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Fusing Overdrive on 77 MGB

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Fusing Overdrive on 77 MGB
#1
  This topic is about my 1977 MG MGB MkIV
7279MGBs Avatar
7279MGBs John Fraioli
Denver Colorado, USA   USA
1972 MG MGB GT "Baxter"
1977 MG MGB MkIV
John Twist says it is imperative to place an inline fuse in the Overdrive wiring. I did this quite easily on my 1972 B-GT. I just spliced a fuse into the Yellow wire found coming out of the harness under the hood found running along the passenger side fender close to the windshield washer reservoir. But I can't figure out where to place a fuse for my 1977 (North American) MGB. I don't see a Yellow or a Yellow with Purple tracer wire anywhere under the hood/bonnet. The previous owner apparently has done away/eliminated the TCSA Switch, TCSA Solenoid and the Inertia Switch as well. Can anyone tell me where is the best place for me to place an inline fuse? I will be driving on an 11 day trip from Denver to San Diego and back in a few days and would like to protect the car from any severe melt downs due to any possible overdrive wiring shorts.
Thanks,
John

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Chalky Avatar
Chalky David White
Coventry, Warwickshire, UK   GBR
You should be able to work out the wire from where the gearbox harness joins the main harness. It's by the large collection of bullet connectors on the right of the engine bay.
The join here would be a good place to add a fuse. I added mine into the gearbox harness I rebuilt after it shorted out...

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
Certainly not imperative to fuse the overdrive circuit, since the factory never did so. Nevertheless it certainly can't hurt. The 4-synchro Type LH overdrive electrical circuit draws only about 800 milliamps, less than one amp, so any fuse will work.

As a general rule, when fusing a circuit, always best to fuse it as closely as possible to the power source. This way the fuse will protect more of the circuit. Remember that a fuse only protects the ciruit downstream of the fuse...

From the Advanced Autowire schematic it appears that your overdrive and electric fuel pump are both powered by a white wire coming from the alternator warning light on the dash. So you might want to locate this white wire and fuse it at the vicinity of the warning light. Do note that, on the plus side, locating the fuse here will protect more of the circuitry. On the down side, if a problem with your overdrive causes the fuse to blow, it will also disable your fuel pump...

Dick



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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Nicecar Avatar
Nicecar Gary (ex "Harv") G
Victoria, BC, Canada   CAN
1980 MG MGB "Red On Red On Red"
My notes for my B show 2 wires coming from Intertia switch; one to fuel pump and one to OD. I fused both near rats nest. Single wire running down to transmission should be easily able to find.

Glad I did, because incompetent "MG" mechanic (though I asked him to fuse, didn't) and two days after install OD kept blowing fuses.

Found this:


Attachments:
Bare OD wires in shifter.JPG    45.4 KB
Bare OD wires in shifter.JPG

7279MGBs Avatar
7279MGBs John Fraioli
Denver Colorado, USA   USA
1972 MG MGB GT "Baxter"
1977 MG MGB MkIV
Dick, Would a short in a line that is only carrying 800 miliamps be enough to causing wiring to burn up?

Gary, My car has had the Inertia Switch taken out. Is the "Rats Nest" you refer to the same as the "Large Collection of Bullet connectors that David referred to? If So what color was the wire going to the OD. Like I said earlier I could not find a Yellow wire there as I did when fusing the OD in my 1972 B-GT.

course2kid Jeffrey Johnson
Fountain Valley, CA, USA   USA
1979 MG MGB "Lucy (Lucifer)"
I think you have the OD Switch on top of the gear shift knob, correct? That is one area that can be problematic because the wires to the switch run up a groove cut into the threaded portion at the top of the selector lever. When you screw on the knob, there's a chance to chew up those wires (they do have an extra protection sleeve over them, but, it still gives me the creeps.

Also, where the wire goes into the solenoid is a bit dodgy and could use some sort of strain relief.

As to your question about whether a short in that circuit could be bad since it normally only draws about 800 mA, yes, it could be bad because a short will draw as much current as it can based on its resistance between the 12 vDC power source and the path it takes to ground. So, a bad (low resistance short) could burn up the wiring anywhere between the source and the ground and when a wire in a loom burns, it can take other wires out as well.

Nicecar Avatar
Nicecar Gary (ex "Harv") G
Victoria, BC, Canada   CAN
1980 MG MGB "Red On Red On Red"
"line that is only carrying 800 miliamps be enough to causing wiring to burn up"

Line DOES NOT "CARRY" 800. Solenoid draws 800. A Short will carry massive current, frying something till that burns out breaking circuit.

You can see my labeled OD fuse to white wire, that then continues on with red.

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Attachments:
OD Fuse to White Wire.JPG    50.7 KB
OD Fuse to White Wire.JPG

ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
Fuse it use it
Or potentially lose it



"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972


Attachments:
odWiringDiagramFused[1].jpg    51 KB
odWiringDiagramFused[1].jpg

7279MGBs Avatar
7279MGBs John Fraioli
Denver Colorado, USA   USA
1972 MG MGB GT "Baxter"
1977 MG MGB MkIV
Gary, Thanks for the explanation about the difference between the 800 ma draw and the line load. Wiring is not my "Forte"

Rob, That is a great wiring diagram....with this old out of shape 65 year old body (eyes starting to go as well), it is hard to crawl around under the car up on jack stands. But I did go under and with a drop light looked for wires running to the tranny/OD area. I found that there are 3 wires coming up to that cluster of bullet connectors from the Tranny area.. I took a picture (see attachment). What I found corresponds to the diagram you posted. I found that there was a green, a green with brown tracer and a white with brown tracer. The white with brown tracer comes out of a bullet that also has a solid white wire. This bullet connector is supplied by a solid white wire. Based on the diagram you supplied it looks like I should splice an "In-Line" fuse at the White/Brown wire coming from that bullet connector.....Does everyone agree?
John

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Attachments:
OD 006 (1024x792).jpg    28.1 KB
OD 006 (1024x792).jpg

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
If you're going to add an in-line fuse for the overdrive, that's a good place to put it. That wire is the only wire for the overdrive going from the engine compartment down to the overdrive. The circuit goes through the gearstick switch, the overdrive isolation switch, the TCSA switch if so equipped, and then to the solenoid and on to ground. So only one wire is needed to feed the overdrive.

the other two wires, green and green/brown, are for the backup light switch. They can be connected to either terminal on the backup light switch, since all the switch does is complete the circuit between the two terminals when reverse is selected.

Also note that the overdrive isolation switch, the switch that only allows overdrive in fourth, or third and fourth, is identical to the backup light switch and, as such, it does not matter which wire connects to which terminal on this switch as well. And, for the record, it doesn't matter if the overdrive wiring goes through the gearstick switch first or the overdrive isolation switch first; overdrive cannot be engaged until both switches are closed and current can flow through both of them and on to the solenoid...

Dick

In reply to # 3535695 by 7279MGBs Gary, Thanks for the explanation about the difference between the 800 ma draw and the line load. Wiring is not my "Forte"

Rob, That is a great wiring diagram....with this old out of shape 65 year old body (eyes starting to go as well), it is hard to crawl around under the car up on jack stands. But I did go under and with a drop light looked for wires running to the tranny/OD area. I found that there are 3 wires coming up to that cluster of bullet connectors from the Tranny area.. I took a picture (see attachment). What I found corresponds to the diagram you posted. I found that there was a green, a green with brown tracer and a white with brown tracer. The white with brown tracer comes out of a bullet that also has a solid white wire. This bullet connector is supplied by a solid white wire. Based on the diagram you supplied it looks like I should splice an "In-Line" fuse at the White/Brown wire coming from that bullet connector.....Does everyone agree?
John



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
In reply to # 3535637 by ohlord Fuse it use it
Or potentially lose it

Rob

A brilliant schematic.!

B



Check your ego Amigo!

Team DNF Avatar
Team DNF David Dobbins
Kansas City, Missouri, USA   USA
Always fuse OD circuit. I made jumpers with bullet connectors so I didn't even have to modify the harnesses. You only have to repair one harness to see the possibilities for damage.

I've lived what I speak.

7279MGBs Avatar
7279MGBs John Fraioli
Denver Colorado, USA   USA
1972 MG MGB GT "Baxter"
1977 MG MGB MkIV
OK, it is done. I wired in a fuse on the White/Brown wire. See photo of completed job. Thanks everyone for your input.
John


Attachments:
fused 001 (1024x684).jpg    47.4 KB
fused 001 (1024x684).jpg

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