In this article, we have a look at the Volvo Penta MDI electronic black box while relocating it off the side of a Volvo Penta D2-40B engine in order to protect it from the heat and vibrations.
IF YOU OWN A FAILED MDI BOX, PLEASE CONSIDER SENDING US PHOTOS OF THE INSIDE AFTER LIFTING THE LID! There have been many revisions of the module and it would be extremely interesting to see what design changes were made.
Volvo Penta began the release of the current D-Series marine diesel engines around 2006. While all the larger models are common-rail, fuel injected electronic engines, the smaller D1- and D2-series engines, up to 4 cylinders and 75HP, still operate with traditional mechanical injection pumps. Volvo elected to interface these engines to its electronic gauges and controls by using an electronic interface module: the MDI (Mechanical Diesel Interface) black box. For some owners at least, the MDI box rapidly gained fame as the least reliable part of an otherwise excellent engine.
The MDI box has had a surprisingly long revision history over the years, aimed at addressing failure modes and on-going reliability issues, with some versions recording high failure rates. The table below, compiled from publicly available information, shows the consecutive model numbers and approximate year of release when it could be determined.
|Volvo Part Number||Year released|
|23231607: kit containing 23195776?||2019|
The role of the MDI box is largely a supervisory one in the sense that it reads all the engine sensors and outputs the information over CANbus for the Volvo EVC gauge/display system, but it does have a minor control function: it switches the engine glow plugs, the starter motor solenoid, the fuel stop solenoid and it energises the alternator about one second after the engine has started. A failure of the MDI box leaves the engine dead, but it can be bypassed very easily to preheat and start using a simple jumper cable or even a screwdriver, with caution. The key issue is the lack of monitoring over coolant temperature and oil pressure afterwards. In fact, should the concern of being unable to start the engine be acute enough, one could opt to install an independent parallel engine start circuit by adding two external relays to connect the “PREHEAT” and “START” terminals to the “BATT” terminal at the push of a button. In this case, the preheating relay must be able to handle a current of about 10A per glow plug, so 40A on a 4-cylinder engine. Briefly applying power to the “D” terminal post of the alternator after the engine has started will cause it to begin charging normally, but increasing RPMs can be enough to achieve the same, due to the residual magnetism normally present in the rotor.
Reliability Factors: Heat and Vibration
The MDI box is factory-mounted to the side of the water-cooled exhaust manifold. When the engine has been running for some time, its temperature is approximately equal to that of the coolant: too hot to keep a hand on. Furthermore, it is hard-mounted to the manifold casting and fully exposed to the vibrations of the engine.
High heat and vibrations are two well-known root causes of premature failure for electronics. Fortunately, I make little use of the engine and I very rarely run it for any amount of time. This may be why I never experienced any issues with my MDI box in 10 years and 230 engine hours. Nevertheless, I always had in mind to relocate it off the engine block to prevent a failure.
Relocating the MDI Box off the Engine Block
A preliminary investigation some time ago had shown that the wiring harness connecting the sensors around the engine to the MDI box was generally long enough to allow remote-mounting the box on the sidewall of the engine compartment, thanks to some extra length in the loom bundled with plastic cable ties. In fact, this extra length almost suggested that relocating the MDI box off the engine had been made possible and favoured by design, at least on early engines. This didn’t come as a complete surprise as it was not the first instance I found of understated, unadvertised superior engineering on a Volvo Penta engine. Another one is remote voltage sensing for the alternator. I would have otherwise extended the cabling by cutting it and splicing it.
I constructed a plywood pad fitted with two M6 studs and epoxy-glued it to the sidewall of the engine compartment to support the MDI box. I also had to accommodate a few other constraints, namely the lengths of the Multilink cable feeding the EVI gauges and the cable to the EVI control panel, as well as the presence of an access panel immediately to the side of the engine. I moved the MDI box down and back to a location close to the rear engine mount, quite low. A low location may be more prone to see water in a rare and improbable event, but it is likely to be cooler too and I decided this would be adequate. I only had to lengthen the coolant temperature sensor wire and I was otherwise able to re-route the loom without issues.
Reliability Factors: Electrical
Many failures, and especially short-term failures, instead have electrical root causes except for some rare instances where engines shipped with clearly defective MDI modules. Some early versions would have failed from exposure to the back EMF of the engine stop solenoid. This can be prevented by adding a diode over the device to short the negative spike out and this is an electrical alteration I will carry out at some point (or I will trace the wiring and add it within the module if not already there).
Many failures of the module, reported or real, have in fact been caused by poor connections to the battery: a high resistance path causes the voltage to drop whenever the starter solenoid and motor are energised, which can easily cause the electronics to reset and lead to starting problems. A negative voltage spike is also induced into the engine electrical system each time at the end of cranking. It normally has very little effect because the very low impedance of the battery absorbs it, but if the battery is poorly connected, it can cause high reverse voltages to appear at the MDI box supply. This event has demonstrably killed many modules, often several in succession, with the blame usually going towards the black box and Volvo Penta, when it is in fact an installation problem. Use batteries with threaded studs and quality disconnect switches and ensure that all the connections are bolted tight.
Don’t even think about replacing the module until the path to the battery is above any suspicion
It also pays to remember that, in the factory configuration, the alternator charges into the main positive supply terminal at the starter solenoid. Anything less than a solid, uninterrupted connection of the engine to the battery while the alternator is charging will cause the voltage of the engine electrical system to spike up with the risk of destroying the MDI module.
Some modules were also lost due to the addition of ground disconnect relays to the installation on alloy boats because of the unclamped back EMF of the relay coil.
Pathways Following an MDI Box Failure
In the light of the poor reputation of the Volvo Penta MDI black box, I always wondered what I would actually do if I faced such a failure, remembering that both reliability and maintainability are important to me in the context of ocean cruising in remote places. I envisioned a few pathways, listed below in decreasing order of desirability:
- Repairing the module, if possible. With an electronic engineering background, this is always the first consideration that comes to mind, but the electronics could be potted in resin and completely inaccessible. The root cause of the failure, once identified, should be addressed however.
- Doing away with the Volvo Penta EVI instruments system once and for good. This would require installing standard automotive gauges for coolant temperature, oil pressure and engine speed, as well as a few stand-alone switches and relays to deal with the glow plugs, starter and electric stop.
- Developing an equivalent replacement module. This would represent more work, but a better third-party open-source module would clearly have a market. Rather than using gauges on CANbus, a simple LCD display could present all the information. Such a module could be much simpler than the Volvo MDI box, which is clearly inheriting technology from the ECUs of the larger electronic engines in the range.
- Replacing the module. It is easy (and quite costly), but I have heard reports of people having gone through more than one module, because replacement alone obviously failed to address the root cause. This would hardly be satisfactory.
The second option has always been highly desirable in my eyes, because it would replace a proprietary system with one that is standard and fully maintainable at low cost more or less anywhere in the world. The field of engine instrumentation seems to be split between European and American standards. Here, any such solution would rely on European gauges and the key challenge could be identifying the factory-installed sensors to select compatible gauges. Worst case, some of the sensors could require replacement to ensure compatibility.
Would the Volvo Penta MDI Box Be Repairable?
While I had the MDI box off the engine and well accessible, I couldn’t resist investigating to what extent a failure would be repairable for my own forward planning; this hinged upon whether the electronics inside were encapsulated or not. I started by unplugging all the multicore cables to facilitate handling, but I left the three bolted heavy wires in place. The housing is made out of aluminium, but the baseplate accepting the connectors is moulded black plastic, held in place by four Torx T-10 screws. A compression rubber seal is present between the two parts. Removing the screws presented no difficulty and then the aluminium cover separated effortlessly.
Underneath the cover, I was very pleased to discover two stacked circuit boards and no potting compound whatsoever. The fully sealed nature of the enclosure also means that the circuit boards are not coated and could be easily worked on if necessary. The top board contains two 40A-rated relays switching the preheat circuit and the starter solenoid. It also includes the power supply for the electronics and additional circuitry with a IRF4905 P-channel MOSFET transistor likely related to the alternator D+ connection, but I didn’t formally trace this. The supply for the logic circuits appears to originate from a NCV4269 5-volt linear regulator, which can handle spikes of up to 60V and reverse voltages down to -40V. The key point of interest here is two electrolytic capacitors rated 220μF / 63V in the power supply section, because these components are well-known to age faster and fail early when exposed to heat. This would make them prime suspects in case of black box failure, because a degradation of these capacitors would result in poor filtering of the electrical noise from the alternator and this could ultimately affect voltage regulation and the operation of the CPU. Here, the capacitors appeared in good condition, without signs of swelling or electrolyte leakage. As far as components go, everything else in the design of the MDI module should generally prove quite durable and resilient. The other enemy of electronics is vibrations and, here, in the absence of encapsulation, some of the larger components in particular could be prone to cracking of the solder joints over time.
The bottom circuit board is the control board, which communicates with the upper board through a 12-pin pluggable header arrangement. While I didn’t attempt a complete teardown of the MDI box, it seems that separating and extracting the top board should be quite easy after disconnecting the three heavy-current terminal posts. This would give access to the solder points to replace the capacitors or even the relays if it ever proved necessary.
The bottom circuit board carries the CPU and crystal as well as more interfacing components to deal with the sensor signals and the CANbus interface to the gauges. The pins engaging into the sealed Deutsch connectors of the MDI box are soldered to it and they should be expected to just pull through the plastic baseplate, same for the auxiliary flat blade terminals. The control board may be less likely to require repairs, but it is not impossible of course. Not dismantling the module only affords a limited view of the circuit. The Philips / NXP-branded CPU is a LPC2119 microcontroller with 64kB of flash memory, quite a powerful 32-bit processor built on an ARM7 core with 2 CANbus interfaces and a fast 10-bit analog/digital converter. The crystal frequency was not readable.
While I took a while before finally relocating the MDI black box off the side of the engine, it should clearly be the first thing done when installing these engines. The presence of electrolytic capacitors in the module looks like a recipe for trouble, even though such capacitors can technically be rated for service lives in the thousands of hours at the temperatures considered. In all cases, the combined exposure to vibrations and thermal stresses promotes the breaking of solder joints over time.
Fortunately, the construction of the MDI box allows access to the electronics. Component replacement or even reflowing the solder over the boards appears perfectly achievable, so a failed module could be repaired. If this was not successful, a replacement module should arguably be able to offer a long service life, provided it is installed in a protected location. The fact that the electronics are not encapsulated makes them both repairable and more vulnerable to failure from exposure to vibrations.
In case of failure, replacing the whole Volvo Penta EVI monitoring system with conventional gauges may not be more costly (in terms of materials) than replacing the MDI black box and doing so would eliminate any reliability issues once and forever. This is a pathway I would seriously consider if I happened to be unable to repair a failed module.