Trunk Release Fuse?

trhardy · July 5, 2010

Have 96 ES300; Trunk release button in door panel no longer opens trunk. Have read thru manual 3 times but find no "fuse" reference for this button. Anyone know what fuse feeds it? Before I pull switch out and start researching, I would like to just change a fuse if possible.Boy do I feel dumb! Had tunnel vision looking for fuse. Wife found switch inside glove box!!!!!!!!!! All is well now.

Thanks, Tom

LoveLearn · June 3, 2011

Our family's ES-300 is a 1992 model for which we purchased the new $137.90 two-volume factory shop manual set. I believe the same "Theft Deterent & Door Lock ECU" is used in this model series at least up through 1996, so these comments should apply to all those cars.

A little background about remote trunk openers. These have been available as options on some expensive cars for at least the last 60 years. So the basic technology is pretty mature. A few systems used vacuum operated motors, but most used electro-magnetic solenoids to overcome spring-loaded latch returns. The simplest used the vehicle frame as the electrical ground path back to the battery. That's still used today. They usually ran the + positive electric feed from the battery, through a fuse, then through a spring-loaded normally-open switch, then to the solenoid-powering terminal. Consider those switches to be equivalent to common house door-bell switches. They are normally held to their non-conductive "open circuit" position by their return spring. But when someone pushes the control button in, it conducts current, closing the circuit, triggering the solenoid's electromagnet to pull the trunk latch open. Very simple and very trouble free.

Now these Lexus ES-300 models are quite different. Start with the big shop manual index and look for trunk release. Surprise, Lexus English-language shop manual writers don't think these cars have trunks. The main index lists a "Body Electrical System," with its own index. No mention of trunk or similar reference there. That sub-section has its own main index, also with no mention of trunk or anything similar. But it does list a sub-sub-section named "Door Lock Control System." Turning to its own sub-sub-index on page BE-355. we finally discover that sub-sub-index lists "Luggage Compartment Door Opener Main Switch and Opener Switch Circuit." Sounds suspiciously like what over 99% of Americans would call the trunk! Finally, you've found the right section! Really poorly worded, just the opposite of good Shop Manual communications.

Volume 2, section Body Electrical System, page BE-364 shows a wiring diagram covering this system as part of a larger system. The body ground connects to the solenoid "motor" which passes power through a PTC unit, identified only many pages away as a "Positive Temperature Coefficient" module, then to terminal number 5 of the Theft Deterrent & Door Lock ECU (mini computer) module's 20-pin socket named T8. This ECU also has a 14-pin socket named T9, but the only two connectors I'll discuss here are to the longer 20-pin socket and matching wiring loom connection plug. So the + positive battery voltage required to power the solenoid MUST come from this ECU which also controls several other functions.

So how does this specific ECU get triggered to send + current to the trunk latch solenoid? One way is having the low-current +12 volts sent from its terminal 19 pulled down close to 0 volts by grounding through a plain old-fashioned door bell type switch inside the driver's side door, physically mated with the push button switch which releases the fuel door. But just to add one more little measure of "security" to your car, the electrical line from that ECU passes through an on/off switch located inside the glove box.

Now we come to an error in this massive manual. Its writers named these two series-wired switches as follows. The "Luggage Compartment Door Opener Main Switch" is illustrated as located in the glove box. The "Luggage Compartment Door Opener Switch" is illustrated as located in the driver's door inside panel. So they named the glove box located switch the "Main Switch." Following is a direct quote from page BE-386: "The luggage compartment door opener main switch is off when it is pushed in, and the luggage compartment door opener switch is on when it is pulled." Well kids, that is exactly "bas-ackwards", as one of my old physics instructors liked to say. The manual wrongly claims that when the main switch is pushed in, the system is off, but in our cars, when that switch is pushed in the push-button trunk release system is switched on. Forum writers correctly observed that when it is pushed in that switch is closed and conducts current.

How this ES-300 system works:

In the driver's door is a white & black wire going to the trunk opening push-button switch. That white & black wire is your ground. When you push the button in, it grounds out the low-current 12 volts sent from ECU pin 19, through the glove box switch. That triggers the ECU to deliver +12 volts through pin 5 to the solenoid, popping the truck lid open. Yoke, sugar, easy! It's a simple but messy system.

I'll scan or photograph illustrations of both the two sockets and plugs to this Theft Deterrent & Door Lock ECU so you readers without the manual can identify which leads to probe if this part of you trunk opener system stops working. I will not discuss how you can also trigger that ECU to deliver +12 volts through pin 5 with your wireless transmitter, but that is another way it can be triggered.

Where is this specific ECU? Right next to your glove box toward the right side. If you see three boxes aligned roughly side by side, you are looking too far forward. This one is by itself and further rearward from the firewall. Observe that it has two red plastic sockets. The longer one is your 20-pin socket and plug, the shorter is the 14-pin socket and plug.

I'll come back later to add illustrations so you can know which pins are #5 and #19 on this 20-pin connection and describe some quick tests to instantly narrow you fault search.

Where do these systems fail? Most common cause is "the nut behind the wheel," also known as user error. Someone turns off the main switch in the glove box and either doesn't know or forgets that they disabled the system. Switches also fail. One ES-300 glove box switch refused to connect. Removed and triggered on & off for about 10 minutes. It works perfectly now. Door mounted switch copper contacts have become green from oxidation and don't conduct. Disassemble, clean contacts, reassemble and it works again. The front door opening and closing flexes the wiring loom cable aligned with the hinge rotation line. One multi-strand wire in that cable bundle is your red +12V low-current wire from ECU #19 that must be grounded to trigger ECU response. But if that door hinge flexing eventually breaks and internally separates that red wire's conductors, no matter how many times you push that door button, your grounding effort never gets to the ECU so the trunk doesn't open. The dealership mechanics would be forced by Lexus rules to replace the wiring loom almost certainly for well over $1000. Just add a new super-flexible super-high-strand-count wire between the door switch red terminal and the glove box switch to restore that function. Don't use common low flexibility low-strand count wire. You can buy the right kind of super-flexible wire from hobby shops for wiring critical high-vibration components. Or if you'd prefer eBay vendors, search for Silicone Wire Ultra Flex. This roughly-1000 strand count, limp like cooked noodle wire, is not cheap per foot, but you won't flex that door often enough to internally break it. Common cheap multi-strand wire can fatigue and break internally surprisingly fast in door opening service. When one of these ECU fails, it usually also takes out other circuits like door locks. If those are working, your ECU is getting power so no need to check ECU power source. I'll explain how to easily test that ECU without removing it in my next note. Solenoids are pretty durable, easily tested and repaired.

Here's a cute trick. If you want the trunk open for an extended period but don't want the trunk light battery load switched on, just push the latch shut leaving the trunk lid open. That action switches off the trunk light. To open the latch so you can lock the trunk lid shut, just use your key or trunk opening circuit button or wireless opener to reopen that latch.

I know this was long and verbose. But now you have a flavor for how this system works. All that Lexus versatility and security control requires complexity.

I'll be back with those connector and socket illustrations.

Good luck and hope this helps a few others when their trunk lids fail to open by electrical command.

John