View Full Version : BJ8 tach???

04-01-2003, 07:36 PM
My tach is reading pretty much double what the expected rpm's are. Any ideas as to what could be causing this?


04-01-2003, 11:45 PM
One of the electronic thingies inside is broken! (It IS April 1st)

Externally: if the point dwell were way off or the capacitor on the distributor were open, it would affect the tach reading. My experience is that the car will run badly if either of these conditions exist. Once you've determined it's not one of these two items, it's time to look inside.

Internally; It could be any of the little thingies inside. After 30+ years, some things are more suspect than others. The Schematics I have for BJ8 tachs show three capacitors. They are suspect. I would start by replacing the 100 uF capacitor, then the .25 uF (.1 on some units) and lastly the .47 cap across the piclkup transformer.

I'd say do all 3 at once, but if you're not profficient at soldering, wait a minute, if you're not comfortable working on electronic thingies, maybe you should send it to a specialist.

If you think the problem is internal and you are brave enough to want to troubleshoot it yourself, let me know, and I'll send you a PDF of the schematic.

Good luck, double check the dizzy capicitor.


tahoe healey
04-03-2003, 11:53 PM
Ok, you got me thinking and I looked in my sources but couldn't find an equivalent for RPM vs MPH at any given gear (including overdrive). Anyone know? It would be nice to compare assuming stock rear end.

04-04-2003, 03:35 AM
Tahoe Healey,
Though I tried I just could not resist the chance to bring forth the rath of the math Gods. So here goes.

((Engine RPM / Selected Transmission Gear Ratio) / Rear End Gear Ratio)* (Tire Circumference in inches / 12)* 60 / 5280 = MPH

I tried to write this different ways but this is the closest I could get it to look like a real formula. This would not be that hard to build into a spread sheet. Just have to know what your constants are for trans gears and rear end and would have to measure your tire.It looks harder than it is.
OK Math Gods did I get it right.
If not, is it at least close?

[ 04-03-2003: Message edited by: CDK ]</p>

Randy Forbes
04-04-2003, 10:52 AM
Spreadsheet already exists.
Dan Masters (MGBGTV8 Newsletter Editor) did one several years ago. It works great, just plug in all the varibles (tire size, trans and final drive gear ratios) and stand back.

Is there anyway to attach it to the board? I have it as an EXCEL File.

04-04-2003, 06:36 PM
I have an electronic ditributor installed by PO. Could that be the cause?

Dave Russell
04-04-2003, 10:22 PM
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by droy:
I have an electronic ditributor installed by PO. Could that be the cause?<hr></blockquote>
Yes, it could be the problem. I have read of several cases where various electronic ignitions caused tach problems. Unfortunately I didn't find a ready solution. Some had to do with slightly modifying the tach. Sorry I'm not more help but maybe it will point you in the right direction.

04-05-2003, 11:23 AM

To add some hope. My Tach works great with a Pertronix ignition system. Who made your dizzy?

Does the distributor manufacturer have recommendations? I can't imagine thay sold many units if it fouled up the tachometer (and maybe they didn't) Check their web site (if they still exist).

One other thought.. If the ignition system has been altered; is the coil polarity correct?

[ 04-09-2003: Message edited by: ThomP ]</p>

04-08-2003, 10:33 PM
Before you tear apart your tach, return your ignition to original condition. Install new points and condenser. Did you switch from pos to neg ground? If so, did you modify your tach correctly? If your tach returns to normal ops then do one mod at a time as you reinstall your changes graemlins/cheers.gif Sky

06-20-2004, 03:39 PM
I have the same tach problem you had. But, I have stock ignition. Did you ever get
it figured out?


06-20-2004, 09:29 PM
Me too. Mine generally runs high, sometimes by 1000 rpms. I drilled a hole in the back of the tach to get to the adjustable pot. However, when I turn the tach down, it works at lower rpm's, but once it reaches about 3000 rpm's, the tach falls back to zero or jumps radically. I would try changing the capacitors, but can't read which one is which. I am a little afraid to start taking too many things out and replacing them. Someone said to try the .22 uf first, but can't tell which one that is. It's beginning to look like it will follow the speedo to MoMa Industries.