the fuel system).

FP power is on a black/yellow wire, both into and out of the connector. Open the connector and test for 12V on the incoming side (female pin). Same as above, voltage lasts only 2 seconds after key ON.
 

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Discussion Starter · #14 ·
I'll check that. A couple of things that are interesting that I observed just now...

Voltage at the pump hops up to about 7V until the immobilizer finishes the key check (I think) and then the voltage drops to 0. There's an audible click when the voltage starts to drop.

When cranking, the voltage at the FP is about 6.5V consistently, then drops down once cranking stops.

Does that indicate the new Main Relay I put in to solve the first set of immobilizer issues may have already gone bad? It's only a few months old.
 

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Does that indicate the new Main Relay I put in to solve the first set of immobilizer issues may have already gone bad?
It's possible yes, but I don't see enough indication yet to justify replacing it again, unless you want to roll the dice to save time.

Somewhere between Fuse 1 and the fuel pump, voltage is dropping from 12V to 7V.

The complete list of suspects in circuit order is:
  • Driver's side fuse box, fuse 1 circuit
  • Main relay
  • Driver's side fuse box, fuel pump pass-through circuit
  • The underbody fuel pump connector
  • Wiring between the above
BTW, have you tested for 12V on both sides of fuse 1 in the driver's side fuse box? Have you benchmarked your meter for 12V directly from the battery?
 

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Discussion Starter · #16 ·
I'll start tracing things back as you suggest.

Perhaps not a super informed question...How do I test for 12V on both sides of the fuse? The fuses are quite small and I'm not getting a reading when putting my Fluke's contacts in the tiny little holes on each side of the fuse.

I have confirmed that I'm seeing 12V on the battery with my meter, if that is what you are referring to.

Also, where is the fuel pump pass-through circuit located?
 

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I have confirmed that I'm seeing 12V on the battery with my meter, if that is what you are referring to.
Perfect. Just had to be sure.
where is the fuel pump pass-through circuit located?
In the driver's side fuse box. Enters on pin 7 of connector O; exits on pin 4 of connector A.
not getting a reading when putting my Fluke's contacts in the tiny little holes on each side of the fuse.
Yep, those little holes are the test points. Maybe use a small nail to extend the Fluke's contacts?

BTW, fuse 1 is located before the timeout part of the fuel pump circuit so there's no necessity to get the reading within 2 seconds of key ON.
 

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Discussion Starter · #19 ·
In the driver's side fuse box. Enters on pin 7 of connector O; exits on pin 4 of connector A.
I'm not entirely sure what this means, but will take a closer look if the weather clears up a bit this afternoon to see if I can make sense of it in the van.

Really really appreciate all the care you've taken with responding to me.

BTW, fuse 1 is located before the timeout part of the fuel pump circuit so there's no necessity to get the reading within 2 seconds of key ON.
Does this indicate that I should be getting 12V when testing Fuse 1 in the driver side fuse box under the dash?
 

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I'm not entirely sure what this means
The driver's side fuse box has a number of connectors plugged into it.

FP power enters the driver's side fuse box on connector O. This one is located on the back of the fuse box so it's difficult to access.

FP power leaves on connector A. This one is easier to access. It's located on the front of the fuse box, just above the group of 2 fuses (12 and 13). Its color is green.

The pins in the connector are in two rows, numbered as follows as you look into it:
[1] [2] <2 empty spaces for the tab> [3] [*4*]
[5] [6] [7] [8] [9] [10]

The corresponding pins on the fuse box are the mirror image of that.

To test: Remove the connector. Contact your meter to pin 4 on the fuse box. Turn the key ON (2-second timeout in effect). Should be 12V.
Does this indicate that I should be getting 12V when testing Fuse 1 in the driver side fuse box under the dash?
Yes, on both test points, key ON.
 
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