P0121 - the 1200 rpm limp mode - Part 1

Today, I will start to go through my efforts to identify the cause for my P0121 - Intermittent Signal from TPS issue. Yes, I know! Another multi-part series?!? Well, that's how it goes when we need to wait for shippers. Anyway, the process I followed is similar to what I did for the other wiring issues I just resolved in that there is a logical approach, but otherwise, this is about solving a 6-wire circuit and really shouldn't be a big deal. But it was and here we are.

Drive-by-Wire

drive-by-wire pedal

For those who haven't suffered from this code before, or may not even know this is the code you are getting, I'll start with a quick summary of what this is about. In modern drive-by-wire cars, your throttle pedal may not actually control the speed of your engine or vehicle directly. If you have watched some of the car shows on MotorTrend, when an older car (pre-fuel injection) gets a modern engine (with fuel-injection), they need to add a throttle position sensor (TPS) to tell the computer how far open or shut the throttle is. In these conversions, the TPS is usually connected to the throttle cable in the engine bay. This converted the pull/push of the cable (which reflects the push / let-up of your foot) into a signal the computer understands so it can figure out how much fuel to throw in, or how to adjust the timing of the spark. Cool, eh?

 

New cars, whether they are burning volts, waste cooking oil or dead dinosaurs, do not have a go pedal hooked up to a cable any more. They skip the middle man and have a TPS wired up to the pedal, so your pedal works like a big fader (light dimmer) or rheostat. The TPS converts your foot push / let-up into different voltage that gets sent to the computer where it does all the things I mentioned above.

P0121 - What Is It

All this is great. I mean, having something that precisely collecting your speed wishes and implementing them without a mechanical component seems pretty fantastic. And, it is, until that day when you are driving along and suddenly the pedal no longer seems to translate into speed. You notice that no matter how you position your foot, the engine speed doesn't vary. In fact, you note that the engine's speed has become static. I can't speak for other makes and model years, but if you are in a VW TDI circa 1998-2004 the engine will be pinned at 1200 rpm. Adding color to the scene will be the illumination of the check engine light. Your first impulse may be "oh snap, the engine is broken". Not so fast. In this case, you are probably getting code P0121: Intermittent Signal Inconsistency from the Throttle Position Sensor.

That sounds awfully scary, but in cars that were not hacked up like Hapy, this is probably caused by your TPS failing. It happens. In VW's, this is one of those rare parts that do not fail super-often, but when they do they are actually kind of easy to replace. You remove the panel under the dash, unplug the flat-connector, remove 4 bolts and it's out. Replacing the one in Hapy is just as easy, but determining that the TPS is the root cause is not. In Hapy, we have 6 circuits running through 7 meters of wire and multiple connectors. In your car, the 6 circuits probably run for less than a foot and run from that flat-connector to a T10 or something similar. No extra connectors, no super-long wires.

Hapy Diagnosing Plan

Before I pulled out my multi-meter, I gave myself a few minutes to think about the situation. I had intermittent P0121 codes getting thrown since I before swapped out the original 1998 NewBeetle one-year-only TPS for the more general-use one for model years up through 2004. The original TPS had the 6-wire bundle pressed against the underside of the bus, and eventually the plastic wire casings wore through. The wires grounded against the body, causing the original P0121 errors. The general-use pedal threw codes intermittently and was one of the drivers for the replacement of those wires with the front-to-back cable. That cable runs from just after the flat-connector all the way to the blue T10 (through another T12 connector). So, either some part of the wiring was done wrong or it was never the wiring all along, and it had always been the TPS or the flat-connector or both.

 

A side note about the wire sizes. The wires coming out of the VW 6-pin flat-connector vary in thickness from 22 or 24 up to maybe 18 (probably more like 20). Since the resistance of the wire is inversely related to it's thickness (thinner = more resistant), by replacing the old thin single wires with a thicker well-jacketed cable, I effectively reduced the resistance of the front-to-back run. According to this calculator, the wires in the new cable should be around 0.03 ohms. The old wires, being 22AWT gauge were around 0.3 ohms. Noting the decimal point, the old wires were 10x more resistant to current. So, we can exclude any concerns about the new cable wire size being a cause; quite the contrary, it should be delivering electronic messages better.

A failing TPS or a bad flat-connector sound like great theories. Let's prove it. I chose to start at the flat-connector and work back to the ECU. I have a second flat-connector in the heap of spare or removed TDI stuff in my garage that is also 6-pin, so I can swap-out the in-Hapy one if necessary. This also provided me the resistance values for a 6-pin flat-connector that is otherwise "normal". With the multi-meter at its most sensitive setting, I tested the resistance of each pin -to- end-of-wire. After the plug was warm in my hand, the resistance value was "3". Prior to that, when it was cold from the garage, the resistance was closer to "8". Since they were all within the same order of magnitude, I wasn't too concerned. That's our baseline: less than 10 on the most sensitive setting. With that, we bundle up and head back out to the bus.

Hapy Wiring at TPS

6-pin to
front-to-back cable

First, I unplugged the flat-connector, peeled back the plastic wire protector to where the wire splices were and then, with a razorblade, cut a small slit in each cable wire just past heat-shrink so I could touch metal with a probe. You can see the repairs to the wires in the picture on the right. The blue wire is probably the best visual example. Then, from 1 to 6, I checked the resistance to make sure they were all less than 10 on the most sensitive setting. To keep variables down, I held the flat-connector in my hands for about a minute first. It is still cold (4*C) here, even if I'm not complaining about it in every post. Apparently, its not a cold winter, I'm just a sissy working outside in it. So be it. Anyway, they were all very low, like at or below a "2" on the most sensitive setting. This told me that the flat-connector was good and the wire connections between the flat-connector and the cable were good. Very good, actually.

 

While the 6-pin flat-connector was out, I addressed the TPS plug socket with DeoxIt D5 according to the instructions on the can. I figured that if our problem was a simple case of the contacts getting worse over time, we would see a some improvement from this. I covered the little slits with electrical tape, and took a picture so I could reference the color-to-color later. I re-wrapped the wires in the plastic covering and moved to the back of the bus. I left the 6-pin flat-connector disconnected so the contacts could air dry.

Hapy Wiring at Engine Bay

T12-to-donor (T10 blue)

In the engine bay, I confirmed that the blue T10 and grey T12 plugs were snug. I would have had other issues if they had not been, but diligence wins electrical issues. Then, I pulled back the plastic cable wraps and took a picture of the T12 wires from pins 1-6 and then a picture of the color-to-color connections from the T12 to the wires which run to the blue T10. I compared these pictures with the wiring diagram I had constructed and believed all of the wires were routing to the correct partners. So far, so good. The next test was to confirm continuity between the blue T10 pin and the originating pin on the 6-pin flat-connector. This would demonstrate that the wiring from end to end is clean.

 

I was not really sure what to expect, resistance-wise. Before I could do anything, I needed to close the circuit, so I grabbed a long solid-core wire I have used to test with in the past. My testing process: I would gently insert a bare wire into the pin-hole in the flat-connector, and run the rest of the wire spool back to the engine bay. There, I measured the resistance between that wire end and a pin in the blue T10. I had difficulty figuring out which pin was "1" versus "4" in the blue T10 and the pattern from wire to pin seemed illogical. Unfortunately, had I really internalized that, I would have potentially found the root issue. When I found a connection, the resistance was down in the single digits in all cases. So, we can rule out the new wiring... ish: I discovered later that when I wired up the blue T10, I crossed up 3 of the wires. I didn't realize this until later (hence the 2-part post as I lost a bunch of time going down a wrong road). So, I continued forward believing that the wiring was right. Since I learned a bunch about the pedal along the way, I felt it was worth retaining all of this content.

 

Hapy TPS

Since I thought there were no wiring issues, I figured that the TPS went bad. I first thought that maybe I broke it by stepping on it all the way to max, and it needs a movement stopper or something. I looked at the way I installed it; the floor and the carpet would have acted like a prevent against over-pressing the pedal. After doing some research, it seems like these pedals are prone to failure if they get wet. Since Hapy is parked outside, and last year his fancy BusDepot cover effectively held water in rather than out, if the pedal had been steadily degrading because of the damp conditions over the winter (and sometimes in the Summer around here), then this seems a fairly reasonable explanation for the persistent, recurring P0121 code. I figured that last winter's super-damp caused the pedal to finally fail so I ordered a replacement.

 

That's it for today while we wait for the replacement pedal to arrive. More next time about resistances, voltages, etc as I examine the pedal and the pedal wiring and, ultimately, fix the P0121 error.