Aristo! Aristo! A la lanterne!: RF bleeds can be a stone cold %$@#!

Wednesday, June 1, 2011

RF bleeds can be a stone cold %$@#!

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Go team me! (So far....)

About 18 months ago I bought Scooter the VunderVagen, right after the wife stole my truck in the middle of the night. Scooter, otherwise known as a 1994 VW Golf, with a 2.0 liter engine and a 5 speed manual trans. It came a fair amount cheaper than market value, attributed to age, mileage, and a smattering of needed repairs. Making parts by hand to repair the door locks, overhauling the brakes, some sensors and ignition work... I soon had it in decent repair. The best part? Fuel mileage good enough to almost be scary. I know how to drive stick.

Except... one nagging powertrain management problem I have been hunting since the day I bought it. I suspect it was also the chief reason I got the vehicle cheaper.

Occasionally, the check engine light would come on, the PCM would drop into limp-in mode, and it would store a code for cam sensor signal issues.

Limp-in means light on, base timing, mileage drops, power down, and performance poor. It's perfectly drivable.... but not as fun and certainly gets crappy mileage in limp-in mode. By crappy, I mean maybe 25 MPG instead of 35+.

Well today I had a lab scope hooked to cam sensor output, and I was getting occasional weird readings. Looking for a 7 (and a bit) hertz signal, I got it... with intermittent jumps to 29 hertz... and occasionally just utterly crazy readings.

I figured.... bad hall effect sensor in the distributor, and I was glad to have finally seen something I could pin these weird symptoms on. Then.... the aging brain kicked into gear.

Hmm... expecting a 7 hertz signal at idle from a hall effect sensor with a single window trigger wheel, but getting occasional bursts up to 29 hertz on the signal wire... and 7+ a bit IS 29 when multiplied by 4 and it's a 4 cylinder engine.... and... and... and........

I took my lab scope lead and probed around the distributor cap.... and %$^$%#@ if I didn't find a 29 hertz signal right above the wiring harness going to the cam sensor!

I pushed the harness away and shoved a wadded up paper towel between it and the distributor. Both the test drive and the drive come were clean, with no more symptoms. It runs like a different car.

Tomorrow I will case the wiring harness and make a grounded RF shield to go around the cap. With luck.... I just found the last nagging drivability issue this car had.

I am.... pleased with myself this evening.

2 comments:

Rev. Paul said...: That's a good bit of detective work, right there. Congrats!; June 1, 2011 at 4:37 PM
og said...: You should indeed be pleased with yourself. Nicely done!; June 2, 2011 at 7:56 AM

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Robert Heinlein

"Do not confuse "duty" with what other people expect of you; they are utterly different. Duty is a debt you owe to yourself to fulfill obligations you have assumed voluntarily. Paying that debt can entail anything from years of patient work to instant willingness to die. Difficult it may be, but the reward is self-respect.

But there is no reward at all for doing what other people expect of you, and to do so is not merely difficult, but impossible. It is easier to deal with a footpad than it is with the leech who wants "just a few minutes of your time, please — this won't take long." Time is your total capital, and the minutes of your life are painfully few. If you allow yourself to fall into the vice of agreeing to such requests, they quickly snowball to the point where these parasites will use up 100 percent of your time — and squawk for more!
So learn to say No — and to be rude about it when necessary.
Otherwise you will not have time to carry out your duty, or to do your own work, and certainly no time for love and happiness. The termites will nibble away your life and leave none of it for you.

(This rule does not mean that you must not do a favor for a friend, or even a stranger. But let the choice be yours. Don't do it because it is "expected" of you.)"