Logo
FULL SITE MENU

1994 Mercury 75 ignition issues

B

bacchus

Contributing Member
I wasn't getting fire to cylinder 1 and sometimes 2. Did a DVA check and 0 volts to #1 and #2. #3 had correct voltage. Ran thru all DVA checks and symptoms pointed back to switchbox. Installed a new Sierra box. DVA checks out now and we have good voltage to all 3 cylinders and spark jumps a 7/16 gap on all cylinders! Yaay it's fixed right? Well not so fast. Put it in the water and she runs pretty good except seems to be idling slightly rough, lacking power to get up on a plane fast(altho it will plane) and at WOT i noticed just slight surging kinda(like its running fine then just kinda backs off then up). Almost like its losing power not really RPMS if that makes sense. Compression is 128psi every cylinder. 99.9% sure fuel system is clean clean. Went thru gas tank, new lines and bulb, filters, rebuilt fuel pump and cleaned carbs twice with fine wire thru holes and compressed air and carb cleaner...really feel like fuel system is good to go.

Any ideas on the symptoms its doing now? Trigger ohms test ok. It has the red stator with the little adapter module that is connected between the stator and switch box. The white/green and green/white stator wires test ok ohms. The yellows test slightly high on ohms at .5 instead of the recommended 0.165 - 0.181. This motor has a separate voltage reg and rectifier block. Tach does jump around some.

Man i need some help lol. Thanks in advance.
 
I'll add that coil dva voltage is erratic when idling on the muffs...dva meter just bounces around all crazy on all 3 cylinders...not sure if that is a clue. Voltage is perfect when spark plug wires are pulled off plugs and just measuring dva voltage at cranking speeds
 
I'll add i don't think this motor has a rev limiter. I'll have to remove the bottom cowl to be 100%...that would actually make the most sense but i don't see one and in the merc documentation it looks like the non oil injected version has it and the oil injection version does not. Mine is oil injected.

I talked with merc customer service and he said the ignition system seems happy given the dva readings at coil are good at cranking speeds. Not sure why it seems to runs kinda weak and bleed power/surge at WOT. We verified the rectifier terminal block and voltage reg have nothing to do with the ignition system only the charging system...so if either one of those were bad it shouldn't affect ignition anyway. Maybe the sierra box isn't that great? Gets pretty good reviews on the web. I do have some new sierra coils i can throw in there just to try.

I'm at a loss to what to try next besides bring it in. I have put countless hours into this motor and want to fix it myself! Any help/ideas of something to look at or try would be greatly appreciated! Thanks!
 
I had a customer with one of those and it had a bad reed valve. Easy enough to pull the carbs and plate behind them to check.

Jeff
 
fastjeff said:
I had a customer with one of those and it had a bad reed valve. Easy enough to pull the carbs and plate behind them to check.

Jeff
Click to expand...


Last time i pulled the carbs i inspected the reeds the best i could using my phone camera and a mirror to see any issues with the pedals. They all looked good from what i could see of them. I did not pull the reed plates. i'm not opposed to doing that but no carbs spit fuel on my hand while idling on muffs. Not saying it couldn't be a reed tho...lol...thing is driving me crazy.
 
bacchus said:
Last time i pulled the carbs i inspected the reeds the best i could using my phone camera and a mirror to see any issues with the pedals. They all looked good from what i could see of them. I did not pull the reed plates. i'm not opposed to doing that but no carbs spit fuel on my hand while idling on muffs. Not saying it couldn't be a reed tho...lol...thing is driving me crazy.
Click to expand...


Gonna throw a new rectifier in there and see what happens. I know that really isn't quite ignition related but i've read it has solved some issues somehow. Cheap enough try...plus it should fix the tach so that's a bonus.
 
bacchus said:
Gonna throw a new rectifier in there and see what happens. I know that really isn't quite ignition related but i've read it has solved some issues somehow. Cheap enough try...plus it should fix the tach so that's a bonus.
Click to expand...

I went and got a rectifier and will try it this afternoon. The mechanic at the shop gave me a reason the rectifier could cause ignition issues...It is ground to the same block as the coils are..if its throwing voltage to the block it could mess with coils...hrm..i do still have a bad oil warning module that is grounded...i just unplugged the speaker part. Maybe its throwing voltage some how. Anyone know how to test the plate the electronics mount to for voltage? i'm thinking positive lead on it and neg on another ground somewhere?
 
I have voltage on the plate all the electronics ground to. How do I find the offending component without burning up other components. Think I'm getting somewhere!
 
If the "ground plate" is not in fact grounded then you will see voltage there. Electricity is a little like water, if it has no where to go it just backs up and sits there.
 
Swagonmaster said:
If the "ground plate" is not in fact grounded then you will see voltage there. Electricity is a little like water, if it has no where to go it just backs up and sits there.
Click to expand...

It's actually showing negative voltage with the meter red lead touching the ground plate. Weird. It increases in negative voltage if key is turned on. It jumps all around when running... Something ain't right.
 
While it is true that the ground side of anything actually has a minuscule amount of voltage due to resistance in every connector, there should not be any quantity of juice be it positive or negative. Even large high draw connections should not drop more than 1/2 volt so if there is more than that with things operating then go looking for a missing, broken or poor connection.
 
Connect a jumper wire connected to the negative post of the battery to the plate while your tester is touching the plate at the same time I bet the voltage reading will go to zip.
 
Swagonmaster said:
Connect a jumper wire connected to the negative post of the battery to the plate while your tester is touching the plate at the same time I bet the voltage reading will go to zip.
Click to expand...

Well its not the low oil warning module..that is a known bad component and all i did was disconnect the speaker wire and left the rest hooked up a few weeks ago...was hoping that was it but just disconnected it totally and still have negative voltage...more with key on. rectifier is next...guess if its not that then could be starter, solenoids, ignition switch/kill switch...something ain't right it seems to me. Hoping i'm hot on the trail tho.
 
When you are doing your diagnosis stop to think what could not cause the problem, the starter could only influence things if it is running, as well as anything that is either on or off. A voltage regulator because it contains diodes - maybe, especially since it takes a/c voltage and rectifies it to d/c and has such a thing as negative voltage to have to deal with, but dont waste time on things that physically cant cause the issue.
 
Swagonmaster said:
When you are doing your diagnosis stop to think what could not cause the problem, the starter could only influence things if it is running, as well as anything that is either on or off. A voltage regulator because it contains diodes - maybe, especially since it takes a/c voltage and rectifies it to d/c and has such a thing as negative voltage to have to deal with, but dont waste time on things that physically cant cause the issue.
Click to expand...

Will do. Praying its not the switchbox...its brand new with one run on it.
 
So what is a fair test to know if the "ground plate"(the plate mounted to the block that all the electronics are mounted to) is being energized(however slightly) by a bad component or ground? Can i set my meter on DC touch the red lead to the plate and the black lead to the negative post on the battery and if i see voltage i have a bad ground or component? If i do this with the key in the "ON" position and meter set to 200m vDC i get about a .5 reading. A new rectifier didn't seem to make this go away. Could it be the ignition switch? How do i test that? Sorry i'm not so great with the electrical side of things.
 
Also, really don't understand why my meter jumps around wildly when black is on battery neg and red on the plate while the motor is running. Can someone verify this is normal or not normal? I don't wanna chase a nonissue.
 
Did you run a jumper wire from negative post to the mounting plate when you tested?
And make sure to touch the battery post not the cable end, a poor connection can't be seen from the outside. You are wanting to be able to eliminate that as any possibly of adding to the problem so don't shortcut.
 
Last edited:
Swagonmaster said:
Did you run a jumper wire from negative post to the mounting plate when you tested?
Click to expand...

No not yet. I will do that this evening. Lets say i do that and the meter still jumps all around when running is it safe to then assume that is normal and i'm chasing my tail?
 
By how much does it fluctuate? It shouldn't show on the plate but the amount could lead you to whatever is putting voltage on what should be a neutral plate. First things first, make sure that there is a definite ground or we are really wasting our time chasing gremlins.
 
It jumps around so fast its hard to capture by how much. I'll make me a ground cable this evening and see what we got.
 
bacchus said:
I'm sitting here racking my brain on if have some wire. I do have cables...good call! thanks. i'll try it after work.
Click to expand...

Jumper from plate to negative post doesn't seem to make a difference. Meter still jumps around crazy mostly in negative voltage range.
 
DVA at the coils are perfect when engine is just being cranked(holds like 200volts). DVA at coils when engine is idling is erratic. Is dva supposed to hold a steady voltage while idling?

I'm just throwing out things i've seen and don't understand. The service manual lists DVA at 300rpm(crank speed), 1000rpm and 4000 rpm making me believe DVA should be measurable while idling....not meter jumping around erratically. If this is a symptom of something wrong someone here has got to have seen this and what causes it or at least be able to tell me its normal. i'm trying to figure out if i'm hot on the trail or if this is all normal stuff that it is supposed to do.

Update: Merc customer service believes if the service manual shows dva readings for each of those rpms it should be measurable and not erratic. I tend to agree. I think we have found something...just not sure how to go about figuring out what it is. Probably whatever it is fried my old switchbox. Don't wanna fry the new one.
 
Last edited:
Mikey N.C. said:
Did u disconnect rectifier/regulator ?
Click to expand...

Haven't tried that yet..i put in a new rectifier block the other day...no change. My motor has a separate voltage reg that i haven't tried unplugging for testing...guess the worse that could happen is over charge the battery for a few minutes while testing.

CDI guy said to dva check the black/yellow wire(kill wire). I did that and meter is erratic. he just said if voltage drops off on it when revving then its the stator..but its erratic not holding and dropping.
 
Back
Top