5.0L 2452 wiring explanation - anyone seen a description of how the ignition operates-gctid432535

[TerryW]

Hi,

I just had a remanufactured engine installed to my 1998 2452 so the old one was winched out and a remanufactured one winched in. Maybe before and possibly during the process the wiring harness or a connection has given up the ghost.

I can "hot wire" the engine by supplying 12v directly to the coil and the engine runs really well like this. I have the wiring diagram courtesy of manual 24. However, I am getting a little confused over the role or the oil pressure sender and alternator and other "interlocking" elements that supply current to the coil.

I wonder if anyone has seen an explanation that starts something like "when the owner cranks the engine the oil pressure sender prevents starting until oil pressure builds up". I suspect this sort of explanation will help me locate the offending wire or connector. The wiring diagram is really difficult to understand without the 'design-intent' of whoever designed it.

Can anyone help please?

Terry

[bhawes]

Damn, where's my manual when I need it?

It works something like: the ignition switch supplies power to the electric fuel pump until the motor starts, and then it's up to the oil pressure switch to continue to send power to the fuel pump. If it does not detect adequate oil pressure, the contact is broken, and the fuel pump stops operating, shutting down the engine. A faulty sensor could be the source of your problems. Remove the 2 wires from the sensor and short them out (connect them). It that solves your problem, replace the sensor. Mine went South last year and I bought a replacement direct from GM. I've a wiring diagram somewhere, but it's in hiding right now!

[nottingham88]

I agree with prev post for theory of operation. Here's what I read from the schematic.

I couldn't find the diagram for the harness to dash panel for ign sw but I'm pretty sure there is an inline fuse in that lead for the 12V it sends via purple wire to pin 5 of the engine harness. That will stop you dead if that blows.

Look for an inline fuse in series with with purple IGN output when key is ON. I think 20 AMPS.

Check that the output from key switch is getting 12V to pin 5 at engine harness when the key switch is ON.

If you have 12V there the path is as follows via purple wire/lead from pin 5 which commons to Alternator exciter lead, Positive of Coil, thunderbolt ignition and the oil pressure switch.

The output of the Oil Pressure switch changes to purple with yellow trace and commons to Starter Solenoid, Choke Heater Coil and the Fuel Pump itself.

The output of the oil pressure switch supplies 12V to the electric fuel pump.

On my own boat that Oil Pressure Switch is on a T which also has the Oil Pressure sender for your oil pressure guage. Note also there is another Oil Pressure Switch associated with the audio warning system if equipped.

Note the difference between sender and switch.

IMO - Hot wiring 12V to any of the purple lead locations in engine bay would rectify blown dash side fuse (temp of course) but would not allow fuel pump to run IF there was an issue with the oil pressure switch. Sounds like yours IS working if the engine runs in that configuration..

I think you have no 12V to pin 5 BUT I've been wrong before.

Summations made from page 4E2: Section D is OP Sender; Section B is Oil Pressure Switch for Fuel Pump etc and Section C is for Oil Pressure Switch for Audio Warning.

I'm looking at a soft copy of M/C man #24.

[TerryW]

bhawes wrote:
Damn, where's my manual when I need it?

It works something like: the ignition switch supplies power to the electric fuel pump until the motor starts, and then it's up to the oil pressure switch to continue to send power to the fuel pump. If it does not detect adequate oil pressure, the contact is broken, and the fuel pump stops operating, shutting down the engine. A faulty sensor could be the source of your problems. Remove the 2 wires from the sensor and short them out (connect them). It that solves your problem, replace the sensor. Mine went South last year and I bought a replacement direct from GM. I've a wiring diagram somewhere, but it's in hiding right now!

Hi Bob,

Thanks very much for that. I searched through manual 24 and the electrical systems manual and whole raft of wiring diagrams and bullettins but I couldn't get any explanatory text.

I spent yesterday afternoon with the engineer who installed the engine and transposed all the ancilliaries. It looks like there is a break in a wire inside the wiring harness. We connected a continuity buzzer on a purple wire and could hear it going off and then stopping as the harness was shaken/twisted by hand. Trouble is replacing the purple wire didn't restore the ignition. My guess, from past experience, is that there are a few dodgy disconnects in there. This wiring loom has seen better times.

We did short the oil switch out but it didn't help. However, I'll short the sensor out again just to be sure. I'll probably have to trace my way around the harness, point to point, checking that every section is continuous. I suspect that is the only way I'll gain enough confidence in the wiring to go do battle with the cruise boats on our stretch of water.

The way you explained how the ignition is energised (initially from the switch and later from the oil pressure switch) is correct I am sure. However, I can't see, from the diagram in manual #24, what device is actually switching the power. You would need a relay or transistor to do the switching (I think). That would mean some device would have three leads and act as a switch. I find this a bit perplexing. I think the diagram may be "generalised" or "simplified".

Many thanks.

Terry.

[TerryW]

Hi Nottingham,

That is how I read the diagram as well. I spent an hour going through it in my mind last night. You filled in the gaps for me. The diagram poses a few questions such as how does the supply to the fuel pump switch from the ignition switch to the oil pressure switch if indeed it does.

The engineer that was helping me yesterday has a box of tricks that mimics the dash. This plugs into the harness plug. The fault is still there using his test-box so it proves the fault off the dash. NB He used his test box when running the engine in the workshop. NB I have the dash diagrams in a document numbered 90-816462 2-695.

I think my issue is akin to your analysis of a fuse being blown but is caused by a break in the purple wire somewhere in its length in some inaccessible spot. I think I just need to get greasy and wet to find it. What else would you be doing this time of year.

Many thanks.

Terry

[nottingham88]

TerryW wrote:
Hi Nottingham,

That is how I read the diagram as well. I spent an hour going through it in my mind last night. You filled in the gaps for me. The diagram poses a few questions such as how does the supply to the fuel pump switch from the ignition switch to the oil pressure switch if indeed it does.

[COLOR]"#0000FF" wrote:
I'm not really sure it does switch over, not from what I can see.You have enough fuelin the float bowl to start and should have oil pressure before you need more fuel is my take on that but I think BHawes said that for a reason [/COLOR]

The engineer that was helping me yesterday has a box of tricks that mimics the dash. This plugs into the harness plug. The fault is still there using his test-box so it proves the fault off the dash. NB He used his test box when running the engine in the workshop. NB I have the dash diagrams in a document numbered 90-816462 2-695.

[COLOR]"#0000FF" wrote:
I did find the location of a circuit breaker on the trophy docs for electrical for our boat- right below the ign switch and 20 amps. I have also experienced problems with that circuit in previous boat with M/C power and found a semi melted inline fuse that allowed intermittent contact - guess when it failed. That boat was twin power and the same semi melted condition existed on the other side but had not completely failed.[/COLOR]

I think my issue is akin to your analysis of a fuse being blown but is caused by a break in the purple wire somewhere in its length in some inaccessible spot. I think I just need to get greasy and wet to find it. What else would you be doing this time of year.

[COLOR]"#0000FF" wrote:
I studied the circuit for a bit before coming to the TS procedure I mentioned. The purple ign lead from pin 5 is quite clear in what it is common to prior to reaching the oil pressure switch whose output turns to purple /yellow. There are 2 splices in that section of the harness so your suggestion to check continuity from pin 5 on engine side is very sound IMO. [/COLOR]

Many thanks.

Terry

Hi Terry, I've commented in blue for clarity. An additional comment is that these engine harness plugs do get quite dirty - contact wise - but can easily be cleaned and fixed by carefully using some needle nose pliers and carefully transferring a bit of the cross hatch onto the male plug and adjusting the female for tighter fit. Especially critical for low current guage sender leads.

Rick.

[2850Bounty]

Actually, (and correct me if I'm wrong or not explaining this correctly)....... the starter motor solenoid circuit over-rides the fuel pump power source (the one that's interrupted with low oil pressure) during cranking ONLY.

This is to allow for pump operation and fuel supply until the oil pressure switch closes the circuit. Once closed, the fuel pump remains energized.

If oil pressure was to drop, this circuit opens, and the fuel pumps quits operating. USCG regulation.

Technically speaking, the ignition switch has no influence other than the S terminal involvement.

But with regard to powering a relay (I believe) that powers the electric fuel pump, the "Ignition" side will have an influence.

I'm thinking that the circuit to for the relay is what becomes either interrupted or NOT interrupted via the low oil pressure switch. The solenoid circuit simply by-passes this during cranking.

So...... if the fuel pump is not being energized during cranking, yet the low oil pressure switch and relay are both OK, look towards the momentary by-pass portion of the entire circuitry. You should be able to test right at the fuel pump's power supply connection.

Crank engine..... ya should see power at the fuel pump + connection.

Crank engine... no power, then chase the system further.

Crank engine... no power, try powering the pump directly, temporarily circumventing the by-pass circuitry.

Once up and running, and with the temporary supply now disconnected, does the system continue to power the fuel pump?????

If so, then the LOS and relay are working, and it's likely an issue with this start-by-pass circuit.

Does that make sense?

.

[TerryW]

2850Bounty wrote:
Actually, (and correct me if I'm wrong or not explaining this correctly)....... the starter motor solenoid circuit over-rides the fuel pump power source (the one that's interrupted with low oil pressure) during cranking ONLY.

This is to allow for pump operation and fuel supply until the oil pressure switch closes the circuit. Once closed, the fuel pump remains energized.

If oil pressure was to drop, this circuit opens, and the fuel pumps quits operating. USCG regulation.

Technically speaking, the ignition switch has no influence other than the S terminal involvement.

But with regard to powering a relay (I believe) that powers the electric fuel pump, the "Ignition" side will have an influence.

I'm thinking that the circuit to for the relay is what becomes either interrupted or NOT interrupted via the low oil pressure switch. The solenoid circuit simply by-passes this during cranking.

So...... if the fuel pump is not being energized during cranking, yet the low oil pressure switch and relay are both OK, look towards the momentary by-pass portion of the entire circuitry. You should be able to test right at the fuel pump's power supply connection.

Crank engine..... ya should see power at the fuel pump + connection.

Crank engine... no power, then chase the system further.

Crank engine... no power, try powering the pump directly, temporarily circumventing the by-pass circuitry.

Once up and running, and with the temporary supply now disconnected, does the system continue to power the fuel pump?????

If so, then the LOS and relay are working, and it's likely an issue with this start-by-pass circuit.

Does that make sense?

.

Hi Rick,

Makes perfect sense.

I tracked down the issue today. The first sunny day we have had for all week. There were two wiring issues:

1) The purple wire between the alternator and the oil pressure switch had a break in it right behing the bullet connector . The insulation was fine but the conductor had snapped inside the wire.

2) The purple wire bringing power back from the ignition switch wasn't making it through the harness plug. I'm not sure why as I haven't been able to under the fastenings yet. I just wired around the plug.

Thanks for all the help. The additional understanding saved my loads of time.

Terry

[nottingham88]

I wasn't content assuming the float bowl had fuel so jumped at the opportunity to see just how the starting overide to fuel pump works.

The purple yellow that leaves the oil pressure switch is spliced to another power source - a purple yellow that comes from the 9 oclock terminal on starter. Similar to the 12V overide for ign coils.

That's where the fuel pump gets it's 12 V during cranking only. I needed to expand section B of said schematic to see clearly. Refer to Page 4 E2. Section B.

[2850Bounty]

Just an FYI here...... if we add or alter this circuit any, just make darn certain that when the oil pressure switch closes (powers the fuel pump in lieu of the start-by-pass) that no current "back-feed" will occur to the start-by-pass circuit.

Typically, this is why a relay is in the loop.

It may not even cause an issue with the starter motor solenoid......, but I'd certainly avoid this potential.

.

[TerryW]

2850Bounty wrote:
Just an FYI here...... if we add or alter this circuit any, just make darn certain that when the oil pressure switch closes (powers the fuel pump in lieu of the start-by-pass) that no current "back-feed" will occur to the start-by-pass circuit.

Typically, this is why a relay is in the loop.

It may not even cause an issue with the starter motor solenoid......, but I'd certainly avoid this potential.

.

I am with you on that one. Whenever I do a work-round (as per the fix with my worn-out harness plug) I keep very close to the original wiring (as it appears on the boat not the diagram).

This is mainly because I have no information about the "design intent" of whoever came up with the original design. You get this a lot in my industry where modern software designers do not write anything down (they work "agile" or is it "fragile"). This just means that you can't change anything at a later date because you can't work out what the "knock on" effects will be. I have the same issue with the wiring diagram. I am sure the wiring design has "features" that are undisclosed that could be ruined by inconsiderate changes.

Many thanks. It is great having the boat back in working order. A lot of the credit goes to you guys on the BOC.