Timm's The BMW M62 and M62TU Engine
The Engine Failsafe Problem - Main 30A DME Fuse Blowing
Repair
The pictures above are from a M62TUB44 where the problem was caught just in time. The connector pair and the ECU pins were given a good
scrub with contact cleaner and a small stiff brush.
This problem was peculiar, it started with the car over-revving a couple of times and then showing 'ENGINE FAILSAFE' on the instrument
cluster OBC readout. The car could be switched off and then back on, and all would be OK for a few more miles. Later, the engine behaviour
was so erratic that it could not be driven any further and the 'Engine Failsafe' was a constant error.
I've seen this happen a few times now and there are a few different error codes that are thrown
by the primary cause:
•
P0120 119 Throttle/Pedal Position Sensor/Switch 'A' Circuit
•
P1622 140 Map Cooling Thermostat Control Circuit Electrical
•
P1633 136 Throttle Valve Adaptation Limp-Home Position Unknown
•
P1634 133 Throttle Valve Adaptation Spring Test Failed
•
P1635 134 Throttle Valve Adaptation Lower Mechanical Stop not Adapted
•
P1636 132 Throttle Valve Control Circuit
•
P1637 130 Throttle Valve Position Control, Control Deviation
•
P1638 131 Throttle Valve Position Control Throttle Stuck Temporarily
•
P1639 131 Throttle Valve Position Control Throttle Stuck Permanently
•
P1643 135 Throttle Valve Actuator Start Test Amplifier Balancing Plausibility
During initial investigation a few things were replaced or went through fault-finding procedures;
the throttle-body was thoroughly tested, but these are extremely reliable (unlike those used on
the M70 engines) and was not the cause of the problem. The throttle pedal was replaced and the
error codes still pointed to a pedal or throttle body problem.
The 'Engine Failsafe' error reports that the engine cannot be controlled correctly through the 'fly-by-wire' system, or EML to give it its proper
acronym. The EML system cannot determine the positions of certain engine sensors and decides to shut down the engine. More information
on the EML system can be found in this document
The source of the problem eventually turned out to be the infamous temperature-mapped thermostat - or more precisely, a failed
heater connector that allowed coolant past it and into the wiring loom. On both the M62 and M62TUB engine this thermostat heater causes
all sorts of problems. On the pre-facelift E38 and E39 you don’t get the Engine Failsafe warning - the engine just stops and won’t start
again. This is because the main DME fuse has failed due to the damage to the heater loom as seen below:
This seemingly unrelated problem has surprising consequences - the coolant works it way all the way up the wiring loom and into the
engine ECU (DME). It hardly seems possible, but the loom works like a pipe, delivering coolant into the ECU where it drips onto the
connector pair. As the coolant contains minerals, these can produce an electrolytic action that actually dissolves the metal tangs:
The thermostat housing was replaced - and that was it, fully working again. However, there have been at least one other case where the
damage was deemed to severe (by an Independent garage) and the replacement of the ECU and loom was more than the car was worth.
The bottom line is, if you get an engine failsafe on the facelift 740i or 540i, immediately check the thermostat housing for coolant leaks.
The DME fuse is hidden away in a separate fuse carrier, the fuses are all 30A - don’t try a bigger fuse as this can cause a car fire that you
won’t be able to put out. The fuse carrier is shown below in the EBox on an E38:
The damage to the loom can also have another side effect, the temperature gauge goes quickly into the RED zone - and it will do this
as soon as the ignition is switched ON - sometimes this is the first warning you get of the wiring fault. The damage shown in the right-hand
picture above is in the engine wiring box above Bank 1 - easy to check and trace if you suspect this is your problem but, as a general rule:
If the 30A fuse blows in the auxiliary fuse carrier in the EBox - you can be 99% sure that the thermostat heater
or its loom has failed
Right, let’s have a look at the problem on the M62TUB and the associated damage that can occur to the DME - here’s the source of the
problem - the thermostat heater connection:
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