1. and 2. Located under the instrument cluster, the Body Control Module
(BCM) for the 1999 GMC Sierra controls many functions. The chart at left runs down the elements the BCM controls.3. When Jeep’s Cherokee got Chryslerized back in 1991, the Engine Controller received Vehicle Theft System functions and moved from inside the passenger compartment above the accelerator to this friendly spot. The old Jeep 4.0L Bendix system (87-90) was dreadfully tricky to troubleshoot.Shadowboxing with boxes
Most technicians were either intrigued or annoyed when those mysterious boxes started showing up under hoods, package trays and instrument panels. These strange components were drawn into our schematics simply as outlined boxes blanketed with the words “Solid State,” as if the innards of those little critters were either too complicated for us to understand or so secret that we didn’t need to know how they worked. The problem was that we needed to understand how these devices worked in order to properly diagnose customers’ concerns. For years now, the only real way to determine if one of these black boxes is bad is to analyze the inputs and outputs as best we can according to the literature available, and then plug in a new one or preferably a “known-good unit,” as the manuals so often say in their pinpoint tests.Fast forward
Without covering all the ground between the lunar landing and vehicle body computers, let’s have a quick look at a few problems that I’ve seen. The first case is a situation I experienced with a Mustang Remote Keyless Entry (RKE) system. Earlier Ford RKE systems had a stand-alone module, but on the ’99 Mustang, it had migrated to the innards of the Generic Electronic Module (GEM) module. The transmitter fob on your keychain sends a radio signal to the GEM, which either locks or unlocks the doors and honks the horn if you so desire, while at the same time operating the door locks and activating the illuminated entry system. On this vehicle, the RKE system was totally inoperative. In many cases, simply reprogramming the GEM to recognize the signal from the fob takes care of the problem. On the Mustang, cycling the key eight times puts the GEM in programming mode; the locks cycle to indicate it has entered the mode. When any button on the transmitter is pressed, the GEM cycles the locks in response to indicate that it has recognized and recorded the frequency of that particular fob. When the key is switched off, the locks cycle again, terminating the programming session. As many as four fobs can be programmed in a session. This Mustang entered programming mode and cycled the locks for me, but when I tried the transmitter after exiting the programming session, it still wouldn’t work. To make a long story short, a ‘Special Service Message’ search and a subsequent call to the hotline revealed that I needed to go beyond regular service procedures. On early ’99 Mustangs, the hotline guy indicated, fuse 38 must be removed for a few seconds. Reinstalling the fuse would bring the GEM’s RKE function back on line. It did. My question was: “How was I supposed to know that if it’s not written down somewhere?”Security issues
I replaced the PCM on a 1997 Grand Cherokee and was satisfied that I had repaired a driveability complaint. The next morning, the owner came wheeling back with the concern that her Vehicle Theft Security System (VTSS) was inoperative.
I plugged in the DRB III scan tool to make sure the VTSS was enabled, but it just wouldn’t work. By the way, don’t do this on a vehicle that isn’t equipped with VTSS, or you’ll have a nasty no-start that can’t be undone without replacing the PCM. Usually, locking the doors with the RKE transmitter fob arms the security system on these vehicles and triggers a flashing indicator light, but this one was as dead as a hammer. After 30 minutes of struggle, I decided to call the hotline. None of the hotline guys had a clue what the problem might be. They forwarded my call to a higher level, where I spoke with a clever and articulate young woman who asked me how many times the Jeep had been started since the PCM was replaced. I navigated the DRB III to the screen that gives that information and told her it had been started eight times.“When the PCM has seen 20 starts, the VTSS will be activated.” It sounded like a rather interesting prophecy, but after I started the Jeep 12 more times, the VTSS responded quite nicely in response to the fob. While this problem would eventually have evaporated on its own, I had burned almost an hour of valuable shop time, and once again, my question was: “How was I supposed to know that if it’s not written down somewhere?”3. This is a shot of the Ford Ranger Central Timer Module (CTM), a low-end version of the GEM module that won’t communmicate with a scan tool. This module controls intermittent wiper functions, battery saver and interior light functions, etc. There were a lot of Ranger CTMs that were replaced with a GEM in the late 1990s due to uncommanded wiper operation. The GEM was a higher-quality unit that didn’t exhibit the same problem.
Most technicians were either intrigued or annoyed when those mysterious boxes started showing up under hoods, package trays and instrument panels. These strange components were drawn into our schematics simply as outlined boxes blanketed with the words “Solid State,” as if the innards of those little critters were either too complicated for us to understand or so secret that we didn’t need to know how they worked. The problem was that we needed to understand how these devices worked in order to properly diagnose customers’ concerns. For years now, the only real way to determine if one of these black boxes is bad is to analyze the inputs and outputs as best we can according to the literature available, and then plug in a new one or preferably a “known-good unit,” as the manuals so often say in their pinpoint tests.Fast forward
Without covering all the ground between the lunar landing and vehicle body computers, let’s have a quick look at a few problems that I’ve seen. The first case is a situation I experienced with a Mustang Remote Keyless Entry (RKE) system. Earlier Ford RKE systems had a stand-alone module, but on the ’99 Mustang, it had migrated to the innards of the Generic Electronic Module (GEM) module. The transmitter fob on your keychain sends a radio signal to the GEM, which either locks or unlocks the doors and honks the horn if you so desire, while at the same time operating the door locks and activating the illuminated entry system. On this vehicle, the RKE system was totally inoperative. In many cases, simply reprogramming the GEM to recognize the signal from the fob takes care of the problem. On the Mustang, cycling the key eight times puts the GEM in programming mode; the locks cycle to indicate it has entered the mode. When any button on the transmitter is pressed, the GEM cycles the locks in response to indicate that it has recognized and recorded the frequency of that particular fob. When the key is switched off, the locks cycle again, terminating the programming session. As many as four fobs can be programmed in a session. This Mustang entered programming mode and cycled the locks for me, but when I tried the transmitter after exiting the programming session, it still wouldn’t work. To make a long story short, a ‘Special Service Message’ search and a subsequent call to the hotline revealed that I needed to go beyond regular service procedures. On early ’99 Mustangs, the hotline guy indicated, fuse 38 must be removed for a few seconds. Reinstalling the fuse would bring the GEM’s RKE function back on line. It did. My question was: “How was I supposed to know that if it’s not written down somewhere?”Security issues
I replaced the PCM on a 1997 Grand Cherokee and was satisfied that I had repaired a driveability complaint. The next morning, the owner came wheeling back with the concern that her Vehicle Theft Security System (VTSS) was inoperative.
I plugged in the DRB III scan tool to make sure the VTSS was enabled, but it just wouldn’t work. By the way, don’t do this on a vehicle that isn’t equipped with VTSS, or you’ll have a nasty no-start that can’t be undone without replacing the PCM. Usually, locking the doors with the RKE transmitter fob arms the security system on these vehicles and triggers a flashing indicator light, but this one was as dead as a hammer. After 30 minutes of struggle, I decided to call the hotline. None of the hotline guys had a clue what the problem might be. They forwarded my call to a higher level, where I spoke with a clever and articulate young woman who asked me how many times the Jeep had been started since the PCM was replaced. I navigated the DRB III to the screen that gives that information and told her it had been started eight times.“When the PCM has seen 20 starts, the VTSS will be activated.” It sounded like a rather interesting prophecy, but after I started the Jeep 12 more times, the VTSS responded quite nicely in response to the fob. While this problem would eventually have evaporated on its own, I had burned almost an hour of valuable shop time, and once again, my question was: “How was I supposed to know that if it’s not written down somewhere?”3. This is a shot of the Ford Ranger Central Timer Module (CTM), a low-end version of the GEM module that won’t communmicate with a scan tool. This module controls intermittent wiper functions, battery saver and interior light functions, etc. There were a lot of Ranger CTMs that were replaced with a GEM in the late 1990s due to uncommanded wiper operation. The GEM was a higher-quality unit that didn’t exhibit the same problem.
4. The
Taurus GEM module is mounted piggyback on the instru-ment cluseter (as
is the F-Series/
Expedition/Excursion.) These modules handle intermittent wiper operation, interior lamp functions and warning chimes. The Explorer transfer case shifts are controlled by the GEM, as are the F Series. The Taurus GEM controls the Variable Assist Power Steering.5. Continentals, Crown Victorias and Town Cars have been using a cousin to the GEM module called the Lighting Conrol Module (LCM) since the mid-1990s. This unit contains relays for the exterior and interior lamps and the horn, including flasher circuitry for the turn signals as well as tone generators for warning chimes as requested by other modules. The electric deck lid release receives its juice from this unit, too. Depending on the application this module can have dozens of possible DTCs and scan tool PIDs.Tool trouble
Frequently updated scan tools can add a new element to a stressful, computer saturated service bay. The DRB III scan tool is a case in point. I had just updated the DRB III via the Mopar Diagnostic System, and I was fighting another computer problem related to VTSS activation. I replaced the body computer on a Jeep Grand Cherokee with a false alarm activation; it was fooling itself into thinking the tailgate was open when it wasn’t. I noticed that, once again, replacing an on-board computer had killed the Theft Alarm system. I couldn’t for the life of me figure out how to activate it on the new body computer. When I went to the appropriate screen on the tool, the menu just didn’t contain ‘Enable VTSS’ any more. The latest update had removed it. It took three calls to the hotline before I found someone knowledgeable enough to tell me that I could boot up the tool on the memory card that was designed for older model years. Then, I could go to the same screen and the ‘Enable VTSS’ would be there. It was. Once again, my question was: “How was I supposed to know that if it’s not written down?”Flashing lights
I drew a repair order for a flashing high-beam indicator on a 1999 Grand Cherokee. Sure enough, it was flashing, but what I was seeing was a flash-out of what looked like some sort of computer-generated binary code.
As I researched the circuit, I found that instead of the high-beam indicator drawing power from the wire leading to the high-beam headlamp bulbs, as in the old days, it was now taking its orders from the body computer. The computer received a signal from the dimmer switch on the column. The body computer commands the lights to switch from bright to dim, concurrently sending a command to the instrument cluster computer, telling it to turn the high-beam indicator on.
Why all this is necessary is beyond understanding. The hotline said to replace the instrument cluster (which was $480), but the problem actually turned out to be in the body computer. Who knows if anybody will ever see that again, but it was a costly lesson.Crazy electronics
Sometimes even the simplest electronics can be aggravatingly devious. The next case is a 1994 Mazda B3000 I encountered at the Ford dealer. With the radio on and the wipers operating on intermittent, the radio would wink off each time the wiper motor swept the blades to the park position. Power and ground feeds to the radio terminals showed no loss of power and no spikes on the oscilloscope. The radio didn’t respond this way to the operation of any other accessory. We searched and actually found a loose ground at the wiper motor, repaired it and thought for a few minutes that the problem was solved. Raising the wiper arms off the windshield appeared to make a bit of a difference, but the problem still occurred with clock-like regularity. Plugging in a new wiper motor and letting it run while holding it in my hand made no difference either. Finally we made a trip to the boneyard and robbed a radio out of an abandoned Contour. When we plugged it in, the problem was gone.Like so many other electronics problems, the only way to be sure is to try a known-good unit. And if we can’t find a substitute for the component in the boneyard, do we go to the trouble of replacing the radio in hopes that it’ll take care of the problem? How are we supposed to know for sure? It’s tough to explain this sort of thing to some customers who think all we do is tighten screws and replace fuses.Burn, “Rurn” and Learn
Donnie drew the repair order for the 1998 Dodge Durango, and it was weird from the start. The “4WD Service” light was on and the vehicle was two-wheel drive. Well, he found a strange Chrysler TSB (08-032-00) which said to replace the HEVAC module – which is what Chrysler calls the control panel for vehicle heating and air conditioning – on two-wheel drive Durango units that exhibited this concern. He ordered the $265 module, installed it and found that the “4WD Service” light didn’t go out. A call to the hotline revealed that the battery cable should be disconnected for 30 minutes, then reconnected and the HEVAC module should be configured with the DRB III. During the configuration procedure, Donnie went through what Chrysler terms a “cool-down test,” which brought the compressor online. He also enabled the ambient air temperature display function while walking his way through the procedure. He still had a flashing indicator on the HEVAC that wouldn’t go away. Another call was made to the hotline, and the guy asked if he had enabled ambient air temperature.“Yes,” said Donny“Do you have an overhead console?” asked the hotline tech.“No.”“Then you’ll need to order another HEVAC unit.”“What? Why?” asked Donny.“Because once the ambient air temperature display has been enabled, you can’t reverse the setting.” Donnie had the same question we have been asking for years now: “How was I supposed to know this? There’s nothing in the book that I could find about it.”“You learn it by making mistakes.”“Let me get this straight: When we replace a HEVAC module for the first time, the procedure is ‘Burn, Rurn [ruin] and Learn,’ is that right?“That’s about it.”Cases closed?Donnie and I could both tell story after story like this. But the point is that the only way any of us learn some of the quirky, non-published idiosyncrasies of today’s high-dollar computerized parts is through sharing what we’ve learned through the process of “Burn, Rurn, and Learn.”What about you? Do you have any stories you’d like to share?The 1999 GMC Sierra BCM
controls the following functions:-Audible warnings
-Indicator warnings
Interior lighting
-
Expedition/Excursion.) These modules handle intermittent wiper operation, interior lamp functions and warning chimes. The Explorer transfer case shifts are controlled by the GEM, as are the F Series. The Taurus GEM controls the Variable Assist Power Steering.5. Continentals, Crown Victorias and Town Cars have been using a cousin to the GEM module called the Lighting Conrol Module (LCM) since the mid-1990s. This unit contains relays for the exterior and interior lamps and the horn, including flasher circuitry for the turn signals as well as tone generators for warning chimes as requested by other modules. The electric deck lid release receives its juice from this unit, too. Depending on the application this module can have dozens of possible DTCs and scan tool PIDs.Tool trouble
Frequently updated scan tools can add a new element to a stressful, computer saturated service bay. The DRB III scan tool is a case in point. I had just updated the DRB III via the Mopar Diagnostic System, and I was fighting another computer problem related to VTSS activation. I replaced the body computer on a Jeep Grand Cherokee with a false alarm activation; it was fooling itself into thinking the tailgate was open when it wasn’t. I noticed that, once again, replacing an on-board computer had killed the Theft Alarm system. I couldn’t for the life of me figure out how to activate it on the new body computer. When I went to the appropriate screen on the tool, the menu just didn’t contain ‘Enable VTSS’ any more. The latest update had removed it. It took three calls to the hotline before I found someone knowledgeable enough to tell me that I could boot up the tool on the memory card that was designed for older model years. Then, I could go to the same screen and the ‘Enable VTSS’ would be there. It was. Once again, my question was: “How was I supposed to know that if it’s not written down?”Flashing lights
I drew a repair order for a flashing high-beam indicator on a 1999 Grand Cherokee. Sure enough, it was flashing, but what I was seeing was a flash-out of what looked like some sort of computer-generated binary code.
As I researched the circuit, I found that instead of the high-beam indicator drawing power from the wire leading to the high-beam headlamp bulbs, as in the old days, it was now taking its orders from the body computer. The computer received a signal from the dimmer switch on the column. The body computer commands the lights to switch from bright to dim, concurrently sending a command to the instrument cluster computer, telling it to turn the high-beam indicator on.
Why all this is necessary is beyond understanding. The hotline said to replace the instrument cluster (which was $480), but the problem actually turned out to be in the body computer. Who knows if anybody will ever see that again, but it was a costly lesson.Crazy electronics
Sometimes even the simplest electronics can be aggravatingly devious. The next case is a 1994 Mazda B3000 I encountered at the Ford dealer. With the radio on and the wipers operating on intermittent, the radio would wink off each time the wiper motor swept the blades to the park position. Power and ground feeds to the radio terminals showed no loss of power and no spikes on the oscilloscope. The radio didn’t respond this way to the operation of any other accessory. We searched and actually found a loose ground at the wiper motor, repaired it and thought for a few minutes that the problem was solved. Raising the wiper arms off the windshield appeared to make a bit of a difference, but the problem still occurred with clock-like regularity. Plugging in a new wiper motor and letting it run while holding it in my hand made no difference either. Finally we made a trip to the boneyard and robbed a radio out of an abandoned Contour. When we plugged it in, the problem was gone.Like so many other electronics problems, the only way to be sure is to try a known-good unit. And if we can’t find a substitute for the component in the boneyard, do we go to the trouble of replacing the radio in hopes that it’ll take care of the problem? How are we supposed to know for sure? It’s tough to explain this sort of thing to some customers who think all we do is tighten screws and replace fuses.Burn, “Rurn” and Learn
Donnie drew the repair order for the 1998 Dodge Durango, and it was weird from the start. The “4WD Service” light was on and the vehicle was two-wheel drive. Well, he found a strange Chrysler TSB (08-032-00) which said to replace the HEVAC module – which is what Chrysler calls the control panel for vehicle heating and air conditioning – on two-wheel drive Durango units that exhibited this concern. He ordered the $265 module, installed it and found that the “4WD Service” light didn’t go out. A call to the hotline revealed that the battery cable should be disconnected for 30 minutes, then reconnected and the HEVAC module should be configured with the DRB III. During the configuration procedure, Donnie went through what Chrysler terms a “cool-down test,” which brought the compressor online. He also enabled the ambient air temperature display function while walking his way through the procedure. He still had a flashing indicator on the HEVAC that wouldn’t go away. Another call was made to the hotline, and the guy asked if he had enabled ambient air temperature.“Yes,” said Donny“Do you have an overhead console?” asked the hotline tech.“No.”“Then you’ll need to order another HEVAC unit.”“What? Why?” asked Donny.“Because once the ambient air temperature display has been enabled, you can’t reverse the setting.” Donnie had the same question we have been asking for years now: “How was I supposed to know this? There’s nothing in the book that I could find about it.”“You learn it by making mistakes.”“Let me get this straight: When we replace a HEVAC module for the first time, the procedure is ‘Burn, Rurn [ruin] and Learn,’ is that right?“That’s about it.”Cases closed?Donnie and I could both tell story after story like this. But the point is that the only way any of us learn some of the quirky, non-published idiosyncrasies of today’s high-dollar computerized parts is through sharing what we’ve learned through the process of “Burn, Rurn, and Learn.”What about you? Do you have any stories you’d like to share?The 1999 GMC Sierra BCM
controls the following functions:-Audible warnings
-Indicator warnings
Interior lighting
-
