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MG MGB Technical - Sorry - another tacho thread :-(

v. sorry as this is always being covered and i have read through the archive, but must be being a bit thick cos i can't seem to relate that stuff...

Very recently my tacho has started 'wavering' usually this is when at cruising speeds, (about 75mph for me). It wavers between about 2500rpm and 3700rpm and then just as soon, stops itself.

This wouldn't bother except for two further developments.

One, i am sure the unit is reading lower than it did - whether it was high or not before i don't know. I have 185/70R14 tyres, 4spd OD box, 75mph normally came up at 3100rpm and now its about 2900rpm. Again not serious but its bugging me as to what the change is.

Two, one the way home sunday night, the suddenly started gently wavering tacho went completely bonkers and repeatedly flew from 0 to about 4500rpm many times, and very rapidly. This worried me the most as it was accompanied by a good couple of misfires and poor running for a few minutes. The throttle had little effect on this at all. Once the tacho had settled it appeared to be fine and we had little further problems.

I've had a scratch about, i have checked the earth point for the tacho, it seems to be ok, but i will double check all connections again to make sure. I'm wondering though if this is the start of something going, but at a loss to think what - voltage stabilizer?

Would apreciate a slight nudge in the right direction if at all possible. Have obviously got complacent in the last few months cos the ol' girl has been very well behaved and i haven't had to diagnose anything for yonks. Consequently i can't think where to start to look.

TIA ~PHIL

Sorry for the essay, just trying to give you all the info!
Phil

If the tacho faltering coincides with a misfire, have you checked your LT circuit thoroughly?

Tim.
Tim Jenner

Tim,
I agree on checking the low tension circuit. Could be loose or corroded connections.
Steve
Steve Brooks

Hi..

The tacho looks at the number of on / off pulses that the distributor's points make, so it is probably a problem in the points / condenser / coil area, as indicated by Tim and Steve.

The earlier models required the coil current to pass through the tacho via the white wire, while the later models take a 'sense' connection (white / black) off the coil.

The voltage regulator isn't part of the tacho circuit.

Don
Don

Some further thoughts...your problems seem to be happening at reasonable engine speeds, I'd check your points for bounce as well as the normal gap and cleanliness checks (though points normally only bounce at much higher speeds, and then consistently not intermittently). If you have electronic ignition, try a set of points to eliminate that as a cause of trouble.

Since you get erratic tacho readings AND erratic running I'd guess that the rev counter itself is probably fine. On earlier cars, as mentioned by Don, the white switched LT lead is looped around a post on the back of the rev counter. Check this isn't too tight and the insulation is OK - you could have a short to earth here since there's lots of metal behind the dash! Similarly with the later cars, except there's no loop because the feed is direct and not induced.

Final thought - I've had a couple of spade connectors fail on me where the LT circuit connects to the coil. Run the engine with the bonnet open and wiggle all the LT wiring to see if there's an intermittent short or break.

Tim.
Tim Jenner

Thanks a lot guys

i don't know why it stumped me but it sure did!!

Been out with the car today, just a short run and then a quick fiddle about as light was fading so i coudn't do much, but i will look at points et al this weekend. It has been 3000miles since they were changed/looked at/adjusted so i think the BGT is calling for some TLC again (how you guys with kids juggle the two i'll never know! ;-> )

Seems to be alot of under bonnet grime despite a right clean up under there a few weeks ago and i have a slightly weeping head gasket between cyls 2 and 3 obviously above dizzy, so its probably all related when i think about it! <smacks head in realisation!>

Thanks again guys, i'll squawk again if it doesn't sort it!!!

~PHIL
Phil

Phil,

Before I switched to electronic, I used to have to clean and regap my points every 3000 miles. Sounds like you have a likely culprit! Good luck!

Tim.
Tim Jenner

Tim

I think you're right

I'm usually dead regular on a 3,000mile oil change and check up, however i've done less miles this year and only just reached 3000miles (normally hit a 1000 a month).

Of course being only a month and a bit until we're off to le mans i was trying to be sneaky and miss out the mini service to get to the big one in june - should have known that would never happen!

Consider myself told! ;->

~PHIL

Phil

Phil,

So you're off to Le Mans? I'm very jealous! :) Really wanted to go this year (never been before) but couldn't find anyone willing to come with me and map read. :(

Tim.
Tim Jenner

Tim

Yep - certainly am! Took my little brother last year as a birthday present (just turned 18), it was the first time for the both of us and also two mates. The two mates took a diesel audi - we took the BGT!!

Faultless run and much antics had by all.

I was persuadeed back again this year and despite my better judgement ( ;-> ) have agreed to take not only my car agin, but my brothers classic vauxhall (*bought just beofre last years trip) and one of the guys who went last year, has acquired a BGT also and he's taking that, so theres three classic cars going, six people!

We must all be mad, it cost me a fortune in 97 octane fuel last year (i think 7 fill ups compared to the audis 2.5! and diesel is CHEAP there too) and we're multiplying that by 3 this year, but if you enjoy seeing exotic cars it is worth it, the drive down from the ferry with the Brits and their supercars, is just an amazing site, and i could bore you all day with stories..! ;->

Think the MGOC might sill have a few places in terms of discounted tickets, otherwise the ferry companies still have some and i know of a cracking campsite just a few k from the circuit, run by MG nuts (H+C running water, cooked food in price etc) - go on!!!! Bag someone to come with you, you know it makes sense!!!

Good luck ;->

regards ~PHIL

Phil

Ooh, just don't even joke about tempting me! :) I have to admit being slightly wimpy about it too - would like to speak to someone (like you!) about things like where to stay, what to take, how do you get from your campsite to the track, etc, rather than just upping and going! The thought only really occured to me recently and it's a bit late now. Maybe next year! Oh sod it, DEFINITELY next year!

Tim.
Tim Jenner

Surprisingly there are people who just get up and go and sort it all out when they are there!! Our first go last year, though i couldn't do that and this year we just followed last year basically.

If you camp at circuit no worries.

If you stay at the campsite i know, then you either drive up (its onlysomething like 5k not far at all) or you can pay for them (the campsite guys) to take you in small shuttle bus.

If you look at this thread tomorrow i will hopefully have the dug the place out of my work internet favorites box and post so you can have a look at it yourself - we were impressed enough to stay again this year, but thats also cos we were too slow to buy tickets to camp at the circuit (which isn't all its cracked up to be anyway, this place is far better). Won't be as tired then either so will make more sense hopefully! ;->

~PHIL
Phil

This thread was discussed on 30/04/2003

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