Need GURU's to diagnose a Mercury Distributor Ignition Sensor "CHERRY"

[CaptainMak]

So I'm trying to diagnose a "slow acceleration" problem I'm having with my 5.0L EFI (TBI) Merc.

I pulled out the Mercury Distributor Ignition Sensor from under the CAP and attempted to test resistance between RED/WHITE and GREEN/WHITE wire connectors. OHM meter showing OPEN LINE. I know the manual states it has to be over 100 OHMs, so OPEN LINE would be defective ?

It's an open type sensor with all electronic components open to the elements. I even checked the 2 soldering points that the wires are soldered to. Still says OPEN.

The question is: Will the motor still run with this condition sensor? Because it starts up fine, but seems line it runs rich and slow to accelerate under heavy engine load. I also maxes out at about 3,000RPM. Or would I not be able to start the motor at all if this sensor was shot ?

 

[alldodge]

Don't know if this is your 2003 5.0 or another, in any case need a engine serial number

 

[Scott Danforth]

Its a 40 ignition trigger. Usuall when the go bad, you simply have no spark

Mine failed because the units from Cherry are not conformal coated and salt air corroded the components

 

[Fun Times]

Some (two) noted possible characteristical symptoms,
Sensor Failures in MEFI-3 Mercury Distributors Used on V6 and V8 305/350 cid Engines

 

[CaptainMak]

Sorry, the Engine serial # is 0M019033. It's not the original motor and the manuals show it as 1999-2001 are the same.
Two wire setup. One wire red/white, the other green/white. Thinking that although I have spark, somehow there is no spark advance info being sent to ECM.

 

[CaptainMak]

From what I understand, these things either work, or they don't. Boat starts up and idles good, just under load it seems the timing is off and lacks power.
Is it possible these things partially work?

 

[alldodge]

Don't see the sensor partially working, its all or nothing. The ECM controls spark advance. Also note your distributor wires change colors going to the ECM



Your running rich and have no power, I would look at the MAP sensor. Also check fuel pressure for 30 psi. Next make up the paper clip and get a resistor LED and check for codes

 

[CaptainMak]

Thanks for the info AllDodge. Got the same in my Merc Manual.

MANUAL #24, PAGE 4B-21 shows the OHM test on the sensor. Specs say has to be over 100 OHM resistance.
Mine is still showing OPEN with my Multimeter. What to make of it ?

 

[alldodge]

MANUAL #24, PAGE 4B-21 shows the OHM test on the sensor. Specs say has to be over 100 OHM resistance.
Mine is still showing OPEN with my Multimeter. What to make of it ?

I don't see an issue, a lot of the resistance measurements has a lot to do with what meter is used. If something like a Simpson 260 (low impedance) was used the resistance could probably be read, but would still be high. The digital meters are higher impedance and therefore use much less power to get the measurement.

If you weren't running I would be able to lean more toward it being the issue. You could buy one for around $50 if you want to rule it out

 

[CaptainMak]

So I checked the timing, and in BASE TIME mode, it showed 14?. So it was off there. BTW, when put into BASE TIME mode, my tach reads 1,000RPM, is that normal?

So undid distributor clamp, blah blah, got it to 8? BTDC while in idle at BASE TIME mode at 1k RPM.
At 1200 RPM - 12?
at 1800 RPM - 26?
at 2800 RPM - 32?

Is this correct ? I can't find an official timing graph in the manual. Please advice.

 

[alldodge]

So I checked the timing, and in BASE TIME mode, it showed 14?. So it was off there. BTW, when put into BASE TIME mode, my tach reads 1,000RPM, is that normal?

So undid distributor clamp, blah blah, got it to 8? BTDC while in idle at BASE TIME mode at 1k RPM.
At 1200 RPM - 12?
at 1800 RPM - 26?
at 2800 RPM - 32?

Is this correct ? I can't find an official timing graph in the manual. Please advice.

Yep, all your numbers are good, and sure hope you didn't rev the 2800 on a hose.

Your timing graph is on page 4B-13

 

[CaptainMak]

sure hope you didn't rev the 2800 on a hose.

You only live once, brotha ! HA . What's a new impeller cost these days? Ice cream money. LOL. All kidding aside, it was only about 10 seconds at 2800 RPM.

I'll record the lake test results when I have it in the water early next week.

 

[achris]

Spark advance is all done in the ECM. The sensor just give the ECM the relative position between itself and the crankshaft/camshaft (done by setting the timing in base mode). I have never tested the resistance of a sensor, only the voltage output, and then on a CRO (but it was actually a PCO :lol.. If the system is generating spark and advancing the timing correctly, then I'd be looking for a 'fault' elsewhere. BTW, there's usually not a published timing curve for ECMs. The advance is driven by more things than just engine speed.

Chris.......