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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What causes a relay to buzz? May have figured it out while typing this - does it make sense?
Posted by: rficalora
Date: December 06, 2011 12:06AM

Symptom - flip the switch controlling the trigger & relay makes a loud buzzing sound. Relay is wired where switch completes ground as follows:

86 wired to switch, other side of switch tied to ground [tests open w/switch off, ground w/switch on]
85 power from harness (headlight power) [tests 12v constant]
30 power from harness (same headlight power wire) [tests 12v constant]
87 wired to dash lights & parking lights [tests open with switch off, ground w/switch on]

Tests were at the relay socket with the relay removed.

They all make sense to me except the 87 pin. It seems like it should read open whether the switch is on or off (remember, the relay was not in the socket during this test so there's no coil to energize to make the 30/87 connection.

That suggests the 87 pin is shorted with the trigger switch, right?


Bill Young
Bill Young
Kansas City, MO
(1337 posts)

Registered:
10/23/2007 09:23AM

Main British Car:
'73 MG Midget V6 , '59 MGA I6 2.8 GM, 4.0 Jeep

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Re: What causes a relay to buzz? May have figured it out while typing this - does it make sense?
Posted by: Bill Young
Date: December 06, 2011 11:48AM

I think you have it nailed Rob. You might read some resistance on pin 87 but that would be through the lights so not a direct short. It appears that pin 87 and pin 86 are shorted together somehow. I found this diagram and pretty good explanation of the standard Bosch relay on line, might be of help. [www.classictruckshop.com]


DiDueColpi
Fred Key
West coast - Canada
(1365 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

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Re: What causes a relay to buzz? May have figured it out while typing this - does it make sense?
Posted by: DiDueColpi
Date: December 06, 2011 08:47PM

Double check the relay Rob and make sure it is laid out in the conventional pattern.
Some of the Bosch and Hella relays won't have the pins where you would expect them.
Relays for fog lights and headlights for example have two #87 terminals.
Relays for BMW have the #30 and # 87 reversed.
Many of the relays will also have a diode or a resistor to collapse the coil faster and the polarity of #85 and # 86 will matter in this case.

Hope that helps.
Cheers
Fred


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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Re: What causes a relay to buzz? May have figured it out while typing this - does it make sense?
Posted by: rficalora
Date: December 06, 2011 10:01PM

Hmmmmm. The plot thickened with a little more testing; hopefully it'll be a clue....
* Dimmer in either low beam or high beam position with just parking lights on - relay 1 buzzes.
* Dimmer in low beam position with headlights on - relays 1 & 2 buzz.
* Dimmer in high beam position with headlights on - relays 1 & 3 buzz

So folks will have the full picture, this is the diagram for what I'm working on...
Headlight wiring.jpg

This all tested good & has been fine for a while so I don't think it's the basic wiring or components. But, I've "touched" a bunch of the wires in the past few weeks -- disconnected/reconnected the leads to the starter & a ground junction under the car; added a ground that was missing for the park/turn lights, disconnected/reconnected a few of the wires on the low & high beam relays. I've also pulled & reinstalled my dash a couple of times (it goes in with 3 nuts, 2 screws, & 3 wiring plugs) to change out my high beam indicator light (it's a blue LED & I broke one of the legs when I was tieing down the wires).

I don't think the dash wiring is the culpret - it is all neatly zip tied, connectors are soldered, etc. The more I've thought about this, the more I'm thinking it has to be something I've reconnected wrong (my garage isn't well lit). I'm off on Thursday so looks like I'll need to break out a heater & start double checking things.



Edited 1 time(s). Last edit at 12/06/2011 10:02PM by rficalora.


DiDueColpi
Fred Key
West coast - Canada
(1365 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

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Re: What causes a relay to buzz? May have figured it out while typing this - does it make sense?
Posted by: DiDueColpi
Date: December 07, 2011 01:28AM

Just as a guess, are one of the 87 and 85s on one of the headlight relays reversed.
This would blow the parklight fuse and result in the buzzing relays.
A missing or loose dash ground will cause the same thing.
Cheers
Fred


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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Re: What causes a relay to buzz? May have figured it out while typing this - does it make sense?
Posted by: rficalora
Date: December 07, 2011 05:01PM

Thanks Fred - that should be easy enough to check & very possible. I just have bullet connectors for the relays right now (have some metri-pack weather resistant relay sockets coming) & did disconnect/reconnect them when bundling the wiring.

My dash grounds are pretty solid. All the wiring in the dash goes to one of 3 multi-pin connectors & all the grounds are tied to a common point under the dash with soldered ring terminals held by a bolt/riv-nut. And, I have a 10GA lead from that same spot to the chassis ground where my battery ground cable terminates.


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

authors avatar
Re: What causes a relay to buzz? May have figured it out while typing this - does it make sense?
Posted by: rficalora
Date: December 08, 2011 09:46AM

SOLVED!

Wasn't anything wrong with the relay wiring! But, Fred's mention of the dash ground was a big help. I checked the relay connections & they were right. But, I made sure they were all seated well & then retested the lights. When I did that, when I flipped the headlight switch on I noticed the light on my keyless ignition's start button dimmed & flickered some. Flipped the switch a few times & noticed the speedo start up sweep was doing weird things too so started thinking maybe my battery was run down from working on the power windows (that was the last thing I was doing before this problem started) so went to connect a charger... That's when I noticed I'd neglected to reconnect the main ground lead that goes to th back of the car - fuel pump, trunk light, and all lights (tail, park, and turn)... Hooked that back up & everything seems to be working again! Since it was working even when disconnected, the only thing I can figure is the ring terminal was touching the IRS frame making enough of a ground, then as it got cold here over night that night, the wire moved a little or maybe the cold causes less conductivity? Anyway it works again so back to figuring out where the window mechanism is rubbing and how to adjust the up travel limit.

Thanks again for the ideas.



BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6468 posts)

Registered:
10/23/2007 12:59PM

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

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Re: What causes a relay to buzz? May have figured it out while typing this - does it make sense?
Posted by: BlownMGB-V8
Date: December 08, 2011 11:50AM

Rob, it seems the length of the screws attaching the window crank to the door is critical. Make sure they aren't too long.

JB


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

authors avatar
Re: What causes a relay to buzz? May have figured it out while typing this - does it make sense?
Posted by: rficalora
Date: December 08, 2011 07:22PM

Thanks Jim. The Moss power window kit I'm using completely replaces the stock scissor & crank mechanism... But the bolt lengths are critical on it too. I got rid of the rubbing today - a couple of the bolts needed an extra washer or two. Also there's a rubber block that limits the down travel. It bolts on from the back & I had to grind the heads to about 1/2 the thickness for them to clear. The up travel limit is a screw with a sleeve that one of the sprockets on the main gear hits. Travel limit is adjusted by moving the screw in a horizontal slot. I had to make the sleeve diameter about 1/4" bigger to get the window to stop w/o tearing up the rubber stop at the top of the wing window channel. Could have lengthened the slot, but it's already pretty close to the edge of the mechanism.


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