Rough Idle/apparant misfire during high alternator load

[MichaelOverfield]

Ok, seems I've got more than my fair share of strange things on my truck... here's a kind of new one that has been getting worse. When the alternator is under high load, say over 90 amps, the truck idles very rough, and appears to be misfiring on at least one cylinder. It goes away as soon as I rev the engine. No smoke of any kind. It's been doing this to some minor degree for awhile, with the grid heaters when they cycle on, and I think I have a different issue there, right now I have that grid heater disconnected so it's not a factor. In any case, all I can guess is there is something that is affecting the injectors (or at least one of them) with the high load on the alternator. I have already verified grounds between the battery and engine, and other locations.

Anyone ever hear of something like this?

 

[mpaulson]

Voltage regulator not kicking in enough supply under heavy load at idle?? Are volts dropping at idle when large load is triggered? Would be my thought.

 

[Ozymandias]

Can you measure the input voltage to the ECM when it happens. I think you have a problem with the wiring. Either plus side or ground, pulling the voltage down under high load.

 

[MichaelOverfield]

Voltage regulator not kicking in enough supply under heavy load at idle?? Are volts dropping at idle when large load is triggered? Would be my thought.

Voltage is normal when it's doing it, 14V or more. It was also my first thought.. the original wimpy alternator would drop voltage with the grid heaters, which is normal.. and it did not do this then. I now have either one or two 220Amp alternators online.. and it does it with either or both running.

 

[MichaelOverfield]

Can you measure the input voltage to the ECM when it happens. I think you have a problem with the wiring. Either plus side or ground, pulling the voltage down under high load.

That was something I considered, but the monitor I have (when it's still getting ECU data, that's a whole other issue) reports normal voltage to the ECU in the 14's.

 

[Alan Reagan]

You may be getting transient spikes on the signal lines. I had a similar experience where my fan clutch was bad and was kicking on and off putting a transient pulse on the fuel load signal. The truck would start missing, most noticeably when climbing a hill. The dealer replaced injectors, and other parts. Finally, the mechanic let me see the computer screen that showed the signal lines. I’m an electronic engineer and noticed the voltage spike on the load line was coinciding with the fan clutch engagement. The clutch was turning on and off at a high rate and that’s not right. They replaced the fan clutch and the problem went away. I would guess that under high load your alternator is probably noisy (electrically) and that could be causing your problem.

 

[MichaelOverfield]

You may be getting transient spikes on the signal lines. I had a similar experience where my fan clutch was bad and was kicking on and off putting a transient pulse on the fuel load signal. The truck would start missing, most noticeably when climbing a hill. The dealer replaced injectors, and other parts. Finally, the mechanic let me see the computer screen that showed the signal lines. I’m an electronic engineer and noticed the voltage spike on the load line was coinciding with the fan clutch engagement. The clutch was turning on and off at a high rate and that’s not right. They replaced the fan clutch and the problem went away. I would guess that under high load your alternator is probably noisy (electrically) and that could be causing your problem.

Thanks, that is the best explanation of a possible cause so far. With the grid heaters cycling the load was excessive.. I have amp meters on output of each alternator and the grids were pulling about 200Amps. The alternators are the 220Amp Denso alternators, not the original wimpy 136A Bosch. I didn't see this kind of thing with the original alternator, so your scenario makes sense. I do have a 83F super capacitor on one battery to mitigate voltage spikes and drops. I wonder connecting that to the engine block might mitigate electrical noise?

 

[Alan Reagan]

That’s a pretty big capacitor. I doubt it will stop transients. Keep in mind, with that capacitor, if there is a voltage drop, it will discharge exponentially and I have seen that cause signal problems in avionics. In other words, the voltage from a discharging capacitor is not a linear decreasing voltage but rather it starts with a sharp drop as it begins to discharge and then the voltage begins to level off at a lower until it begins to charge again.

First I would make sure noise is the problem. Disconnect the grids and any other load that won’t affect engine operation and see if the misfire goes away. The alternators are operating at near max capacity and may just be noisy (electrically). See what it does. There used to be line filters that you could buy at Radio Shack that would eliminate line noise on electronics but they are out of business.

 

[MichaelOverfield]

That’s a pretty big capacitor. I doubt it will stop transients. Keep in mind, with that capacitor, if there is a voltage drop, it will discharge exponentially and I have seen that cause signal problems in avionics. In other words, the voltage from a discharging capacitor is not a linear decreasing voltage but rather it starts with a sharp drop as it begins to discharge and then the voltage begins to level off at a lower until it begins to charge again.

First I would make sure noise is the problem. Disconnect the grids and any other load that won’t affect engine operation and see if the misfire goes away. The alternators are operating at near max capacity and may just be noisy (electrically). See what it does. There used to be line filters that you could buy at Radio Shack that would eliminate line noise on electronics but they are out of business.

I made it from this item on amazon: Amazon product ASIN B07WFFZ4ZS
I've used line filters before, and generally they are just inductor/capacitor units that mitigate voltage fluctuations. My understanding is that a capacitor resists change in voltage, by either taking a rapid charge on voltage increase (spikes) and putting out current when voltage drops (mitigating the voltage drop). An inductor resists a change in flow of current. I would think the Capacitor resisting voltage changes would mitigate noise.

There are pre-made super capacitor units for cars, and even some large capacitors that some have used to replace the battery entirely. It's only recently become afordable as the cost of super capacitors has dropped, they are also getting use in EVs for the ability to take a rapid charge for things like regenerative brakes.

 

[MichaelOverfield]

That’s a pretty big capacitor. I doubt it will stop transients. Keep in mind, with that capacitor, if there is a voltage drop, it will discharge exponentially and I have seen that cause signal problems in avionics. In other words, the voltage from a discharging capacitor is not a linear decreasing voltage but rather it starts with a sharp drop as it begins to discharge and then the voltage begins to level off at a lower until it begins to charge again.

First I would make sure noise is the problem. Disconnect the grids and any other load that won’t affect engine operation and see if the misfire goes away. The alternators are operating at near max capacity and may just be noisy (electrically). See what it does. There used to be line filters that you could buy at Radio Shack that would eliminate line noise on electronics but they are out of business.

Oh, and BTW, I've already tried the disconnect of large loads like the grid heaters.. no large load, no missing or rough idle. It is a direct correlation.

 

[Alan Reagan]

At least we are on the right track. So it looks like the alternators are noisy at max output. It could be just one. You might be able to get them diagnosed at a car electrical shop. Or possibly a local vo-tech school that may have a oscilloscope.
One more thought....clean your grounds. I recently had a problem with my nav system dropping out. The dealer said I needed a new radio. The problem I saw was that that the FPS time kept changing. I cleaned the under the hood frame grounds and the problem cleared up.

If there is a “Pi” filter on those alternators for noise reduction, they have to have a good ground in order to take out the transient spikes.

Of course, an alternator can definitely be a problem if it’s going bad.

 

[MichaelOverfield]

There used to be line filters that you could buy at Radio Shack that would eliminate line noise on electronics but they are out of business.

What do you think about this alternator capacitor? Amazon product ASIN B001QV2W6M

 

[MichaelOverfield]

At least we are on the right track. So it looks like the alternators are noisy at max output. It could be just one. You might be able to get them diagnosed at a car electrical shop. Or possibly a local vo-tech school that may have a oscilloscope.
One more thought....clean your grounds. I recently had a problem with my nav system dropping out. The dealer said I needed a new radio. The problem I saw was that that the FPS time kept changing. I cleaned the under the hood frame grounds and the problem cleared up.

If there is a “Pi” filter on those alternators for noise reduction, they have to have a good ground in order to take out the transient spikes.

Of course, an alternator can definitely be a problem if it’s going bad.

I do have an O-Scope. Think I'll hook it up this weekend and connect grids so I can manually initiate a large load and take a look at what is going on.
Grounds were already checked, and considered suspect. I pulled the passenger side and it was clean and tight, but I cleaned it more and bolted it back on, diver side is under the starter, so did not pull it, but appears tight and clean. One thought I had was to run an additional heavy ground wire from the alternator cases to the chasis or negative battery terminals directly. I was thinking the high current going through the engine block might induces some EMI in the ECU or assoctiated cables to the injector control wires... thoughts on that as a possible?

The issue happens with either alternator online, or both. They are both pretty new, as in about 2017. The are REMY remans, but appear to actually be new units from a detailed inspection. Seems unlikely they could both have issues, but not impossible. I could get another from Rock Auto for about $140 last I checked.

 

[Alan Reagan]

The capacitor may work (the pic above).

Regarding the alternators, they are obviously both charging. Try checking for bearing play on the pulley. If a bearing is going bad or has some play, that will cause noise.

Look at the wiring around the alternators and across the front of the engine. If possible, move the harnesses a little to see if you can stop the miss.

I can’t think of any way to shield the wiring up there but if the problem has just recently started, I’m really thinking it’s the alternators being driven to saturation.

 

[Alan Reagan]

Check this:
https://www.knowyourparts.com/techn...starters-and-alternators-common-misdiagnosis/

 

[Alan Reagan]

http://www.pvv.org/~syljua/merc/TooSeptST07.pdf

This would be some good things to look at.

 

[MichaelOverfield]

http://www.pvv.org/~syljua/merc/TooSeptST07.pdf

This would be some good things to look at.

Excellent stuff there.. I'll try those measurements and see what I find. I have long had strange computer issues.. perhaps it's all related.. though it was present with the OEM alternator as well.

 

[petersonj]

I pulled the passenger side and it was clean and tight, but I cleaned it more and bolted it back on, diver side is under the starter, so did not pull it, but appears tight and clean.

Visual inspections are good, - looks "clean and tight" are good, too..., but it does not guarantee a good electrical connection. Performing a voltage drop test will give you very accurate information regarding the condition of any electrical connection. Since you have a means of providing a heavy electrical load, you could perform a voltage drop test in the ground circuit while under an electrical load.

- John

 

[Tuesdak]

diver side is under the starter, so did not pull it, but appears tight and clean. One thought I had was to run an additional heavy ground wire from the alternator cases to the chasis or negative battery terminals directly. I was thinking the high current going through the engine block might induces some EMI in the ECU or assoctiated cables to the injector control wires... thoughts on that as a possible?

The engine block is a solid short, period. Running ground wires to the alternator, etc. is useless. Further it is risking the possibility of a ground loop. Not only can these small wires go up in flames if the bolt falls out of the main battery ground strap (any failure including corrosion) forcing the new added wire to carry the full starter current... There is Noise. The loops of wires to the noisiest component on the engine, alternator, can transmit the AC noise to other things. Where you bolt a ground to also affects noise. Easy button of adding a ground strap to an existing bundle of grounds can add noise to the system. An example is a (Big 3 kit) engine to frame, frame to body, and engine to body ground strap dropped on a OEM ground bundle on the fender. This generated alternator whine on the OnStar mic. Moving the new ground strap 1" away on the fender eliminated the new noise.

Again one common example: if a high current connection fails by cable corrosion, bolt falls out, etc. the current may have a way to run back to the battery over the smaller ground loop wires. Best advice is to only have ONE connection to the (negative) battery terminal. Yes, I am aware RAM has many positive terminal connections for the grid heater and this may be the place to start.

I would load test each battery. I would disassemble that ground behind the starter because looks don't mean anything. Corrosion or oil hidden under the bolt is normal for a bad connection that looks "OK." Battery cable internal corrosion is a suspect. Also clean all the connectors on the alternators. Then you start tracking down the positive cables like the other end of the alternator charge wire. A voltage drop test may help find a high resistance connection.

Is there an amp draw test for the grid heaters in case one is shorting out or suffering from that loose engine killing bolt issue?

What wires have you added and where? Was the 2nd alt a factory installed option and have you routed the wires exactly as the factory would? One TSB comes to mind on exact wiring and routing:

GM for some 5.3L V8 engines in the 2002 era would cut the long alternator regulator voltage sense wire from the main bus junction and put it on the alternator output lug. (Making it now useless.) The long sense wire was causing an interaction and generating enough noise it was messing up the ECM during shifting.

Oil on the damn connector...

The worst electrical problem I ran into took an auto electric shop all summer to find. SES light popped on with an 89 Olds Cutlass and the V6. At first there were no stored codes and the bad ECM that wouldn't store any codes, even with the light on, was replaced. (With no codes I took it to a shop.) Then the code that started all this indicated ignition module failure as it wasn't responding to ECM timing requests. DI ignition with waste spark. So the shop went NUTS looking at: ECM, ignition module, coils, ECM wires, grounds. Even the easy stuff like spark plugs and new wires. Kept saying ign module timing problem. I recall the shop even had an ECM tester/simulator of some sort.

Turned out the code really meant there was a valve cover gasket oil leak. The oil leak dripped on the knock sensor, in a PIA location firewall side of the engine, and had insulated the electrical connector. So the ECM would no longer get a knock signal from it. To set the code the primitive ECM would run a random test: step timing to max advance listening for a knock. When it didn't get a knock signal it coded that the timing couldn't be advanced rather than knock sensor fail...

This problem was right up there with the positive battery cable that would short out (just sparking with engine vibration vs. dead fire causing short) on a heat shield only when the engine got hot causing the engine to shut down on a grade. Somehow it didn't burn it to the ground. Word was that wiggle test was very exciting...

 

[Ozymandias]

On mine the driver side ground strap is attached in front of the ECM to the block. There is a 1/2" thread, look for it.