Jul 262019
Volvo Penta MDI Black Box Overview

In this article, we have a look at the Volvo Penta MDI electronic black box while relocating it off the side of a D2-40B engine in order to protect it from the heat and vibrations.


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.

Volvo Penta MDI Black Box Overview

The Volvo Penta MDI black box once separated from the engine and most of the wiring harness.

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 EVI gauge/display system, but it does have a minor control role: it switches the engine glow plugs, the starter motor solenoid, the fuel stop solenoid and it energises the alternator a few seconds 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. The key issue is the lack of monitoring over coolant temperature and oil pressure afterwards.

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.

Volvo Penta D2-40 Factory Engine

Volvo Penta D2-40 engine in its factory configuration with the MDI black box attached to the water-cooled exhaust manifold. Note that this photo shows no extra length in the wiring harness to the MDI box.

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. 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.

Volvo Penta D2-40 engine with MDI black box relocated off the engine

Volvo Penta D2-40 engine with MDI black box relocated off the engine to protect it from the heat and vibrations.

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.

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.

Underside of the Volvo Penta MDI Black Box

The underside of the Volvo Penta MDI black box reveals three sockets for Deutsch sealed connectors and terminal posts for the battery supply and cables to the glow plugs and the starter motor solenoid. This baseplate is held in place by four Torx T-10 screws.

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 start from a NCV4269 5-volt linear regulator. 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.

Internal view of the Volvo Penta MDI Black Box

Removing the lid of the Volvo Penta MDI black box reveals two stacked circuit boards. The top board contains relays and transistors for power switching, as well as the power supply section with two electrolytic capacitors and a 5V regulator.

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.

Volvo Penta MDI Black Box Stacked PCB Construction

The heavy current terminal posts on the left connect directly to the switching board. The control board is hidden underneath it with the CPU and crystal oscillator well visible near its edges.

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.

Volvo Penta MDI Black Box CPU

The CPU used in the Volvo Penta MDI black box is a LPC2119 microcontroller with 64kB of flash memory.


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 achievable, 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.

  2 Responses to “Engine Reliability: A Look at the Volvo Penta MDI Black Box”

  1. Thank you for this interesting and informative article.

    On my D2-75F, the +12V power supply to the MDI is sourced from the starter motor rather than from the battery. This would seem to make the unit vulnerable to voltage transients owing to the inductance of the starter motor. When you looked within the MDI, did there appear to be adequate protection in the MDI box against such transients?

  2. Dear Jeremy,

    As the only supply from the battery is terminated at the starter motor solenoid, this is likely to be valid for all the engines using the MDI module.

    Looking at some of the key components like the NCV4269 linear voltage regulator producing the 5VDC supply, and the 63V-rated filter capacitors, the module seems to have been designed around a 60V peak supply voltage limit. A number of other parts I identified had higher voltage ratings. I didn’t find any large capacity transient voltage suppressors anywhere, so it seems quite clear that Volvo opted to design for a sufficiently high peak voltage and I also think it is the most robust option in this context. 60V should be ample considering that the cables to the battery are normally both quite short and substantial. Longer and/or undersized cables could allow the voltage to spike more at the engine, but even then the available margin seems quite considerable.

    One potential scenario that could lead to the electrical destruction the MDI box is an alternator load dump caused by the loss of the battery while charging at high current. In this case, the surge could easily exceed 60V, especially with the standard Volvo wiring which also connects the alternator output (B+) to the started motor solenoid.
    In most cases, it makes sense to run a new dedicated heavy cable from the alternator B+ post to the battery, or – better – to a battery isolator for charging separate banks. This normally mitigates or even entirely eliminates this possibility.

    Kind regards,


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