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MG MGB Technical - tach short?

A few weeks ago attempted to start Ole Red, '71 MGB. Extensive amt of smoke emitted into tach face and from under dash. Pulled tach and inside was extensively melted (white wires) also melted ~3-4" of white wires to harness. Still have continuity on both white wires, one to the switch and the one to the coil after repairing the melted wires. Car runs fine with the two white wires connected behind the tach. I'm running a Petronix with 3.0 ohm coil.

I've installed a fuse in the white wire from the switch to the tach with a 10amp fuse since the occurance.

Opinion is that the tach malfunctioned for some reason internally and shorted.

Any other suggestions or am I safe to install a replacement tach?

Gene

It would appear that you are correct and it was a short inside the tach
John H

Have to disagree, or at least urge caution in this assessment. There is no electrical connection between the whites from ignition switch and to coil and the tach, only magnetic. If both whites showed heat damage, particularly where they were *not* closely side-by-side, then the short would have been in the white between the tach and the coil. If only one white showed damage then possibly where it went through the pickup loop it shorted to the body of the tach.

Strangely enough this is the 2nd time this has been reported in a couple of weeks. In the first case the heat in the whites was so great that it did actually damage the pickup coil, and so a replacement tach will be required in that case. So yes, you may well have to replace the tach, but this will probably be due to 'collateral damage' (that wonderful euphemism) and not a cause, the important point being that whatever *did* cause it is still out there. However your fuse will protect against an reoccurence, although I'd go for a standard 17 amp rated, 35 amp blow (or near relative) as that will be fine to protect the wiring, and you should have a couple of spares in the fusebox.
Paul Hunt 2

Thanks Paul,
Good info. Will take your recommendation on fuse size. I may have repaired the shorted white when I heat shrinked the white wire, may have an intermittent short. We'll see.
Gene
Gene

After posting and reading the responses to my tach issues and also Rob R's if the last day or so I looked a little further and think I located the source of my short? Pulled the cap on the distributor and the red wire's (pertronix) is abrased to thru the insulation.
Found my short?
Gene

Paul, I currently have the fuse in the white line between the switch and the tach. Appears that to protect the tach I should have the fuse between the tack and the coil?
Gene

Gene - quite possibly, I believe the red wire from the Pertronix goes to the coil +ve, hence if it shorts to ground in the distributor it will damage the white.

Having the fuse as close as possible to the supply protects the most wiring, so between the tach and ignition switch is fine.
Paul Hunt 2

Well darn. Doubt anyone looks back at this but I'll add to anyway. Friend provided a tach, installed today and it wobbles. Initally at idle it's steady then begins bouncing up and down about 250 rpms, will steady a moment then bounce somemore.

I replaced the Pertonix/Flamethrower coil with a backup, no change.

I jumpered around all the white wires coming and going to the tach, no change.

I jumpered the ground directly to the battery, no change.

I have now started cleaning connections in the green wire circuit including at the the fusebox.

We'll give another go tomorrow.

arboangler

If the engine note is steady through all this then I'd suspect the tach electronics. However electronic ignition of various tyoes can cause tach problems, especially on current pulse tachs (used before 1973). After that the voltage tach was fitted which seems to be less susceptible to electronic ignition foibles. One recommendation for earlier tachs with electronic ignition is to change the wiring through the pickup to a single pass instead of the two passes as originally fitted.
Paul Hunt 2

Gene @ Paul: Thanks for the fuse idea. The red wire from my pertronix was also burnt, with most of the damage being at the point where it connects to the coil. I do plan on replacing it or maybe going back to points. Am now waiting for a new wiring harness from Advance Auto Wire and will install a fuse in the new tach wire as you have said...
Bob R.

Well I am stumped. Followed all directions cleaning all connections that lead to the tach, no change. Replaced the Pertronix unit and coil with backups, no change. Jumpered around the white wires/black wires and green wire from the battery, no change.

Still have the flicker. I'm reaching now. I don't have another tach to try, my '73 is an RVC so I'm trying other things, you folks can jump in anytime and tell me that "that's not it".

Tonight I'll jump around the fuel pump ie. take it out of the white splice under the steering column.

Tomorrow night I'll pull the belt off the alternator.

Next night I'll replace the voltage regulator with a back up.

If these don't do it I'll start the search for another tach. Unless someone else has a suggestion.

I'm bumfuzzled.

Wish me luck....I'm open for other suggestions.
Gene

Gene; The RVC tach is more stable with an electronic ignition than the OE RVI tach. You can try your 73 RVC tach in place of your OE type tach. Disconnect your tach white wires from the tach and connect them together to supply power to the coil. Run a temporary wire from the negative side of the coil to the RVC tach connector, as I recall it has a bullet type connector and a white/black wire on it. Run a green wire to the tach for power, the normal wire is green as you know you can use what's in your too box to test it, add a ground and it should work. For testing just hook it up on you fender cover, only problem is you have to remove it from your other car. I converted one car to the RVC because I had problems with the RVI when a Crane Allison system installed.

I have two RVI tach's but as I recall when I tested them several years ago they were not very accurate. I don't remember if they had a wobble. If you want to try one of those I'll send it for the cost of shipping, no charge for the tach. You can email me at flash1929@yahoo.com if interested.

Where is Ridgeland, I grew up in Laurel, but left in 1948 to go into the army for 27 years. My brother and sisters still live in the laurel area.

Clifton
Clifton Gordon

Bob or Gene? - if the red to the Pertronix was burnt then it was the Pertronix that was shorting to ground. Check the distributor ground wire as well, if this isn't sound you will get a misfire or cutting-out when you move the throttle and change the vacuum signal.

Gene or arboangler? Little point in fiddling around with the pump, fanbelt, voltage regulator etc. they won't affect the tach unless the tach itself is faulty anyway. What Clifton says about the RVC is good.
Paul Hunt 2

Clifton, Thanks for the info on conversion. I'll likely give that a try. I really think after isolating everything it's most likely the tach.

Ridgeland is a bedroom comm. on the northside of Jackson. I grewup in Covington County, just north of Collins.

I'm currently assisting a lady in Laurel who's selling her '48TC that's been in storage for 25 yrs.

Laurel was in my stomping grounds area during the early '70s.

Gene
Gene

We'll after being in the doldrums with the tach, 1st shorting then unable to get rid of the bounce/wobble with the replacement tach's my resourceful brother made the fix. Replacing one of the capacitors did the trick. His opinion is that overtime the things break down due to heat/electrical spikes....He checked for calibration againest the tach in his car and seems to be pretty dead on with my dwell tach.

Just an FYI for future inquires.
Gene

This thread was discussed between 02/11/2006 and 13/11/2006

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