P0121 - the 1200 rpm limp mode - Final

Today I complete the saga of trying to resolve the P0121 error that persisted after I re-wired the front-to-back cable, the fuse box (main donor) harness and the fuse box / relay box. It is terribly ironic that this post arrives on Groundhogs Day. I have had drive-by-wire (P0121) issues so much since I did this conversion, it was like my own personal Groundhogs Day every time I dropped into that 1200 rpm zone. Queue the Sonny and Cher song. With only one error code, you would think this would have been easy. Nope. At least, after all the wiring changes, I only had the one code.

Pedal Pedal

new on bottom

When I left off my last post, I had concluded that the wiring was not the issue, and ordered a replacement pedal. These aren't cheap, but I found one for around $100US (which was a really good price). It took a while to get here from the east coast, but once in-hand, I thought it would be interesting to compare the new and the old. Physically, they are very similar, but the old one has a removable panel. I pulled out the multi-meter and started testing the resistance between all of the pins on both pedals. This was illuminating.

Then, I popped each pedal into a bench vice so I could depress the pedal by tightening the vice. This allowed me to measure how the resistance changed across the pins as the pedal was depressed. Generally, both pedals demonstrated the same direction of resistance change relative to the direction of the pedal movement, but there were some definite differences. For example, the resting resistance between the pins (which had a corresponding resistance on the other pedal) was lower on the old pedal than the new one. I was able to correct for most of the difference by shifting the little cover on the old pedal forward (front-is-front / away from the footpad). Prior to moving the cover, it was near the mid-point of the front-to-back adjustability.

The biggest difference I found was that on the new pedal, there was infinite resistance between pins 2 and 4 when the pedal was 0% depressed. The old pedal, however, had a measurable resistance (1760 on the second-to-highest sensitivity). Once the new pedal moved even a little off of 0% depressed, resistance was measurable. I thought this was the big tell that the old pedal was the cause, but before I get into the in-situ test, here are some tables and notes around my testing of the 2 pedals. I suspect there will be readers who are only here for that.

Testing Static Resistance between Pins 

inside the removable
cover

In the table below, the multi-meter was set at the second-to-most sensitive resistance measuring. The number on the top is from the new pedal, and the number below it is from the old one before I tweaked the little cover position. 

The word "flash" means that even though I tested these pin resistances 5 or 6 times, on the new pedal, the resistance between these pins would flash a quick 4-digit number and then go to infinity. I tried to measure it with different settings on the multi-meter, but for my purposes, it was clearly very different from the "old" pedal. Since it happened every time, I concluded it was not caused by my probe accidentally touching pin 3 while approaching pin 4. I did experience intermittent issues with my cheap $5US Harbor Freight multi-meter where the whole plastic probe would fall off the lead. The multiple tests ruled that out as a cause for the "flash" as well.

The "fix" for the probe-fall-off: Strip back a little bit extra off the end of the probe wire and push it back into the probe. Check continuity between the probes. Down to 0 or nearly 0? Perfect. Hold the wire and probe steady and tape the wire to the probe, and then run more tape up the wire a couple of inches. That wire won't pop out easily again.

Where were we? Oh, yeah.. while there are 6 pins, there was not a measurable resistance between most of them. That is why some pins are not on the tables at all and some pin intersections have a blank square. In both cases, there was no measurable resistance. Pin 1, for example, had no measurable resistance to any other pin. Neat, eh?

	3	4	6
2	1320 900	flash 1760
3		1010 996
5			1070 945

 

Testing Transient Resistance between Pins

Next, let's look at the throttle testing in the vice. So, consider these values as "when the pedal is depressed, the resistance between the pins..."

hunting gremlin

	3	4	6
2	unchanged unchanged	*drops (1590, 1480) drops (1570)
3		rises (1422, 1700) rises (1300)
5			infinite infinite

The * is to show that the resistance between the pins suddenly changes from infinite to something (high) 4-digit measurable once the pedal is starting to get depressed. The resistance between pins 5 and 6 goes to infinite as soon as the pedal starts being depressed. This is all really cool stuff, but are we closer to having an accelerator pedal again? Well, I plugged the 6-pin flat-connector into new pedal and started the engine. Idle... and then 1200 rpm mode. So, nope. So, next I checked the voltage of the 6 pins on the flat connector, and put my findings in another table below. Perhaps this would have been a good thing to test while waiting for the pedal to arrive. Or better yet checking before ordering one. Hindsight is great.

Voltage at the Flat Connector

The "should be" column is from comment #7 on this posting on the TDIClub. Fred's TDIClub is such a fantastic resource. Highly recommend for the knowledge, and they are generally nice folks too, unlike some other boards where so many replies to questions are "use the search function". Helping not helping.

	should be	no key	key to run
1	0-90%: 9+V	no V not ground	..8V
2	5v	no V not ground	5V
3	ground	ground	ground
4	.35V -> 4.5V	no V not ground	2.7V
5	0%: 0V >0%: 2.758V	no V not ground	8V
6	ground	ground	ground

So, I looked at this table and some of the values look spot-on, like the 5V reference voltage on pin 2 and the grounds on 3 and 6. The other 3 look like I wired them incorrectly. Like, maybe wire 5 should be going to pin 1, wire 1 to pin 4 and wire 4 to pin 5. At least then the voltages would be in the right ballpark. So, before I assume that my ECU is fried (the only other reason I can come up with for this), I decided to change some of the wiring to reflect this re-arrange first. Considering the difficulty I had determining the logic at the blue T10 I mentioned in the last post, this felt very much like a real possible cause.

Change 3 Wires

more hunting gremlin

Since the wiring on the fuse-box / relay-box harness was very hard to get to, and the 6-pin flat-connector is very easy to get to, I decided to change the wiring at the 6-pin flat-connector. This will force an update to my wiring diagram, but this was the fastest route to testing the theory while minimizing the potential "bump risk" (you bump something and now you have a whole new set of problems). Anyway, this re-wire was very easy: label the 3 wires, cut the heat-shrink, unwire, re-wire, re-heat-shrink.

Of course, any final test couldn't be without it's own challenges, as the electrical acted batty for about a day while I chased gremlins again. Something was creating a vibe, making the system act like the battery was dead. For example, the hazard switch would cause the relay to buzz, lights wouldn't come on, etc but a voltage test on the battery was fine. After several hours of tearing the dash board apart, believing I had bumped something when doing the flat-connector, I removed the battery from the bus and verified it had over 13.5V (full charge). So, I put the battery on the bumper and hooked up the battery cables, omitting the float charger, and tried the hazard. Success. Then I turned the key to run and everything acted normal. Love you, Hapy. Root cause: current owner error not tightening the positive battery cable on the battery post after changing the wiring. Sigh, sometimes are better than some times.

I pulled the positive cable off the battery and re-assembled the dash. While I was there, I cleaned all of the fuses and their connections with rubbing alcohol, and then zip-tied the fuse box up out of the way (it has never been screwed to the bus since I bought it). With the front-end cleaned up, I put the positive cable back on the battery (still sitting on the bumper) and checked again that things were normal. With the key turned to run, I checked the voltages on the flat-connector, and now the pins aligned with the table above.

Test Fire

battery on "bumper"

Feeling confident, I turned off the key, plugged in the new accelerator pedal and test fired the bus. Engine started, sat at idle, threw no codes... and working the pedal with my hands revved the engine. Hazah! To complete the loop, I removed the battery cables from the battery, and put the battery back in the bus. While I was there, I resolved some wire routing, and implemented a battery strap-down with the webbing I used to hold the speaker box in the MGB trunk (See Speaker Box Install Finish). This also is holding the battery-top fuse-box on top of the battery. I returned the battery cables to the battery posts (this time tightening them down) and did another test fire. Start, run and rev. With all the wiring and cables now sorted, I was able to re-install the original engine hatch prop, which means no more holding it open with either my head or a stick (like in the picture on the right, here, you can see a stick all the way to the left). More winning. And, of course, I bolted the pedal to the support post and tested the foot feel before heading inside for a celebratory smoothie.

The next day, I set out re-test the old pedal to see if that small resistance difference between pins 2 and 4 was meaningful. It isn't. The old pedal works correctly now, just like the new one: no codes, engine rev's. Since the new pedal is bolted in, I decided to leave it, and put the old pedal into my parts bin. In the end, the sporadic P0121 error that Hapy and I have suffered for the last few years was ultimately caused by something in the wiring that I replaced with the front-to-back cable at the start of the Chasing Electrical Gremlins saga (See part 1). Still, I will be putting the old pedal in a large Ziplock baggie under the rock-n-roll bed in case this new pedal fails on the road. So, this completes my efforts with the P0121. I hope the resistance and voltage (courtesy of TDIClub) information above is helpful to others as you diagnose your own P0121 error. 

Thanks, as always, for following along-

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.