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MG MGB GT V8 Factory Originals Technical - VDO Gauge Wiring Problems

Hi

I have installed my VDO gauges. And I am having some difficulties and hoping some one could help.

When I installed the VDO tach I used the existing tach green wire in my 74.5 MGB.

Now just turning the ignition on causes the tach to jump.

If the wipers are on when I turn the ignition on, the tach goes up to @1500 (and stays there) with just the ignition on.

Looking at the information provided at Paul’s’ site (The Pages Of Bee & Vee) “Wire Colours Fuses Terminal Numbering” I can see why. Almost all the gauges run via the green switched wire.

Basically how can I isolate the tach and still have a fused switched wire? I do have an upgraded fuse box with several more slots available

Can I tie into the turn signal system and get my switched fused power from there without creating the 'surge? Or, is it anything to even worry about if the gauge stabilities after the engine starts?

I did try running a different ground and that didn’t make a difference.
And the gauges are still wired through the voltage stabilizer

Thanks

Bruce
Bruce Mills

"And the gauges are still wired through the voltage stabilizer"

Don't do that. VDO gauges don't need the stabilzer, in fact, they will do funny things if you use it. Wire the gauges directly to the "green" wire circuit.
Dan Masters

Ok, thanks Dan. I will pull the wires to the voltage stabilizer tomorrow after work and see how I get on.

Thanks

Bruce
Bruce Mills

Hi

Here are the results of my test tonight

I am using the Fig 10:31 Typical Wiring Diagram 1973-74 N American Models. At the instrument voltage stabilizer I have 2 joined green wires and one LGG

1) With the ignition on and the wire disconnected:
I turn the heater on the tach jumps up and then back to zero.
I turn the wiper on (high) and the tach jumps up to 1400 and bounces between that and @1200 corresponding with the sweep of the wiper.
I have no gas gauge or temperature gauge.

2) With ignition on, G wire disconnected LGG connected:
Same as #1

3) With ignition on LGG and G connected together:
Same as #1 but I have gas gauge and temperature gauge.

4) With the car running and the wire connected to the instrument voltage stabilizer:
Turning on the wipers or heat produces no fluctuation with the tach needle.

5) With the car running and the G wire disconnected from the voltage stabilizer:
The fuel gauge and temperature gauge don’t work but there is no fluctuation in the tach needle when the wiper or the heater is turned on.

6) With the car running and the G wire and LGG wire joined together:
There is no fluctuation in the tach needle when the wiper or the heater is turned on.

Installing the VDO gauges wasn't that hard. All the wiring is there for the temperature gauge (I used the required sender) as well as the tach and gas gauge. The only additional wiring is adding extra RW & B wires for the dual light bulbs. Simple enough.

I didn't have a problem with the Smith gauges I wonder if the additional bulbs are drawing too much current and causing this?

Bruce
Bruce Mills

Bruce,

I don't know if this will help at all, but I had a bouncy tach issue a few weeks back, with an orginal 72 gauge.

Problem was :

1. I had it wired into the Black/White circuit (should have been white)

2. I accidentally left a temp 2v supply to the coil + in place when it actually got all it needed from the white tach/ignition circuit. Poor tach was trying to deal with v's that were all over the place.

Wouldn't recommend wiring it threough the white ignition-coil circuit unles you know what you are doing though!

Liam
Liam H

This thread was discussed between 07/05/2007 and 08/05/2007

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