How to determine what resistor code for used bcm

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Feb 12, 2020 | 12:33 PM

HappySalesman

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From: Noblesville, IN

How to determine what resistor code for used bcm

Need to buy a replacement bcm. Looks like all "new" options are gone unless I spend stupid money at the dealership.

There any way to verify what resistor code a used bcm is programmed to? Or just grab a bunch of resistors and see which one turns off the security light?

Feb 13, 2020 | 07:52 AM

WhiteBird00

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From: Jacksonville, FL (originally from Toronto Canada)

The best way is to try to get the original key that came with the BCM. You can sometimes do that at the junk yard and some sellers online are sophisticated enough to know that having the key is useful. Failing that, you'll just have to try different values until you find the right one. There are a few ways to do this:

You could buy a variety of resistors that will cover all 15 possible values - an inexpensive solution as long as you don't have to buy packs of resistors to get all the right values.
You can buy a VATS kit from DEI (the people who make Viper and other alarm systems). It contains 15 resistors that match each of the possible values - no need to find combinations to put in series and/or parallel to create the right total resistance. The kit number is 652T and it's available on Amazon for about $10.
You could get a 10K potentiometer, wire it to the VATS wires then turn the **** until you get the right value, measure what resistance you got on the pots terminals and buy the corresponding resistors. That's the geek solution but it's inexpensive as well and doesn't involve multiple reconnections looking for the right resistor combination. If it still doesn't work when turned all the way up then you need #15 11,800 ohms (the only value above 10K ohms). Total cost about $11.
The really easy solution is the Gator VATS Bypass Module. It is a small box that plugs into the original wiring with no splicing necessary. Under the cover is a series of 15 DIP switches representing the possible resistor values. Turn one on and try it. If it doesn't work, turn it off and turn on the next one and try again. One of the 15 will work then you just put the cover back on and leave the module connected - problem solved. This is by far the easiest way to do it but it costs $40. Having done several bypasses the old fashioned way with multiple resistors, I think the ease of use, convenience, and the fact that no wires get cut is well worth the price.

Here is a chart that shows the different VATS values and some combinations of common resistors which will produce those values:

Combinations with + are in series (one after the other end-to-end) and combinations with || are in parallel (side-by-side resistors with their ends joined together).
The percentages are how much the combination is off from the actual value but VATS has a 10% tolerance so all will work.

Note: All 15 possible resistance values for GM models are listed even though 4th generation F-bodies used only 14 of those possible values.

Now the big question... what makes you think you need a new BCM? There are some common problems with BCMs that can be repaired with only basic soldering skills so it's actually quite unusual for replacement to be necessary.

Last edited by WhiteBird00; Feb 13, 2020 at 08:06 AM.

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