D12A missing on no load 1550 rpm to 1900 rpm

Hi guys, just joined up here after discovering this forum while researching a problem for mate of mine.
First up, gotta say I'm impressed - wish we had something like this in Australia, but we don't so here I am.
Ok, so here is the drama- have a D12a in a '94 FH12. ECU is a Ec93 spec at 420 hp. 24 volt system, so I believe euro wiring specs (ie insulated guitar strings).

Now for the guts of it - A short while ago the engine started carrying on like a second hand lawnmower and coded a camshaft and a crankshaft position sensor. The engine was shut down, offending connectors cleaned and started back up again. A little further down the road the engine starts clunking a bit - turns out it has now broken a tooth off the crank idler gear. Amazingly everything inside the motor survived as it didn't skip any teeth owing to the fact of next tooth just starting to mesh up.

Truck goes into reputable workshop and is repaired. Camshaft sensor replaced but crank sensor found to be ok so left in service. ECU does not show any further fault codes associated with these sensors, or any others apart from a 41 and 46.

When the engine is now started it is all happy - starts with very little cranking, indicating that both sensors are doing their job. But if you spin the engine up to 1500 rpm and upwards, it will run smoothly except it appears to have a hard miss in it as though the injectors were just shut off. Let it drop below 1500 and all comes good again. No codes.

Here is where it gets real interesting - while the engine is running, if you unplug the crankshaft sensor it will code it, but after a short miss in the engine it will run sweet as a nut all through the rev range. The results are the same if you unplug the cam sensor and leave the crank sensor in operation. It will start from either one of the sensors as well, just takes longer to crank.

Truck has a non computer controlled manual synchro gearbox. No workshop manual available at the moment either.

Does anyone have any ideas please?, ' Cos even Volvo are stumped on this one...

 

Hi Pablo,

Thanks for the reply. Here is some more info.....

Injectors were rebuilt approx 10 months ago
We suspected the fuel control valve at the back of the motor so a new one was installed.
The guys initially doing the repair then said it's the fuel pump so they changed that out - no difference.
I decided to inspect the system for evidence of air, so I pulled the return line off the tank and placed it into a large glass jar so I could visual the return fuel - not the slightest hint of air in the system.

When the guys doing the initial repair let us know they were stumped, I raised the question of whether the valve engine brake (not the epg) may be getting a voltage hit on the control solenoids, thereby causing a valve to be held open momentarily - they thought that was a good question so they pulled the top cover off again to double check the solenoids and oil exhaust ports to ensure it wasn't pressuring up and holding a valve open....... They didn't find anything untoward and to be fair, I will trust their work on that, especially since they couldn't find anything else and they were looking hard into it........

One other intriguing aspect of this is that you put the engine under load, it seems to run fine.

I suspect this may be due to the fact that the amount of injected fuel it takes to spin a free running
diesel up is miniscule, so a 50% variance (just a theoretical number here) of that amount shows up as a big problem.

If, however, the diesel is running on full load, the injection quantity by comparison to free run at the same rpm is huge. This means that the larger full-load injection quantity would mask the 50% variance that was present at no-load conditions........ I hope I have made sense here...

 

Hi Snowwy, thanks for joining in.......

I would have thought the same thing first up as well. Especially when you scope it (digital oscilliscope).

For example, the crank sensor shows a steady train of pulses at idle with a recurring 4 millisecond break in the pulse train - this is usually the "reset pulse count" info for an ecu to start counting afresh, thereby eliminating accrued count errors. Quite often it is also used as the reference for other "counting" type sensors and therefore becomes a critical component for the system.

In volvo's case it certainly doesn't have to work that way as evidenced by the fact they will still start and run just based on a camshaft count by itself or even the other way round ( a crankshaft count by itself ).

I then started to wonder if the system uses the two sensors to operate on a "comparative count basis", but as Pablo has pointed out, this is not the case also.....

 

Haha, I asked Volvo man about VCADs pro - He started talking drug money large on that one.....As for the injectors I am not sure whether they are lucas or bosch. The cam and crank sensors are bosch however.

I'm kind of hoping the injectors are not "Lucas -The Prince of Darkness". I keep having an argument with myself about the injectors though - you would think that if it were injectors it would certainly show up under load, unless it is related to the "light load - small injection quantity" issue i posted about