MGB mystery wire

[Svcmangler]

Good morning everyone. I have a 1978 MGB with a mystery wire that is in the loom that goes to the temperature sensor. It is a heavier gauge than the temp sensor wire, but is also dark green. It has power after the key is turned on. Anyone know where this might go?

 

[NutmegCT]

Just a thought, but could it at one time have connected to a temperature controlled electric cooling fan?

Tom M.

 

[Svcmangler]

well the car does have an electric cooling fan. but since it is hot after the ignition is on, that would mean it was on all the time.
The vehicle does have a electromatic switch at the radiator that I assume would control the fan.

 

[DrEntropy]

IIRC, that heavy green (+12V) goes to one side of the fan and the "Otter" switch (in the radiator) has the other side at the fan and one to ground. Those should be black, I believe. When the Otter switch closes it supplies the ground leg of the fan circuit.

Disclaimer: This is from memory, haven't worked on that version MGB in a while. These guys: Advance Auto Wire have wiring diagrams for the MGB at every change point in production. Look through and find the year and compare.

 

[DrEntropy]

Just looked thru the schematics and the Otter switch was introduced in '77 from the looks of it. It does supply the ground leg to the fans, that heavy green one should indeed be the voltage supply side for them. The diagram calls the Otter switch a thermostat.

 

[NutmegCT]

Just as we suspected!

SVC - does the electric fan you have now ever come on?

 

[Svcmangler]

no the fan does not come on. Now I just have to find where that thermostat switch is

 

[DrEntropy]

Just as we suspected!

SVC - does the electric fan you have now ever come on?

If they do, someone has "modified" the wiring!

 

[DrEntropy]

no the fan does not come on. Now I just have to find where that thermostat switch is

There ~should~ be an Otter switch IN the radiator, at the back top starboard side. Round thing about the diameter of a US quarter-dollar coin. Two male spade connectors. Black wires connected, one goes to a ground point, the other to the fans.

 

[NutmegCT]

Note - there could be a reason that green power cable isn't connected. Frozen motor causes blown fuse, etc.

Just sayin' ...

 

[Svcmangler]

yes I have that switch and it already has two green wires coming from it. Could this mystery wire plug into the fuse box somehow?

 

[Svcmangler]

no the fan does work if i short it out

 

[DrEntropy]

no the fan does work if i short it out

Short it how? At the switch? If that's the case, likely the switch is bad. But those green wires to it must have been "added" along the way. There should be NO voltage through the switch, BTW. It should strictly be a grounding device. Voltage should be supplied to the fans with your errant green wire. Have you checked to see if the power is being routed thru the switch? A V.O.M. would be a good thing to have. A 12V test light would work, too.

yes I have that switch and it already has two green wires coming from it. Could this mystery wire plug into the fuse box somehow?

ABSOLUTELY NOT!!! That ~IS~ the fan's "hot" lead. As stated above, the Otter switch is a grounding device. Running +12V thru it is wrong. If the switch body contacts a ground (like if the rubber retainer had a spot where it has been rubbed thru) you could have a dead short. It's a thermocouple device. Bi-metal strip inside.

 

[Svcmangler]

is this what you call the otter switch?

 

[Svcmangler]

Yes I shorted it out at the switch, and the fan turned on. You might be right about the switch needing to be replaced. However that still doesn't help with the mystery wire. Would it go to the fuse block? Here is a pic of mine

 

[Svcmangler]

If that is the fans "hot lead" then where does it plug into? If you look at my switch on the fan, it already has two wires coming from it. There is no other place to plug this wire into. Not that I can see anyway

 

[NutmegCT]

SVC - always possible that the mystery wire fed a *different* device, or maybe an earlier electric radiator fan?

 

[DrEntropy]

Someone has been "creative" in the past, from the looks of it. Can you put a test light on the wires at the switch one at a time to see if one of those is "hot"?

Looking at the diagram, you have it correctly wired, that errant green wire is now a mystery to me as well.

 

[NutmegCT]

Also, note that the second fuse down from the top of the block has no connection on the left side.

 

[Mickey Richaud]

If that is the fans "hot lead" then where does it plug into? If you look at my switch on the fan, it already has two wires coming from it. There is no other place to plug this wire into. Not that I can see anyway

Late to the party here, but I think you'll find this helpful - scroll down to the '78 model year diagram.

http://www.advanceautowire.com/mgb.pdf

Mickey