Connection failures

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Steve Smith
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Joined: Sun Aug 25, 2013 7:22 am

Connection failures

Post by Steve Smith »

The saying “your measurements is only as good as your connection” cannot be ignored and the same of course goes for any sensor or actuator especially where load is concerned. I know we deal with connections every day and rectify such issues often with no more diagnosis than a wiggle test. However, I wanted to go through the havoc behind the scenes of “bad connections” looking at nothing more complex than a heater blower assembly

Below we have the heater blower assembly of a Peugeot 307 1.6 HDI (With Automatic Temperature control)
Customer complaint: Heater blower inoperative
Diagram
Diagram
The heater blower motor is supplied with +12 V on a red wire via pin 2 (M7)

The earth path for the heater blower (pin 1 M7) is controlled by the heater blower control module on a black wire @ pin 4 (A176) and finally to ground via pin 3 (also A176)

The heater blower control module has replaced the typical heater blower resister pack which traditionally varied the resistance of the ground path to give 3 or 4 fixed heater fan speeds

The heater blower control module includes a power transistor (instead of a resister pack) controlled by a single wire command from the AC control panel (Pin 2 @ A176)

Here we benefit from a number of advantages such as multiple fan speeds, progressive fan start with controlled stop, quieter running, prolonged motor/circuit life and reduced load upon the alternator, so improving fuel consumption which ultimately reduces emissions (all this from a power transistor)

So, what went wrong here with the above “near perfect” solution to cabin ventilation?

A connection error has raised its ugly head resulting the cooling fan becoming inoperative accompanied with a high-pitched whine from the fan motor housing!
Image 2
Image 2
Image 3
Image 3
Image 4
Image 4
The connection in question is pin 3 (earth return) from the blower motor control module which is integrated into the heater blower motor assembly (for cooling purposes) as the byproduct of varying the voltage across the fan motor is “heat”.

The size/gauge of the black cables in the images above give an indication of the expected current flow during fan operation. It is therefore no wonder that a reduction is terminal contact pressure will soon generate heat that cannot be dissipated, resulting in overheating and melting of the connector. Below we capture the heater blower control module connector under the microscope where pin 3 is damaged beyond recognition (It should resemble pin 4)
Image 5
Image 5
So how does this look using PicoScope to capture the blower motor circuit whilst inoperative?
Image 6
Image 6
I guess we should start at the beginning with the power supply to the heater motor remaining fixed at approx. 14.5 V (Channel A) as expected

Interestingly, this heater blower fan would not spin until the engine was running!

Channels B & E monitor the current flow in different sections of the circuit;

Channel B captures the current flow from the heater blower motor to the blower control module (Before the power transistor)

Channel E captures the current flow from the blower control module (after the power transistor) to ground. Theoretically, these current clamps should read identical values as current flow is the same at any point throughout a series circuit (This could be a current clamp zero error and so for now let’s err on the side of caution)

Channel C captures the command signal from the AC Control panel to the blower control module.
The image below reveals how this voltage signal is pulled down in sync with the cyclic current flow

Channel D captures the voltage between the blower motor and blower control module (motor earth)

Looking at the zoomed viewport on the right above, our power supply to the motor remains stable whilst the earth path for the motor (channel D) behaves in a cycling fashion at a frequency of approx. 3.2 kHz.

Following channel D closely, the voltage begins to fall below 14.5 V whilst the current (Channels B & E) begins to rise in an attempt to spin the motor (Note the difference in current between channels B & E?)

Here we have a momentary differential voltage (cyclic in nature) across the heater motor, and where there is a “differential” there will be current flow. Using a math channel, A-D we can graph the differential voltage across the blower motor. Note below how peak current flow occurs when the differential voltage reaches 2.868 V and when the differential voltage is 0 V, current flow is at 0 A
Image 7
Image 7
Whilst momentary current flow was evident, the fan motor made no attempt to spin but would emit a high frequency whine! This is possibly due to insufficient current to commence motor rotation or, characteristic behavior of the integrated power transistor/blower control module circuitry in response to high frequency back EMF?

To qualify the frequency of the audible whine, the opportunity to apply NVH was too much to resist

Below we confirm the audible frequency (3.35 kHz) matches that of the cyclic differential voltage across the blower motor (approx. 3.2 to 3.3 kHz confirmed using the time rulers and math channel A-D)
Image 8
Image 8
To go one step further using a x10 attenuator, the earth path from the blower motor to the blower control module was connected to channel B of PicoScope and configured as a microphone within NVH. The results below qualify beyond any doubt the whine frequency is attributed to the frequency of the differential voltage. N.B 3.35 kHz is the fundamental frequency of our whine whist the captured multiples of 3.35 kHz (6.70 kHz & 10.1 kHz etc.) are the associated harmonics. More information regarding harmonics can be found here topic19731.html
Image 9
Image 9
An audio recording captured by the microphone used on channel A can be heard below (headphones are recommended for playback)
Motor whine .wav
Audio file
(901.91 KiB) Downloaded 29 times
Whilst we have discussed differential voltage, what about the induced voltage (back EMF) exceeding 20 V, created as a result of switching the current flow at high frequency?

To put this induced voltage in perspective, we have current “switching” through the fan motor armature winding at a rate of over 3000 times per second (3+ kHz) Such behavior will generate the induced voltage captured above and no doubt introduce Electromagnetic interference (EMI) into other components and circuits

Quite often, one fault leads to another and so on & so forth; the "desired" repair therefore was a new heater blower motor (which includes the blower control module) and wiring harness. Total bill (including labor) for genuine parts was approaching £800 for a vehicle that is worth not a great deal more

After liaising with the customer, permission was granted to remove the offending connector, modify the wiring harness with new termination and finally secure & insulate with a potting compound. (See repair below)
Image 10
Image 10
Proof is always in the pudding and the capture below confirms the repair as we cycle through all 8 heater speeds. Note, peak motor current at approx. 40 A (top speed) and how current flow measured at different points within the blower motor circuit (Channels B & E) are the same as per the theory of series circuits
Image 11
Image 11
Whilst the peak current measured approx. 40 A, the blower motor was already spinning, the real test of “current handling” comes from spinning the motor from rest to top speed where below we capture a peak inrush of nearly 80 A. Note how the differential voltage reveals the fan speed “progressive control” thanks to the correct operation of the blower control module / power transistor
Image 12
Image 12
During top speed, current flow though the motor averages out to approximately 26 A which when you think about it, is a huge amount of continual current flow!

It is no surprise therefore that terminals and termination form the weakest link in the majority of circuits and none more so than where continual loads are present

The thermal image below captures the blower motor circuit (post fix) where the connections and motor brush housing are revealed as the circuit “hot spots”. (Note, these connections are all good too) This is proof indeed that the reliability and efficiency of any circuit is only as good as the integrity of the included connections.
Image 13
Image 13
All the above “drama” because of a poor earth connection!

I hope this helps provide an insight into “connection error” and the looming consequences thereof in the form of heat damage, terminal/multiblock failure, harness damage, control module failure and EMI to name just a few

To conclude, when our customer collects the vehicle and ask’s “what was wrong?”, you reply “a poor earth connection” to which they respond “is that all it was?” You can hold your head up high knowing the implications behind the full story

A big thank you again to Kevin Ives @ Ives Garage for his support during this mini adventure

Take care…..Steve

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