Slow Car Fast
October 31, 2010 - I've been learning about MG tachometers.
It's fairly normal to have trouble with older tachometers and LS conversions due to the signal from the GM computer. The usual problem is a voltage one, and the solution is a pull-up resistor. That didn't work.
So I asked the folks on the Grassroots Motorsports forum, and it turns out it's the tach used in the MG. Up until sometime in 1972, they used an inductive loop inside the tach to measure the ignition pulses - that's it with the white wire and the copper winding. Well, this works well with points but not with newer ignitions, it would seem. And of course, our 1972 MG has the old style, identifiable by an RVI part number on the face.
One piece of information I found suggested that dropping the number of loops of the white wire would help, but that didn't work. I suspect that's for when you change from points to an electronic ignition, and the distributorless setup from the GM computer is just too different. I guess this problem is well known in MG circles.
Luckily, there's no shortage of MGBs that have been turned into parts cars. There's an "RVC" tach on eBay right now with an identical face design, and it'll play nicely with the GM computer. I may still need the pull-up resistor, but we'll see when it gets here.
An interesting little side trip into gauge electronics, I have to say. And one that can be sorted out fairly inexpensively.
entry 418 - tags: tach, instruments, wiring
November 8, 2010 - Amongst all the excitement of the first drive, the new tach showed up.
I'll wire this into the car in the next couple of days and hopefully have a functioning tach. Fingers crossed!
It's in excellent shape, can't complain there.
entry 436 - tags: tach
November 10, 2010 - One step forward, one step back.
Good news - the new tach works! Well, it works once you remember to actually connect the GM tach wire to the MG harness. Interestingly, it seems to read accurately. I didn't have an OBD-II scanner plugged in to read the exact engine speed to confirm, but the car seemed to be idling right around 1000 rpm cold and that's reasonable. I checked the idle at right around 900 after the drive the other day. Interesting - I'd heard the LS engines put out a "four-cylinder" signal, but I figured that was just because a lot of new fours use a waste-spark ignition and thus look more like an 8. Regardless, I'm not going to complain about it! That was easy.
I also spent some time on the brakes. A bit of fooling around and I managed to get rid of a big air bubble in the front of the master cylinder. Voila, a hard pedal! Nice and hard. But the front fitting on the master was still leaking. This will not do. So I pulled it off and yup, the flare looks deformed.
I figured this was probably the case on Sunday when I was bleeding the brakes, but I was hoping that tightening the joint would solve the problem. Of course it didn't, and in doing this I also damaged the seat in the master cylinder - that's it in the picture. No surprise here. Luckily, the seats can be removed so I'll just call Wilwood and order a replacement.
At the same time, I noticed that the master had a big F and R on the outlets. Now, from my understanding, there shouldn't be any difference between them. I did read a blurb in Hot Rod or Car Craft a while back claiming there was and that swapping the F and R lines around solved a soft pedal problem. I still don't understand how - but since I had to rebuild one of the lines to fix that flare anyhow, I figured I'd swap them. This means new lines for both the front and rear outlet, both pretty short but relatively complex. So now I'm back to no brakes at all. Still, I have a clear vision of the way ahead.
entry 437 - tags: brakes, tach, wiring