| [GMCnet] Return from GMCES Dillard (Pt.2) [message #92067] |
Thu, 15 July 2010 19:17  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma:
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After John & Linda got away Tuesday morning, I went back to the GMC. The
ground fault was still there, but when I looked more closely, I realized
that I really had NOT created a short there. It was soon apparent that the
short was between the power and ground leads to the VSS, which I had mounted
on the governor 3+ years ago.
On the workbench I discovered that the power, ground, and signal leads in
the VSS cable were all firmly melted together -- no one could be separated
from the others without baring its wire. After stripping down to the last
3/4" of wire before the potted VSS, I decided I'd better open the VSS's case
to replace the wires completely.
The four rivets holding the case together drilled & punched out easily,
revealing the guts of the thing. I've always wondered how it worked; now I
know: There is a shaft all the way through the case, with a 1/4" thick, 1"
round ferrite magnet secured to it. As the magnet rotates, the N & S poles
pass the end of a nylon tube into which is potted a Hall Effect sensor at
the inner end and the 3 wires at the outer end. Since I could not find a
compass, I made one from a straightened paper clip magnetized by stroking it
with a pick-up tool's magnet. Hung from a string, that confirmed that the
rotating wheel contains only one magnet -- single N & S poles.
Since this is a 4-pulse VSS (produces 4 pulses per revolution), producing
4000 (4 times the US standard 1000 speedometer cable revolutions per mile),
and the magnet has only two poles, the production of the four pulses is
handled by the sensor electronics. There are several methods by which that
could be accomplished, but the HOW is really of no consequence to us.
Since the wires were potted into the sensor module, I had to solder new
leads to the short stubs left when I trimmed the original wires; fortunately
there was no short among them. I added new high temperature wires, potted
them and enclosed them in 3 layers of heat-shrink tubing. Long 6-32 screws,
bradded to prevent loosening, replaced the rivets holding the case together.
I've driven the GMC around the house to its regular home, but I haven't
tested it on the road to confirm the efficacy of the repair. There should
be no problem.
This exercise did clear up a curious situation I've had ever since I added
VSS to the EFI: My VDO speedometer can be set for any pulse rate up to
115,000 pulses per mile, so calibration was easy -- I matched the GPS's
readout and didn't worry about the pulse rate. When I added a divide-by-two
circuit to provide the 2000 ppm required by the EFI, I was surprised to find
that the What's Up Display speed indication (part of the EBL Flash upgrade)
was about 15% higher than that on the VDO. For several months I've been
trying to figure out how I could get such consistent noise into the signal.
Yesterday, while hunting a short speedometer cable to allow me to move the
VSS to a cooler location, I came across an item I didn't reinstall when I
installed the VDO speedometer: The ratio adapter which corrected the
mechanical speedometer for the change from 3.07 to 3.55 final drive ratio.
Ah! Ha! 3.55/3.07=1.156! That's where my 15% WUD speed error came from!
Gosh that's embarrassing! Such a simple explanation to cost me so much
un-affordable hair loss! The latest version of the EBL Flash software
allows the VSS input pulse rate to be set to other than the standard 2000
ppm so I'll be able to eliminate the divide-by-two circuit AND the speed
error.
All's well that ends well.
Sorry for consuming so much bandwidth, but I had a LOT of fun to report. :-)
Ken H.
Americus, GA
'76 X-Birchaven
www.gmcwipersetc.com
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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