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-