Purpose of 4.7Ohm resistor

[Philip Lochner]

What is the purpose of the 4.7Ohm resistor in the EXTERNAL wiring diagram? : http://www.msgpio.com/manuals/mshift/wiring.html

[Bernard Fife]

From that page:

Note that you need a 4.7 Ohm, 5 Watt (or more) resistor (such as Digi-Key 4.7W-5-ND, 34¢) on the pressure control solenoid wire from the trans connector pin D to the GPIO board's pin 33, and a 1N4001 diode placed between pin C and D on the 4L60E connector (the closer to the transmission connector the better). These cut the flyback spikes from the solenoid to manageable levels.

[Philip Lochner]

Thanks Lance

Yes, RTFM.

I need to report that this resistor holds serious danger IMHO. In my case it could have burnt the transmission harness - if I had mounted it in the harness- or possibly even the car.

The GPIO in my car was getting hot, VERY hot, and I found that it was this resistor that was heating up the GPIO to that extent. I had it mounted on the solder side of my GPIO, immediately below the heat sink, having cut the circuit between Q3 and the AMP connector and mounting the resistor to the PCB after exposing suitable lengths of PCB track and soldering the resistor to the board.

Having looked into this issue over the last 3 days, I concluded that the heat was the result of
1) tuning for the lowest line pressures required (meaning the highest PC duty cycles required);
2) the low value of this resistor (which, combined with the 4-5Ohm of the PC solenoid would result in currents high enough to cause serious heating up of this resistor.

I have now overcome this problem by:
1) tuning for the HIGHEST line pressures possible (meaning lower duty cycles and hence less current);
2) increasing the value of the resistor to 15Ohm. Doing so seems to have no impact on the behaviour of the transmission.

I did try running without the resistor but that immediately caused VSS issues. It was VSS issues (only) that motivated my move to the Extra TransControl FW but now I know that my VSS issues were RMI related, not FW related. So, yes, it will "cut flyback spikes" but it seems to me that its MAIN purpose is to reduce RMI resulting from these spikes that clearly holds very real potential for corrupting other signals, where VSS seems to be the most likely one to suffer from this RMI noise.

[Bernard Fife]

Philip,

Thanks for sharing your experience.

A higher resistance will help reduce the heat, as will as higher power rated resistor. The recommended resistor is 5W, but higher is certainly better if heat is an issue.

That being said, I put many, many hours on a 4L60e with a 4.7 Ohm resistor (& diode) without any heat issues at all. The 4.7 Ohm resistor, combined with the approx. 4 Ohms PC solenoid (and the maximum 60% PWM) results in a current of about I = V/R = 13.5/8.7 * 60% = 0.93 Amps (but it is actually less because the PC solenoid is an inductor). So the dissipation in the resistor is P = I²R = 0.93²*4.7 = 4.06 Watts max., and a 5 Watt resistor seems about right.

But there is certainly no harm (other than cost) in going to a 10 or 15 watt resistor, and mount it on a heat sink, if you like. You absolutely don't want to be less than a 5 Watt rating on that resistor.

And yes, flyback spikes cause a lot of noise everywhere on the board. The noise is a symptom, but the potential damage for unseen to components that aren't rated for the voltage of these spikes could result in mysterious and unpredictable failures that are difficult to diagnose and repair. So the flyback protection is about more than just cleaning up signals (though that is a nice side-effect).

[Philip Lochner]

I have since realised that the 15Ohm resistor DOES have an effect that is tangible. Shifting from N into Drive now comes with quite a harsh engagement despite commanding the lowest line pressure possible.

This has prompted me to add another 15Ohm/5W resistor in parallel to the first. So now I have 7.5Ohm/10w in effect. Have not driven the car like this yet.

Shifting from Neutral to Drive is now as it should be. GPIO seems to only get luke warm now.

[Philip Lochner]

So the dissipation in the resistor is P = I²R = 0.93²*4.7 = 4.06 Watts, and a 5 Watt resistor seems about right.

If you dump 4W into a 5W resistor, that baby is going to get seriously hot.

[Bernard Fife]

Yes, it is going to get warm (especially if run near the theoretical maximum for long periods), but most resistors do get hot to some degree. My PC power resistor certainly never caused any problems (but I didn't run at min pressure/max. PWM for long periods). Mounting the resistor on a proper heat sink is probably the best way to go. I will edit the docs to recommend a higher wattage and a heat sink.