Bosch European Motors: Mercedes Sprinter Service and Repair Redwood City - Sprinter Glow System and Trans Modifications - Bosch European Redwood City

Sprinter Glow System and Trans Modifications

T1N-platform Mercedes/Dodge/Freightliner Sprinters are susceptible to a couple of unusual issues as they age. One is an internal failure of the glow plug relay that results in a check engine light with code indicating a glow plug fault even when all plugs are good. The other is a syndrome known as “rumble strip noise,” which is an NVH phenomenon that manifests as a loud rhythmic booming that is audible inside the van in certain circumstances. Each can be permanently solved with effective and relatively simple modifications. I had a chance to work on a van that showed up with both problems, and took some pictures as I carried out the diagnosis and repairs.

Sprinter Glow Plug Relay Failure

Rather than a single external high-amp glow system fuse, as most diesels use, Sprinter vans feature a glow plug relay that has five small internal fusible links inside it. (In fact, there are not five but six fusible links in the relay, since this same part is also used for the six-cylinder variant of this modular engine as found in the MB E320 of similar vintage, but for the 2.7L Sprinter application only five of the links are used.) Each of the five individual glow plug circuits is monitored by the glow controller, and if a fault is found with one or more of the plugs, the controller reports the problem to the ECU which turns on the check engine light and sets a DTC (usually both a P0380 general glow circuit fault and a P067x individual-cylinder fault where x = 1 through 5, identifying the number of the cylinder with the failed plug).

If a plug fails open, replacing it and clearing the code is sufficient to restore proper function. However, if the plug fails shorted, it will blow the fusible link inside the GP relay for that cylinder, resulting in an open circuit that will remain even after the faulty glow plug is replaced. Consequently that new glow plug will receive no power and provide no preheating in the affected cylinder, and the relevant trouble codes will continue to set even without any externally traceable glow system faults.

The standard repair in this situation is to replace the glow plug relay. Unfortunately, list price can exceed $250 for the relay alone, less installation, so this is not a cheap repair. Worse still, if the system suffers another shorted glow plug in the future, the new relay will be damaged the same way the previous one was, and yet another replacement will be required. For most owners, repeatedly replacing expensive glow plug relays is not a practical option.

The alternative is to modify the original relay and relocate the glow plug fuses to an external location, where they can be easily replaced individually in the event of a glow plug short without affecting the relay itself. This is the only way to salvage a failed relay, and additionally it improves ease of service and dramatically reduces cumulative repair costs in the event of a future glow system short.

This van had a chronic check engine light with the typical glow plug failure codes stored; replacement of glow plugs had yielded no improvement, so it was a likely candidate for relay failure. Sprinters have the glow plug relay located on the driver’s inner fender, tucked up underneath the battery tray. Easiest access is obtained by removing the grille and LH headlight assembly. The battery should be disconnected before attempting to remove the glow plug relay, since the relay’s large B+ feed wire has a non-insulated terminal and requires tools to detach. (If your Sprinter has its original radio, at this point you will want to make sure you have your radio security code on hand.)

After removing the relay cover, the failure is easy to see. Evidence confirmed the hypothesis that this van had suffered a shorted glow plug sometime in the past, resulting in the burned internal fuse link that was causing ongoing problems.

The relay has two banks of fusible links, three on each side. The link running parallel to the right of each fuse link, connected at the bottom but leading to a different terminal at the top, is for the system’s diagnostic feedback; it allows the glow controller, integrated into the relay, to monitor the actual conditions in each glow circuit and determine whether that plug and circuit are functioning correctly. If the fusible link is blown, current will not make it down the left side of that link to reach the bottom, where the glow plug harness wires begin, and additionally no current will be fed back to the controller’s monitoring system through the right side link, signaling that that cylinder’s glow circuit has a problem.

Second bank of fuse links ready for wire installation.

The relay modification consists of soldering a large power feed wire onto the switched hot side of the relay itself, which then supplies power to the “in” terminal of an external fuse panel. The fuse panel is then set up with five individual, replaceable fuses, each with an “out” terminal feeding power through five separate wires back into the glow plug relay. The end of each of these wires is soldered to its respective glow relay “out” terminal, which the original fusible links would have fed power to (and which is still being monitored by the feedback link on the right hand side of each terminal, as designed).

Wire ends soldered to GP power out terminals.

The end result is that the entire system functions just as before, except that now in place of the internal fusible links are five standard external fuses, with associated wires added for powering the fuse panel and returning the fused power back to the relay. Any faults resulting from a shorted plug can now be remedied by simply replacing that plug and its corresponding fuse. No more expensive glow plug relay failures.