How 'Bout that ECM

On a 454XL (carbed) engine, what does the ECM do and how does it do it ? Thinking it principally controls ignition timing but would like to know what else. Will the engine revert to "limp mode" if the ECM dies ?

ITs not an ECU...its a Spark Controller....I doubt I has a limp mode...Like the TBI systems they were orphaned short order...

If the fail, I'd b inclined to run the engine off just the module.

Thanks for the comeback Mako. Just want to better understand how the MSTS works. The ECM has 11 wires going into it. Would really like to know where all the wires go and what they do. Does the ECM utilize an oil pressure or temp input ? I know it has a 5000RPM rev limiter. Does it limit revs by retarding timing ? Don't know how else it can do it. Believe the MAP can provide input to retard timing. Also see reference to a knock sensor - but can't find it anywhere on the block. Can't find any decent tech data on MSTS anywhere. Is there any good info in the 454 XL manual ? Would welcome an authoritative primer on the operational modes of the ECM and how it responds to engine rpm/loads. Understand a new one must be programed to the engine. What is that all about. Is that just a matter of loading an rpm/timing advance schedule on to an Eprom or is there more to it ? Will the engine run if the ECM is unplugged at the distributor (and no timing advance) ? Give us light Oh Wise Ones !!

Being as it only controls timing, I'd doubt that it is concerned with temp or oil pressure....The MAP can only be an input to the timing map used as is the knock sensor.

As far as tech reference, if one exists, it would be a crusader pub because its their system....I haven't ever seen that pub so can't tell you if it exists or not. I'd suspect the programming is nothing more than tailoring the spark curve to the engine in question....of course with all the extra adjustments that the extra inputs enable. (timing maps are timing maps.) If the box is one time programmable, it would use a PROM class of device - if it is reprogrammable, it would use an EEPROM (or one of the newer r/w memory technologies).

You can make an EST distributor work without a computer.....its a bit more than just unplugging it but not much more. And if the MSTS box failed, that's the path I'd endorse...

I don't know the unit but looked at the parts diagram. As Mark says, it is pretty basic and is meant to control spark advance. It seems to be a precursor to later ECM units which have more bells and whistles on the spark side, and which also control the fuel flow. In your case, there is no crankshaft sensor so the ECM does not know where TDC is. This means that base timing advance is set with a light on the crank pulley and rotating the distributor, just like a non-ECM engine. Whereas in non ECM engines the spark advance is purely a function of rpm (rotating weights in the distributor), in this case the ECM "knows" the RPM from the distributor pick up coil, and also "knows" the manifold pressure from the MAP sensor. Usually the design engineers have, though testing, determined the best advance for all combinations of these two variables and programmed a "look up" table in the ECM to determine what advance to use. More modern ECMs do it exactly this way, but then add some adjustments based upon other inputs you do not have.

You also have a knock sensor. If the sensor shows knock, the ECM will retard timing until it goes away. When knock occurs it sounds like a tapping to you, but in reality the engine block rings like a bell. The sensor is tuned to the expected ringing frequency of your engine and the ECM notes this input and does it's thing. The knock sensor will be screwed into the block somewhere (usually on the side, but it could be anywhere). Knowing that the ECM will adjust for knock allows the engineers to be more precise and aggressive in programming the advance, since unusual conditions (heavy loads or bad fuel) will not lead to prolonged destructive engine knock.

You also have ECM inputs for water temperature and oil pressure. Water temperature may be used to adjust the timing on a cold engine (or maybe not) but is probably used to control the electric choke. Oil pressure is likely an engine safety input, and if you don't have pressure I would expect the ECM to shut things down.

I don't know about the "limp home" issue but I would expect that if the whole thing failed you might lose all of your spark advance but still have spark. I don't know what rpm can be achieved with only base advance, but I am pretty sure you would be "limping"!!

That's probably the story, but others with actual experience on the engine may have some corrections.

Mako, Cabo – thanks for the responses. My problem is my right engine runs greatidling out of the marina and powering up to 3500 rpm, however, after about tenminutes, the rpm starts rocking between 3500and 2500 until I throttle back to about 3100. Performance from 3100 down to idle is pro forma. The problem isremarkably repeatable. It feels like therev limiter is kicking in and is limiting me to 3100 rpm. Already did all thestandard fuel stuff, distro cap, rotor, ICM etc. It is like the ECM is dropping off and I haveno timing advance. The problem seems tobe heat related since the engine runs great until fully warmed up. Really curious about the wires going to theECM so I could back check them for positive connections and consider other componentsthat may be at fault. At some pointwill try swapping ECMs. Don’t think it is a bad pickup coil or ignition coil sinceperformance below 3100 is unaffected – but who knows. Welcome any theories.

I suppose it could be some sort of ECM timing advance failure, but 3100 is not too bad and certainly is too high for any kind of programed power reduction (which it is unlikely to have in any case). I would go back and reconsider the "standard fuel stuff" you have done. It sounds like the system is not sending enough fuel to the carb to support any higher rpm. Partially clogged pick up screen would have exactly these symptoms (good for a little while after shutdown and then crud reattaches and limits flow) . Also maybe failing fuel pump, collapsing suction hose, clogged tank vent or ??. Lots of things go wrong with boats.

I don't believe the coolant temp and oil pressure go to the timing box....

I'd get a timing light out and check it, though it be tough with the "computer" controlling the timing.... You have the other engine as a reference.

You can also use the advance function built into the module by just unplugging the four-pin connector (engine off).

The MSTS box most likely tries to maximize the timing within the envelope as defined by the sensor inputs. Since the load seen by the engine doesn't vary like a car, the timing optimizations probably won't matter much.

2x on double checking the fuel supply system...

makomark said:

Being as it only controls timing, I'd doubt that it is concerned with temp or oil pressure....The MAP can only be an input to the timing map used as is the knock sensor.

As far as tech reference, if one exists, it would be a crusader pub because its their system....I haven't ever seen that pub so can't tell you if it exists or not. I'd suspect the programming is nothing more than tailoring the spark curve to the engine in question....of course with all the extra adjustments that the extra inputs enable. (timing maps are timing maps.) If the box is one time programmable, it would use a PROM class of device - if it is reprogrammable, it would use an EEPROM (or one of the newer r/w memory technologies).

You can make an EST distributor work without a computer.....its a bit more than just unplugging it but not much more. And if the MSTS box failed, that's the path I'd endorse...

Click to expand...

If the MSTS 4-wire plug is disconnectedpermanently at the distributor, and the shunt doomaflage is used to adjust thetiming, is the ignition system now exactly like a Delco Voyager setup ? This would help me eliminate the MSTS MAPand knock circuits as a source of my 3100+ rpm cutout after warm up.

As long as the module in the distributor is a 'regular' EST module (V-8), yes. I see no reason why it won't work like that. the 'loss' is the dynamic management of timing.

Sounds like a plan....had a similar situation years ago.....started after the 'mechanic' that did my gear had to visit the boat (he had to swap the orientation of the pump cause he didn't pay attention to the friggin arrows). Anyhow, the checkout cruise went fine but later that day had that engine falling off after getting fully warmed up.....turned out the idiot helper of the mechanic had stepped on the HX hose going to the manifold and cracked it...when it got hot enough, the crack would let a tiny spray out....and it landed on the coil's high tension lead and dribbled back to the coil....after enough accumulated, the spark would get shorted out....took longer to find it than to eliminate it.....live and learn about who you hire....

Problemsolved !! CaboJohn was closest to themark suggesting fuel starvation. Apressure gauge teed into the carb fuel line told the story. Upon start had 7 psi – on spec. At 3500 rpm maintained 6.5 psi for aboutthree minutes and then began to slowly decrease. At 2 psi the surging started until Ithrottled back to 3000 rpm and the pressure rose to 5 psi. Thought the problem may be the fuel pump butdelayed taking the bait until more testing was accomplished. Attached a volt meter to the fuel pump CB anddiscovered the voltage dropped as the pressure dropped. Then attached two voltmeters to the fuel pumppower supply wire; one before and one after the oil pressure switch. As I bended throttles on the bridge my FirstMate reported pressure and voltage readings from the engine room. She reminded me of Mister Scott, “ Aye Kaptan, that’s all she can take cause she can’ttake no more”. No kidding, she wasawesome! Voltage to the switch remained at 13 volts while voltage out startedat 13 volts but slowly decrease to 4 volts. Pulled back the throttle to 3000 rpm at the surging onset and the voltagerose to 10 volts. This seemed to clearlyconfirm my one year old $110 oil pressure switch was defective. Finally, real progress in solving theproblem. Changed out the oil pressureswitch and motored out for what I hoped would be the final test run. Bothengines spooled up like a pair of J79s and never missed a beat. When I was convinced the problem was solved, Icut donuts in the calm waters of Choctawhatchee Bay, overwhelmed by theeuphoria. The clue bird had landed and Iwas Neil Armstrong, the 3500 rpm barrier was broken and I was Chuck Yeager – I couldgo on but I think you get the picture. Myyear-long nautical nightmare was over !! I motored triumphantly back to the marina and envisioned fire boatsputting up red, white, and blue arcs of water saluting my technicalaccomplishment. A little over the top,but it was a rough year and, what the hell, it was my vision. I later searched all the car and boat forumsand did not find another instance of the gradual switch failure I hadexperienced. My initial suspicion thatan electrical component was overheating and failing under the thermal burden provedto be correct, however, I was concentrating on the wrong system. The first fork in the troubleshooting decisiontree is the question – is it spark or is it fuel ? New water seps, filters, year old fuel pumpand pressure switch, fresh ethanol-free fuel, clean vents, no anti-siphonvalves, problem occurring with main tank, saddle tank, both tanks selected ,problem happening only after warm up – all suggested problem was spark and notfuel. As the old knight in The LastCrusade said, “He chose poorly”. I spentevery weekend of the summer toiling under the Florida sun wringing out theignition system which proved to be fully functional. As myeyeglasses pooled with sweat I envied the young studs having a blast on the waterwith their 18-foot runabouts and young honeys clad in bun-flosser bikinis – butI digress. The real lesson learned istroubleshooting must be thorough and the temptation to randomly change partsshould be controlled. It took a longtime to solve but I learned much about the Crusader ignition and fuel systems,and feel good about not resorting to a professional parts changer. Thanks to all the forum members who providedmuch good advice and insight – truly an excellent resource. Fish - be aware that I am back !!
FL Panhandle

Congratulations! Strange sort of failure since the switch seems to "un-fail" as you lower the rpms. But then boats are full of strange failures, and the important thing is you are back to boating.

2x on the congrats....

What the voltmeter was telling you showed Ohm's law at work....the contacts inside the switch have failed and in an uncommon manner....sad part is its not likely Crusader will warranty the switch....if they don't, I'd suggest whacking off the pigtail at the switch and saving it for a Holley 12-810 or equivalent....you can buy 3 or 4 for what the OEM part costs....

Finally, I think the bigger benefit is the value of troubleshooting.....real troubleshooting....I'd bet this one will pay dividends in the future....

Mako,
Checked out the 12-810 option and like it. Looks like a $30 direct 3-wire conversion using the harness from the OEM switch. Two big $$ OEM switches in two years cuts it for me. Do you know how the Holleys hold up in the salty marine environment ?
Thanx

I know of three aftermarket OP switch 'mods' in use and there are all in the Chesapeake (tidal but not ocean) and one uses that specific part. Three years and never missed a beat. I'm sure a few good ring terminals and some liquid tape, along with a good coat of paint, and you should see five years without an issue....the only risk is room to install it....worst case scenario is you tap a different spot in the oil galley to install it....