1988 Mercury or Mariner 175-HP: Electrical Problem

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Author Topic:   1988 Mercury or Mariner 175-HP: Electrical Problem

swebre posted 05-14-2014 11:08 AM ET (US)
[I] have a 1988 Mariner 175 Mariner on a 21' Revenge. I have been chasing (what I thought were) [problems with the] new Flowscan 7000 tachometer, but I noticed:
--at idle and up to about 1,800-RPM, the tachometer and charging system work fine. Accurate RPM and about 13-Volts shows on two meters;
--once on step, system voltage increases to about 14.5-Volts, but the tachometer drops to to 0-RPM.
--engine at about 3,800-RPM, and activate the engine trim (or load system), and charging system drops out, but the tachometer starts working and shows a stupid high reading of about 8,500-RPM. Charging will not start again until engine RPM is dropped below 2,000-RPM, at which point the tachometer start working properly and system voltage returns to about 13 to 14-Volts
This is repeatable. Cruising, the tacho,eter shows 0 and system voltage shows 14.5-Volts. Hit the trim and voltage drops to 12.5-Volts and stays, while tachometer comes alive but shows silly readings of 8,500-RPM. But drop to idle and charging starts again and tachometer becomes accurate.
The dashboard voltmeter and voltmeter on multi-function display agree. The Flowscan tachometer is set up properly for engine and has been replaced with the same results.
The wiring from battery forward is new and solid. All new gauges have everything properly secured and terminated. I have gone thru the engine wiring harness looking for shorts, cleaning and tightening all terminals, but no [obvious causes of the problem were found].
I ran a new temporary tachometer wire from voltage regulator tachometer output directly to the tachometer and got the same results.
I am starting to think the stator is the [cause of the problem]. I [checked the resistance of the stator and found it to be] within specification. Also, earlier I bypassed the tachometer output (gray wire) from the voltage regulator and hooked into a stator output (yellow wire) per Flowscan install manual. No change.
Next step is to start changing expensive parts Any words of wisdom?
swebre posted 05-15-2014 05:22 PM ET (US)
Update: I spent some quality time [diagnosing the problem] today [with] a [Volt-Ohm Meter] after a good friend said the voltage [fluctuation] sounded like a bad ground between the regulator and the block. The VOM didn't find any obvious problems, but, out of desperation, I added an additional ground wire from the voltage regulator body to a main ground on the block.
While [running] the engine on the hose adaptor, the tachometer started working properly and the charging system came on line as expected. Trimming dropped the system voltage a bit but came right back up.
I think it's fixed. Will splash the boat tomorrow to confirm. Life is good.
jcdawg83 posted 05-16-2014 04:02 PM ET (US)
Hope it's fixed. I was going to suggest a ground problem. My electrical knowledge is very limited but I have learned that a bad, loose, and poorly connected grounds can create some bizarre electrical symptoms.
sosmerc posted 05-18-2014 11:55 PM ET (US)
You should do a voltage drop test on both your main positive and main ground cables. If these are the original 1988 wires they are most likely due to be changed. Also, if there is a battery master switch in use, check the voltage drop across its terminals as well.
jimh posted 05-19-2014 07:43 AM ET (US)
The wire you added to carry the rectifier circuit negative to the engine block should be selected to have a conductor size that is large enough to carry the full charging current of the alternator. You should refurbish the original ground connection between the rectifier assembly and the engine block so good electrical contact is restored.
swebre posted 05-19-2014 08:14 PM ET (US)
First, sincere "thanks" for the responses.
jcedawg83 - Yep. Grounds, or lack of them, can do very strange things. Have gone through them all again.
sosmerc - Had re-terminated both ends of main cables during the re-wire. Starter now spins the engine w/ authority but will check actual voltages as suggested.
jimh - I had sized the additional ground wire for the full 40 amp output. (Excellent reminder however.) The original ground path for the water cooled rectifier is by direct contact w/ the block. That gasket line does look rough from the outside and I'll clean it up shortly.
Unfortunately, launched the boat today and the same gremlins are present. Charging system did not seem to drop out during EVERY trim change and the tach functioned properly as before during low PRM. But once on step and cruising @ 30 mph, tach was erratic and showing either 0 rpm or ~7700 rpm.
With the 19" pitch prop and assuming 20% slip, RPM's should be ~3900 at 30 mph.
Now double 3900 rpm is 7800 (pretty close to 7700) but that doesn't explain why it is accurate at low RPM. (Again, the new tach has been changed and the new replacement mimics the original.)
I'm starting to think I have two separate issues and thinking of running a new tach wire outside the wire bundle directly from the engine to the tach.
Any other ideas would be appreciated!
jimh posted 05-19-2014 09:29 PM ET (US)
It sounds like heat may be involved since the onset of the problem seems to come when the engine gets above idle speeds. The engine block may be getting warmer.
The circuit has some passive components and some active components. Usually, I would look first for trouble in the active components, which are the rectifier and regulator. They are more likely to fail due to heat. The passive components are the stator and the wiring, including all the connections. The stator could be a problem if there has been too much heat. The stator is usually wound with wire that is insulated only with an enamel (called Formvar in the trade) insulation. If there is too much heat, the insulation melts away. You can get short circuits in the stator windings as a result. Those might come and go with heat.
The electrical wiring and connections could also be a source of problems that might come on with heat. A little expansion and something stops making a connection.
Intermittent electrical problems can be a pain to diagnose. Sometimes you just have to start replacing parts and see what happens. Start with the least expensive and easiest parts to replace, and work your way toward the most expensive and hardest to replace parts last.
ASIDE: My wife's car would suddenly not start about once every six weeks. It took me about a year to find the problem. I followed the method I described above. I replaced the inexpensive and easy stuff first. Finally I worked my way to the real cause of the problem (the crankcase position sensor). Sometime there is no alternative to this method. (I was glad to find the problem as it saved me having to buy her a new car, about $30,000 these days.)
jimh posted 05-19-2014 10:02 PM ET (US)
Also: some of these older Mercury engines have dual stators. I don't really understand how the dual stators work--and neither does anyone else, apparently, as I have never seen a good explanation of them. But there is, apparently, some sort of relationship between the stators and engine speed ranges, as it seems like sometimes these dual stators are called the "low speed" stator and the "high speed" stator. If someone can expand on that and explain how these dual stators are supposed to work, it might shed some light on this problem, since there does seem to be a bit of a relationship between the occurrence of the problem and the engine speed.
Tom W Clark posted 05-19-2014 10:08 PM ET (US)
Jim -- What kind of car [did you repair]?
sosmerc posted 05-20-2014 01:47 AM ET (US)
Jimh, I think you may be referring to some of Mercury's Alternator Driven V-6 2 strokes that have dual regulators.
The stator is divided such that each regulator receives a max of 20 amps, such that the combined output of the charging system is 40 amps. The stator also has low speed and high speed windings (or power coils) to run the ignition system. The low speed windings provide voltage up to around 2000 rpm and then the high speed windings begin to "add" to the voltage such that the low speed and high speed windings together put out 200 to 300 volts at high speed. The voltage at each cylinders ignition coil is about 160 to 180 volts and as the engine rpm comes up the voltage comes up.....this is completely separate from the stators battery charging coils.
It's a pretty simple and reliable system provided the proper battery (conventional flooded marine starting battery) is used. Proper battery cable size and clean tight connections are also important. One benefit is that engines with this system can be started with a dead battery and will continue to run with a dead battery.
jimh posted 05-20-2014 11:18 AM ET (US)
ASIDE to Tom: The car is not important. The method of solving the intermittent electrical problem is important. My mention of a particular instance of applying the method was cited as an example of how, in some situations where the electrical problem is only manifested at the most occasional and sporadic times, there is no other good method than to just start replacing components until one stumbles onto the component that is actually causing the problem.
Let me give another example: about a decade ago a new diesel electric generator was put into service for emergency power at a large plant where I was employed. The new generator was test run once a month. It had a bad habit of not starting. Since it was new, and under the manufacturer's warranty, the vendor who installed it was out there trying to fix the problem. They spent about six months reaching a resolution to the problem. They used the same method I mentioned: since they could not identify any particular part to be the cause of the failure, they began to replace components that could be the cause. They used the same strategy, that is, they replaced the least expensive and easiest to replace components first. Eventually, the fault was corrected when the entire wiring harness of the engine was replaced. Because this was a costly and time consuming operation, this component was not replaced until all other possible components that could have caused the problem were replaced. This same method may need to be used in finding a remedy to this Mercury outboard engine electrical problem, if the usual diagnostic methods cannot located the fault.
jimh posted 05-20-2014 11:38 AM ET (US)
sosmerc--Thanks for the explanation. I see that there are really several stators in the alternator: one (or more) for battery charging, and then these paired two stators---"high" and "low"--for generating the primary voltage to the spark ignition coils of the engine.
Do you know from which circuit the tachometer signal is being derived? I have seen in other engines that the tachometer signal is coming from the battery charging circuit. If that is how Mercury is developing the tachometer signal, then it seems like the "high" and "low" stators of the spark ignition primary circuit would not affect the tachometer.
In the present problem under discussion it seems like the symptoms occur in both the tachometer and the battery charging circuit at the same time. This suggests to me that the tachometer signal is probably in the battery charging circuit. If that is true, then we'd rule out the "high" and "low" stators as being a source of the problem, and concentrate on the stator windings associated with battery charging and tachometer signal.
The tachometer signal usually relies on the rectifier to be working. And, of course, so does the battery charging circuit. This suggests the rectifier could be involved in the problem.
By the way, it is a common problem in a permanent magnet alternator that a single winding may not be able to work well across the very wide range of rotational speed found in an outboard engine. We want the alternator to provide electrical power when the engine is at idle speed, say 500-RPM, and also at full speed, say 5,000-RPM. If the stator coil is designed to provide useful voltage at 500-RPM, it may not work optimally at 5,000-RPM. This may account for the use of dual stator coils for generating the spark ignition primary.
swebre posted 05-20-2014 03:52 PM ET (US)
Interesting discussion folks. SOSMERC - You gave a much better explanation of the high and low stator system than the mechanic did.
JIMH - The tach source lead comes directly out of the sealed [here an acronym was used, which I hope meant "voltage regulator assembly", because in all following instances I have changed it to be "voltage regulator assembly" or "voltage regulator". If that was not intended, please let me know--jimh] so [the tachometer is] counting stator pulses. At one point, I had also tied the tachometer pickup into a stator leg as an option recommended in the Flowscan install manual. Results were the same so it's plugged back into the [voltage regulator assembly] tach lead.
SOSMERC - Out of the sealed [voltage regulator assembly], there are six wires total. Three heavy wires (two stator inputs, one [voltage regulator] output) and three smaller wires: One gray tachometer output, one red that ties into the larger [voltage regulator] output, and and one "maroon" wire that does [something not known]. I can't find a wiring schematic for this particular engine that shows its purpose. Have a idea?
Today pulled the [voltage regulator assembly] and cleaned the mating surfaces between it and the block along with all previously cleaned connections. Re-installed everything and then ran a separate wire from the [voltage regulator] tachometer signal lead (gray) directly up front to the tachometer, eliminating the original wiring harness. Hopefully will get a chance to splash the hull this evening and report back.
Thanks for the discussion!
sosmerc posted 05-20-2014 06:57 PM ET (US)
You must have the water cooled [voltage regulator assembly]. The purple [violet] wire is switched positive and it tells the [voltage regulator] to turn on.
You should always post your engine serial number, because, without it, it's a guessing game as to exactly which system you have. For example, on the V-6 Mercury they have used a 9-Ampere stator witn no regulator, just a rectifier; a 15-Ampere system uses a separate rectifier and a separate regulator; and the 40-Ampere system uses a combination regulator-rectifier, some with single water cooled units, and some with dual regulator-rectifier units.
In all cases, the gray lead gets its pulse from one side of the alternator and should put out 9-Volts read with a peak reading voltmete or DVA meter.
jimh posted 05-21-2014 10:44 AM ET (US)
The typical configuration for an outboard engine tachometer is to take its signal from a tap in the rectifier assembly that will provide a half-wave rectified signal, which looks like a pulsating direct current. This gives the tachometer a string of pulses. The frequency of the pulses vary with engine speed. The tachometer does not really count the pulses. The tachometer measures the frequency of the pulses. It probably does this by integrating pulses across a time base, which is probably set by the time constant of a resistance-capacitance (RC) integrator. The voltage developed across the RC time base is then used to drive the meter movement. For calibration and to make the tachometer more universal, some resistive voltage dividers are provided to make the dial face calibration work for pulse rates of different integral values, according to how many poles there are in the stator windings.
The tachometer also functions as a sort of canary in the mine to provide a warning of problems in the battery charging circuit. This is possible because problems in either the stator or rectifier will affect the tachometer signal.
Although it is not possible to directly observe a flow of electrical current, it is very valuable in electrical diagnosis to make a very detailed and careful visual inspection of electrical devices and circuity because very often there will be some visual evidence of a failure or problem in the circuit. For this reason, the Mercury outboard engine with the electrical problem should be very carefully looked over, concentrating on the electrical components and wiring, and looking for any sign of distress. Look for evidence of too much heat, such as a melting of insulation or a blackening or charing of the wire, the wire insulation, or some nearby surface.
If the rectifier and regulator are combined into one assembly, it is hard to anticipate what sort of resistive measurements can be made on the assembly when it is out of the circuit that might be a guide in diagnosis. If the rectifier is a separate component, you can generally check it rather easily to see if it is working by making resistive measurements, but even that can be misleading, as sometimes a failure only occurs when there is actual operating current flowing and the device is at operating temperature.
One problem with electrical part replacement is that generally you cannot return an electrical part--there is too much risk that installing the part into a non-working circuit might have damaged the new part. By the way, that is also a risk in general with the part substitution method. If you replace a part that has failed because some other component is also failed and caused that part to go, too, then you have the risk that you will cause the replacement part to fail immediately. Such is the joy of electrical and electronic repair and diagnosis.
jimh posted 05-21-2014 10:52 AM ET (US)
When a technician has the luxury of having had a lot of experience in repairing a limited number of devices, the technician develops insight into the possible causes of problems based on the history of the devices he has seen and the failures that they have presented. In this regard, advice from someone such as SOSMERC is extremely valuable. Having no doubt seen many failures in the electrical system of Mercury outboards brought to him for repair, SOSMERC probably has the necessary experience and gained knowledge to be able to give good advice about which component in this electrical sysem is most likely to be responsible.
swebre posted 05-23-2014 08:52 PM ET (US)
Again, sincere "Thanks" for all the discussion.
Latest run produced same voltage/tach results and now at least one cylinder that didn't wake up until ~5 minutes into a three hour ride. Engine ran flawlessly after but w/ voltage and tach mis-behaving as before. For most of the trip voltage was stable ~14.4V w/ few excursions but started to fluctuate between 12.5V (battery) and 16.5V last hour heading home. Considering all the engine electronic components are original and now 26 years old, have thrown in the towel and have parts coming. A new stator, trigger, VR and 2 power packs will arrive next week. All coils ohm within specs so will keep them for now. Will report back after next outing.
Thanks again folks!
SOSMERC - You're right, I should have included the serial # for the beast from the gitgo. It is "OC100863" sporting the single water cooled regulator/rectifier for the 40 amp system.
sosmerc posted 05-24-2014 08:48 PM ET (US)
Yes, ignition issues and electronics have always been Merc's weakness. (creates alot of work for shops).
You probably should replace your plug wires as well...there was a recall back in the day and I am pretty sure 1988 was the model year affected. You should also probably remove each coil and pull back the rubber cover and inspect the coil cores......you may find them cracked.
You mentioned that you are getting a new stator and VR. Are you doing a conversion to the later style dual regulator system? If not, you may want to consider that CDI electronics now has a new design VR for the water cooled 40 amp system that you have. I have a similar engine in my shop right now that will be getting a new regulator (CDI) next week. Over-voltage was the issue....motor ran fine, but voltage climbing above 16 volts could wipe out guages, lights, trim motor, etc. Not good for batteries either.
'91 2.0 litre 135hp is the motor I'm working on.
swebre posted 05-28-2014 08:13 PM ET (US)
Spent better part of yesterday changing out all the electronics under the cowl. Takes a while when everything is color coded and your partially color "deficient".
New trigger, stator, dual regulator/rectifiers (as suggested) and both switch boxes, all CDI. Re-routed and secured all wires, removed and inspected several coils (all looked fine), and re-adjusted carb linkages. Will confirm timing tomorrow and launch when the weather clears. We'll see!
swebre posted 05-31-2014 05:45 PM ET (US)
Just a quick update. Engine ran like a top today for two hours w/ no misses and showing ~14.2V at all times.
Tach still has issues, but likely just proper settings after the regulator changes.
SOSMERC - Each regulator has a tach wire and I just plugged into to one of them. From the tach behavior, I'm assuming that each regulator is pulling from only 1/2 of the stator poles. If so, I'm assuming the tach should be set to the "6 pole" or three pulse/rev setting. Correct?

swebre posted 05-31-2014 06:07 PM ET (US)
Hmm--I just checked the Flowscan install manual and there is no setting for "3 pole Pairs". Can/should the tach leads from the regulators be "Y" together for the tach input?
sosmerc posted 05-31-2014 08:30 PM ET (US)
You just use one gray wire from one regulator, and it gets its signal from a 12 pole alternator (6 pulse tach setting). And yes, sometimes you have to "fiddle" with that adjustment screw just a tiny bit to get a steady correct reading. Be sure that you using conventional marine starting batteries--no AGM or Gel batteries recommended with your ADI ignition.
sosmerc posted 05-31-2014 08:33 PM ET (US)
Also important on your engine: doublecheck your flywheel torque--should be 120 ft. lbs. Also, I recommend you frequently check your magnets to be sure they are not coming loose ! This is a common problem on the 40 Amp RED flywheels. When those magnets come apart they can destroy all your hard work !
swebre posted 06-01-2014 03:34 PM ET (US)
SOSMERC - Yep. Have some first hand experience w/ wet-cell batteries and how they can take the "abuse" these older charging designs can give.
Interesting tidbit about the RED flywheel (which I have) and the magnets being launched. I can see how that could be a real mess, real fast. Any specific "test" besides just gently tapping them and listening for a "thud"?
Sincere Thanks!
sosmerc posted 06-03-2014 12:30 AM ET (US)
Re: checking the magnets: look at the space between each magnet. It should be equal. Also, a broken or loose magnet can actually be moved with your fingers...so try pushing on them...you should not be able to move them.

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