HDM & Cooling Fan Module Operation

[TJBaker57]

For quite some time I have wondered about the HDM and Cooling Fan solid state modules operation. So recently I decided to get some readings. What I got was not at all what I expected to see.

I began with the DRL/Lo beam operation. Here is what I saw reading the control signal from the BCM to terminal #5 of the HDM to chassis ground:

Key Off Engine Off, Lo Beams OFF: battery voltage DC, 0 VAC, 0 Hz, 0%

Key Off Engine Off, Lo Beams ON: ~30mV DC, 0 Volts AC, 0 Hz, 0%

Key On Engine Off, DRL ON: ~2.15 VDC, 3.8 Volts AC, 128.3 hz, 17.8%

Note that to activate the DRL I had to shift out of Park.


Note: when I measured with the engine running the voltages with DRL active were a bit higher but the frequency and duty cycle were the same.

It appears the control signal from the BCM to terminal #5 is always battery voltage whenever the Lo beams are OFF, even when the vehicle has been parked and the network is in sleep mode. I left mine parked and key out for close to 20 minutes and the signal never turned off. I think a loss of this signal would result in the Lo beams turning ON since the power for the Lo beams to the module is hot at all times. Seems like I have read posts where users reported not being able to turn OFF their headlights.




Moved on to the Cooling Fan module and saw similar behaviours as expected since they are the same module.
I'll need to do some testing while driving as testing at idle engine rpms did not always get a response since the fan was already turning faster than the desired rpm would go. But what I saw when increasing the fan speed was the control signal from the PCM would be battery voltage when fan desired speed was less than fan actual speed resulting in a fan clutch OFF condition, a momentary drop to roughly 6 or 7 volts while activating the fan clutch solenoid valve (saw no difference between a 50% command or a 100% command), and the frequency would change from 0 Hz to about 1.99 Hz while the "%" (duty cycle?) went from 0% to about 98.x to99.x%.

 

[budwich]

good info. I guess it depends on what type of FET is in the module (N or P) along with the characteristic required, ie. mostly ON or mostly OFF in terms of duty cycle. Good to know which "stimuli" causes which result. Makes for easier testing for faulty component and as you indicated surprisingly, I would have thought "ON" would have been "full voltage".

 

[TJBaker57]

It appears the control signal from the BCM to terminal #5 is always battery voltage whenever the Lo beams are OFF, even when the vehicle has been parked and the network is in sleep mode

Well,,, not quite accurate.

Why is it that when I do these things it is frequently some days later when a light goes on? (Get it? Light?... HDM?... nevermind)

It occured to me today that I assumed the battery voltage I saw was a control signal from the BCM. Nope. It's the HDM module that is the source of the voltage. They even put a picture right on the darned thing and I still didn't get it!..



Clear as day is a line with a resistor from terminal #2 to terminal #5. The BCM takes that low to effect control !

 

[mrrsm]

This Thread may have some additional useful information on this topic:

NEED HELP - Engine dies with DRL on

I have a 2006 trailblazer I have several problems but the main one is every time i start the truck it dies unless i turn the daytime running lights off. with the lights off it runs fine as soon as you turn them on it dies. 2nd problem is i noticed that when i turn the key forward my cel does not...

gmtnation.com

 

[santon]

This is very valuable information. It helped me to understand how the infamous "quad beam mod" works. This mod allows a simultaneous operation of the high and the low headlights. One of the variation of this mod is to put a diode between the terminal 85 of the high beam relay (relay #43) and the pin 5 of the HDM module (#46). From the GM diagram, it is not clear how the HDM operates - the left side of the HDM symbol looks like a coil with the terminal 6 grounded and the terminal 5 receiving a control signal from the BCM (please see attached). So, looking at the diagram, one may think that the low beams are turned on when the high signal is applied to the pin 5 of the HDM. Then, the above-mentioned quad mod must not work - when the high beams are on, the terminal 85 of the relay #43 is clearly grounded (since the terminal 86 is hot at all times). The fact that the HDM turns on a low signal explains the quad beam mod. The diode is basically grounding the pin 5 of the HDM when the high beams are turned on.

 

[mrrsm]

Amplifying on @budwich 's information in Post #2:

This attached PDF is a combination White Paper and U. S. Patent Application that discusses the Industry Wide accepted principles of "Fixed Frequency - Fixed Duration" for controlling numerous DC Circuits versus the present PWM (Pulse Width Modulation) such as those underlying the function of the Low Beam Daylight Running Lights in use on GM-GMC Vehicles.

This comparative information is definitely 'WIWIG' (Worth Its Weight In Gold) for improving our understanding of this Topic and invites the use of either a PICO-Scope Model# 2204A or a Hantek Model #1008C Oscilloscope and a Laptop or Tablet Computer to perform System Diagnostics when any problems arise:

Source:

US8604709B2 - Methods and systems for controlling electrical power to DC loads - Google Patents

Fixed Frequency, Fixed Duration power controls methods and systems are described for application of power to electrical loads. FFFD techniques according to the present disclosure utilize power train pulses with fixed-frequency fixed-duration pulses to control power applied to a load. The load...

patents.google.com