'84 500SE TPS Bypassed

[VaiFanatic]

For the last 9 years that I've owned my car, I've always had an uncharacteristically low idle, but the car seemed to run fine except for the AC compressor occasionally killing the car in traffic due to the higher load and low RPM.

I went about troubleshooting this by disconnecting the idle speed control module on the firewall and saw the RPMs rise. Figuring it may be the control module itself, I ordered a freshly rebuilt unit. The result was the same. Low idle.

When I say low, here's what I mean, after the car warms up the idle speed in Park will drop to about 550, or just a hair above the 500 mark. When in gear, the idle will drop to about 350RPM. No sputtering, no rough running, nothing. That's why I've lived with it for so long, but driving in the hot summer weather with the AC and the compressor basically shutting off the motor if I blip the throttle in stop and go traffic has gotten annoying. The idle just shouldn't be that low for it to dip down to 200 and play catchup or just flat out die.

This car has been a victim of federalization and independent mechanics who've had no idea what they were doing. From the aux fan being bypassed to always be on (rather than trouble shooting the cooling system properly), to a washer used as a method to seal an injector in a cup, I've fixed a lot of wrong over the last almost decade.

I loaned my fuel pressure gauge set out and never received it back, so I'm waiting on another set I ordered so I can test the control and working pressures and adjust accordingly, but I also decided to check up on the electrical side of things.

For as long as I've owned the car, this 2-pin plug was just sitting here:



It wasn't until just recently (say the last few days) I decided to follow the wire and discovered it goes to a rudimentary throttle position switch meant to feed that data to the idle speed controller. Why has it been unplugged this whole time, and how is the speed module compensating for the lack of appropriate signals? I couldn't find the missing plug to connect the wires, but it was right in front of me the whole time.


Someone pointed out that it may be poking out underneath the headlight, and sure enough, there it was, jumpered!





Before I get into details on the various tests I've done, let me mention one other thing. Regardless of outside temp, my car has a "warm-up" phase, where the RPMs in Park will be 750-800, and about 650 in gear. After the car reaches the first tick mark between 40°C and 80°, that's when the idle will drop down to 550 in P and 350 in gear.


So here is the CliffNotes version of the tests:


Test 1)
TPS plug Jumpered
Park = 550RPM
Drive =350RPM


Test 2)
TPS Jumper removed
Park = 750RPM
Drive = 650RPM


Test 3)
TPS actually plugged into the pigtail
Park = 550RPM
Drive = 350RPM


So, actually plugging in the plug that has been disconnected for well over a decade yields the same results as the jumper wire. This either means the TPS does work correctly, and there's an issue with the fuel/air mixture resulting in a low idle, or the TPS is bad and jumping the pins on the plug send the same signal to the speed control module.


I've tried several other warm up regulators, tested two other known good ICVs, and I've replaced various temp sensors with no discernible change. In fact, discovering this jumpered pigtail has been the closest thing I've come to making any headway in this.


Why would the TPS be bypassed if it's giving the same result as being plugged in? Was the idle high at some point and they jumpered the plug to bring the idle down (I've read someone having done that in a 560), and then something else was messed with and the idle dropped further to what it is now and it just so happens that the TPS failed without even being in the equation this whole time?


Unlike the 3-pin TPS in the gen 2 cars, I'm not sure what to make of the measurements I got when testing the switch.


I get a .3Ohm reading at 0% throttle, and an Open Loop reading at partial and 100% throttle.


The 3-pin should apparently read Closed at 0%, Open at partial, and Closed at 100% throttle.


I've contemplated just driving with the jumper removed now as the idle seems to be within spec, but I'm not sure if with prolonged driving scenarios the idle will begin to creep up as is common with an unplugged or stuck ICV.

[VaiFanatic]

Just went for a quick drive with the jumper removed entirely and the car drove much better. Better throttle response, AC cools better when not moving, and there didn't seem to be any indication of RPMs creeping up. The revs remained stabled the entire drive.