MG Sports Cars

engine swaps and other performance upgrades, plus "factory" and Costello V8s

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rficalora
Rob Ficalora
Willis, TX
(2764 posts)

Registered:
10/24/2007 02:46PM

Main British Car:
'76 MGB w/CB front, Sebring rear, early metal dash Ford 302

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Electrical trouble shooting...
Posted by: rficalora
Date: May 13, 2013 04:15PM

Not MG specific, but wasn't sure where to post it...

I have a nuisance problem & am not sure where to start to find the source... My gauges all work as they're supposed to most of the time... but, my speedo jumps about 2/3 sweep every now & then. Only seems to happen when I'm sitting at idle. And, while I haven't seen a "for sure" pattern, it seems to happen shortly after coming to a stop.

My speedo sender wire has limited area -- 8-10" top where it runs parallel to my main harness. And, it is twisted the entire length with a seperate power wire which I thought would help ensure it wasn't very susceptable to picking interference.

How would you go about tracing down what's causing the problem?


lars49
Larry Barnes
Colorado Springs
(177 posts)

Registered:
06/11/2009 02:12PM

Main British Car:
1980 MGB GM LA1 3400 V6

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Re: Electrical trouble shooting...
Posted by: lars49
Date: May 13, 2013 06:12PM

I wonder how much common-mode rejection your speedo has? Have you considered ground bounce?


hirot
Ian Hart
Ashbourne UK
(88 posts)

Registered:
06/01/2011 05:15AM

Main British Car:
1971 MGB GT (conversion) Rover 3947 R380 gearbox

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Re: Electrical trouble shooting...
Posted by: hirot
Date: May 14, 2013 05:32AM

Could be stray RF maybe picked up from your fuel pump which maybe pumps the odd pump at idle. It could be caused by any power cable that runs close to or crosses it.

You could run a separate shielded cable from the sender to the speedo to see if it goes away.

Is it EFI or AFB as the EFI might be sending out the odd pulse which is strong enough to induce a pulse.
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pspeaks
Paul Speaks
Dallas, Texas
(698 posts)

Registered:
07/20/2009 06:40PM

Main British Car:
1972 MGB-GT 1979 Ford 302

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Re: Electrical trouble shooting...
Posted by: pspeaks
Date: May 14, 2013 08:00AM

Rob, I don't pretend to be an expert on such things, others know much more, but every T-Bucket our company built (22) had electronic speedometers, and every one of them had similar issues. With fiberglass cars, the proximity of the alternator and wiring, and the distributor less than a foot away (all chevy's) our problem was much worse. We used a shielded harness on all speedometer wiring as well as the pulse generator and the speedometer itself. This seemed to help but it didn't fix every car; I guess it's just magic. It was so frustrating to me I used a mechanical speedometer in my GT.


Paul


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6469 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

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Re: Electrical trouble shooting...
Posted by: BlownMGB-V8
Date: May 14, 2013 08:34AM

Rob, the power wire isn't helping you any. If you can, disconnect that wire at both ends and ground the speedometer end to the speedometer ground. Let the other end float, don't connect it to anything. Run a new power lead to whatever you are powering with that one and get your speedo pickup lead as far away from all other wiring as you can. That might help some. If you still have a problem try a full shield on the wire, connected the same way.

Jim


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