I have a 1998 c230 sedan with a remote trunk release problem. I have tried some diagnostics listed below but cannot get the system to work properly.

The trunk release works first thing in the morning with the remote when the red light on the center console trunk release button is not lit. Once the doors are unlocked the trunk release button on the center console lights up red. Then the problem starts. I can get into the trunk with the metal key, but it now will not work from the release button in the center console and the remote key will not operate the trunk release.
I think what is going on, the car thinks the trunk is open ( the red light on the release switch in the center console ) and will not let the remote or center console switch work. I have tried to replace the Vacuum pump assembly with a used one with no luck. I have also tried to look into the trunk closed sensing switch at the trunk latching assembly. I have tried to test the sensing switch with no luck. There is no difference in ohms readings between an open and closed trunk latching assembly. Both positions give me an infinite ohms reading. Looking over a Mitchell electrical wiring diagram and the trunk closed sensing switch looks like a grounding type of switch. ( yellow wire into the switch and solid brown wire out to ground ) I have tried to short the two wires at the switch with no luck. I have left the wires to the sensor disconnected and the system did not work. The red light stays on. I have tested the wire from the vacuum pump to the trunk latching assembly and the yellow wire tests good. The grounding wire at the other end of the switch tests good also. I have tried to both ground and infinite ohms the yellow sensing wire from the vacuum pump bypassing the wiring that bends when the trunk opens and closes with no luck. I am unsure of what to try next? I think the trunk closed sensor switch is bad but I am unsure if there is something wrong with the center console switch because I cannot get the system to work by manually connecting the sensing to the tow different conditions that the wiring diagram says is should see?
One other funny thing it does is that if the red light in the center console is off and the door locks are open the trunk remote will not work and the red light will be lit after trying! Not sure why this happens
Does anyone have any experience with this system and knows what to try next? Thanks John

PS the trunk wiring has given me problems in the past. The car used to arm the alarm sometimes when closing the drivers door. The problem was fixed by cutting the black wire that connects the the locking pump to the trunk locking sensor. I think the wire was grounding itself somehow when the drivers door was closed. Once the black wire is grounded the car thought the car was locked from the trunk and is should set the alarm is my best guess as to why this solved the problem.

You are correct that the issue is related to the red light being on. The car thinks the trunk is open so will not open it again! German efficiency...

You are on the right track checking the sensor (the white plunger that turns on the trunk light). Does the trunk light go on when you open the trunk, and off when you push (or pull out) the plunger?

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

I just acquired a 1998 C230 which is exhibiting the same EXACT problem as you are. Did you ever figure out what the solution is?

Does the trunk light go on when you open the trunk, and off when you push (or pull out) the plunger?

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

No...or I should say, usually not. It's clear that if the red LED is on the truck will not open. Here is the description I gave on another forum:

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The problem I'm trying to resolve now has to do with the intermittent ability to open the trunk. It appears to be tied directly to the vehicle thinking the trunk is already open therefore it will not activate the truck latch mechanism. Here's what I know:

1. The trunk release button on the center console does not work. If you lift it under any condition the trunk will not open. Sometimes the red LED is lit and sometime it is not. Yes, even when the trunk is closed the red LED is still lit sometimes.
2. It would appear that when the red LED is lit the remote control button will not open the trunk. I assume the red LED being lit means the vehicle thinks the trunk is already open and therefore will not activate the latch.
3. When the vehicle is unlocked and the red LED is lit pushing the button on the trunk will not open the trunk.
4. When the LED if off, then pushing the trunk button on the trunk will open the trunk. This is also true when you push the trunk button on the remote control.
5. If the LED is off and you open the trunk the LED will come on and stay on regardless of if you close the trunk.
6. If the LED is off and you open a door the LED will come on and stay on regardless of if you close the door. Again, while the red LED is lit you can not open the trunk by pushing the trunk button or pushing the trunk button on remote regardless of if the car is completely unlocked.
7. If you wait some period of unknown time between 10 minutes and some time longer the red LED will go out. This is true if the car is locked or unlocked. I literally have to wait 10, 20 or 30 minutes with the car sitting in the parking lot to get the trunk to open...usually using the remove control button.

Somehow, I believe my problem is two-fold: First, why doesn't the center console truck release button ever work?...clearly it has power to it as the red LED turns on and off. Second, what is causing the red LED to come on and stay on. I believe if I can fix that problem then I'll be able to open the trunk as needed using the remote button or the button on the trunk. There is still the issue of fixing the center console button.

I get the impression that this vehicle may use some sort of vacuum system to activate door locks and the trunk. Is this related to a vacuum leak? If so where would I find the vacuum system controller and the hoses? Any other ideas fellas?

I found the fix and thought I'd post it here in case someone else has the problem. I always hate it when folks ask for help and many never come back to post that they fixed the problem and what the solution was. Anyhow, in case the OP still has the problem or someone else here is what I found:

1. I purchased a brand new trunk lamp switch button and replaced it as the vehicle kept showing the trunk as being open when it was not (red RED on center console trunk release button). It made no difference. Because the trunk lamp didn't work I was never able to play with the trunk switch button and watch the trunk lamp go on or off.

2. I looked at the trunk lamp bulb. It's is fine. I took a resistance measurement to make sure it was not "open". It had a small resistance which tells me that if it got 12V it would light up.

3. I measured the voltage going to the trunk lamp bulb. With the trunk open you get a reading of 0.01V which is not enough to light the bulb up. Oddly, when you push the trunk lamp switch button in to simulate a "closed trunk" the voltage actually goes up to 0.11V. You also get a reading of 0.11V when you pull the trunk lamp switch button out which is supposed to turn the lamp out.

4. Since I was having "electrical issues" with the trunk lamp I thought I'd also look at the rear seat dome light which didn't light up either when it was supposed to. I used the button in the overhead console in the front to try and turn it on but it never came on.

5. I removed the bulb for the rear seat dome light and inspected it. The bulb was fine and would light up if 12V were applied. I checked the continuity and resistance as I did the trunk bulb and got similar readings. With the volt meter on the rear dome light fixture I observed 0v when the dome light is supposed to be off and 2.3V when the dome light is supposed to be on. It takes 12V to light the bulb properly so this explains why the bulb never comes on.

6. Here's my biggest discovery...when the button for the rear dome light is push "on" I experienced the problem with trying to open up the trunk. Apparently, the dome light button had been on all this time but I wouldn't have known it because the rear dome light never lit up. So, with the rear dome light button turned off I was able to use the FOB remote to open the trunk at will regardless of if the doors were open or closed. However, with the dome light "on" I experience the trunk problem as previously described. One thing I should point out is that with the dome light off the center console switch still did not work, regardless.

7. Since I was able to verify the problem wasn't the trunk switch button and performed the diagnostic work above it allowed me to move on to the electrical issues. When you determine that you are having electrical issues the first step is to check the relays and fuses. I checked the fuses first.

I found the fuse box in the engine and also the one in the trunk under where the spare tire is stored. The engine fuse block didn't have a listing for a trunk light but it did for the rear dome light (only). Weird thing is they had the #9 fuse in the engine area listed along with the #2,#4 & #6 fuse in the trunk area as all being for the rear dome light. When I got to the #6 fuse which is a 5A fuse I noticed it was blown. I replaced it with one of the spares in the engine fuse block and voilą the rear dome light works!...AND the trunk light works AND the trunk opens at will with the key FOB! Interesting that you can leave the rear dome light on (now) and the trunk will open whereas when the fuse was blown leaving the dome light on prevented the trunk from opening with the key FOB.

The center console trunk release button still doesn't work but I'm going to assume it is worn out and needs to be replaced so I'll order a new one and hopefully I'll be good to go.

Anyhow, it was the blown #6 fuse in the trunk that was causing the rear dome light and trunk light issue which contributed to the trunk release problem. Weird but true.