Hello DP'rs.

I have an interesting tale of PMD treachery. As many of you know, I have converted to WVO, using a Frybrid kit. One of the first things you must do is relocate your PMD. I purchased a used (tested good) PMD, and some other parts from ebay, and mounted it on the skid plate. The PMD worked perfectly, for about 60 days. We'll call this PMD1, which has a #4 resistor in it. This PMD would on occasion, cause the motor to just die, classic PMD failure issue. I contacted the fellow who sold it to me, and he was gracious, and sent me another (PMD2) while in the meantime, I tightened the PMD1 bolts, and voila, PMD1 stopped killing the motor randomly. It never threw a code, but had a slight stumble going up a long hill at big throttle. Very slight, hardly noticeable.

Enter PMD2 (with resistor #6). One weekend, I had some time on my hands, and switched to PMD2, which ran perfectly, but would throw codes at me, 35, 36 and 42. The darned thing ran perfectly, but I'll be darned if I could get the codes to stop, which most importantly wouldn't allow cruise control. PMD2 would never cause a stumble, and the thing would run great, straight up hill, on both veggie and diesel.

The SES light has been killing me, so after much review, counsel and discussion among other veggie or biodiesel DP'rs, I realize that the resistor value may be causing trouble (I did a relearn) in relation to the viscosity of the veggy. "Of course," I think, grinning with joy, the fuel rate change is just enough, between the veggy thickness to trip a code, one way or another. I think, hmmm, I'll take the good PMD (PMD2) and switch to the #4 resistor from PMD1 and voila, have a super running rig, with absolutely no codes thrown. Makes sense doesn't it?

Got her all together yesterday, and she fired right up. Still threw a code, so I undid the batteries for lunch, hoping a relearn would have me going. Ate lunch, and went for a roadtrip. About 15 miles from home, all the while throwing codes (so I'm alread disappointed that my switch didn't work), bango she dies. Wait 2 minutes, she fires up we drive 2 miles, bango, dead. Continue restarting every few minutes, driving a few blocks at a time, finally dead and no restarting her. I MacGyver in PMD1, with the resistor from PMD2, both of which I thought were my problems, and low and behold, she runs perfectly with no codes.

The moral of the story is twofold. Follow the advice of the DP, and don't buy used PMD's. Secondly, when you think a PMD is bad, it could well be the resistor itself. I have, through double blind testing confirmed that the #4 resistor from PMD1 was my problem all along. How silly am I? Knock on wood, but she's running perfectly, and not throwing codes, on either veggy or diesel. Veggy 6.5's deserve a spanking new PMD I think.

Take care all, and when you think your PMD is bad (in my case offering all the typical classic failure signs) it could just be the resistor.

Don't you have to put the same No.4 resistor in an absolutely proven good FSD to know its the resistor and not the FSD itself. They can be flakey and sporadic no way to 100 % know the resistor caused problems in the 2 suspect FSD's.

I read once that the calibration resistor only comes into play at max fuel output ?????

I don't know. I know its some calibration bench test correction at 70 degrees F. Or something like that.

I absolutely agree that fuel viscosity/lubricity does affect precise fuel metering performance problems in a big way.

I should have been more clear. I did put the PMD1 resistor in a known good PMD. PMD2 ran perfectly, no stumble etc., but would throw codes and once I put that resistor in it, which made me think PMD1 was bad, it all went to heck.

PMD1 - Used, ran good 60 days, died, sputtered etc. no codes #4 resistor

PMD2 - Used, ran great always, throws codes. #6 resistor

Tried - PMD2, with #4 resistor thinking that fuel metering would fix my SES codes. Assumed the problems were PMD1 and #6 resistor.

Actual issue - #4 resistor.

The calibration resistor is only read during the process of re-learning. This is the reason "on the fly" setups to change this value aren't actually "on the fly". A Relearn in neccesary after any change.
Therefore seems unlikely to be the resistor(but resistor could be faulty and open circuit - see note below). Once the PCM has read the value it no longer needs it! Theoretically you could remove the resistor after a relearn because it is no longer required.

If there is no resistor or if the resistor is faulty and open circuit then the PCM assumes resistor value #1 and programs for this value. The vehicle will run but you will notice economy and performance decrease.

Yep ... I'm with him... if you change the resistor 'on the fly' (without a re-learn) nothing happens... as a metter of fact, you can take the damn thing right out (like some idiot I once knew ... worked fine until I tried to start it on a cool night in the boonies... must have been the 50th KO-KO cycle ... D-Oh!)

On the other hand, A1 thinks that resistor-change-on-the-fly is a good thing.

Maybe he's been on veggie, too.

If I have no resistor in my PMD and I disconnect the batteries and reconnect them, will it run?

Bill

I doubt it. If the thing re-learns without one, It won't go.

Then you'll have to put one in and do the battery thing again.

CRAP. Can I get one from GM, or will I have to get it online? My resistor is still in the PMD on the pump. I'm using a spare that I got from a diesel repair shop and they pulled the resistor. :(

Bill

Probably not a GM dealer available part, it's mfg by Stanadyne, Kennedy has them plus a couple of other vendors on the web, you can also check if your local injection shop has one, a used one will work they toss them out regularly.

So you are all saying that I'm full of crap? I might be, she died yesterday. Ordered a new harness for the PMD extension. It's the only other part that could possibly be my issue. Dang things, what a stupid design.

mr goodwrench, Disconnecting the batteries will clear any codes on the PCM but should not clear the memory for the calibration resistor. The resitor value it learned everytime a TDCO relearn is performed. Removing the PCM has been known to clear the TDCO figure but most will keep this figure even with power disconnected.

If the TDCO figureis lost the PCM relearn and default to resistor value #1 if you have no resistor. Vehicle will run but no economy or performance.

So if you have not cleared to present value the PCM should keep the value of the resistor in your failed PMD.

Get a new resistor when you can. OR pull the resistor from the one still on the car. It might be a little fiddly but with a paper clip/piece of wire with a small right angle hook at each end and you should get it eventually.

Got to disagree with you a little on last statement Noel if he relearns without any resistor in the driver it won't default to #1 value, it won't start and will set a code, been a while since I've done it so I don't remember the code # but all it will do is crank no start if a learn was done with out the resistor. IIRC it will start on the stored value if he installs a resistor after the fail attempt without one

Turbine, This was the way I have understood it to work. Having never tried it personally and not being able to produce documented evidence of my "theory" I can not go against someone with more experience.
When I get my new FSD, I'll give it a try.

It all depends on when the PCM reads the value of the resistor after clearing the TDCO, as to whether the vehicle will start or not.

Once I did it just to satisfy my doubts of claims that a nameless vendor that LURKs forums said value could be changed "on the fly" for "fuel delivery improvement".

A second time I did it when I forgot to reinstall it in my "spare driver", before I converted to the Heath driver I run.

A 3rd time I left it out on purpose to see if PCM would "self teach itself a TDC offset". I ran 2 months without a resistor installed, truck never did a self learn, nor noticed resistor was missing from the driver, that is until I did the Key on/Key off APP to floor to command learn.

Instantly after that I got a no start condition and a code, now what I don't know is if this is a OBD-II only trait, or a just unique to my truck anomaly; as some of the other guys report ability to self learn a offset value after 50 or so warm up events >170F ECT seen.

I am intersested to see how this all works out. Took a road trip last week and had slight stumbling but only under extremely long grades. I assumed it was another pinhole leak in my Frybrid lines but maybe not.

Let me know if you figure it out.
Thanks.

simple question: how do you tell which resistor you have?

Take it out and read the number on it.....or use a scanner....

I should have been more clear.. where is the resistor located?

between PMD and connector that plugs into it

Well, if in doubt, take the resistor OUT!
there are hundreds of 6.5L in this area running without them, with no problems!
Every truck i have done that only got a pMD replacement, also got the old resistor left out of the new pmd. I know probably a 100 or more people are gonna jump on here and say it will never work without it. But it will, I have tons of first hand knowledge that it will run fine without the resistor.

iirc the ECM defaults to a #5 without a resistor in place.

Depends on vintage of PCM, OBDII or at least the few I've messed with will run indefinately with the resistor out, but once the TDCO learn is initated will no longer start.