Strange clock related Voltage drop

[Matt Wild]

When I got Winnie, he had a habit of draining the battery overnight.

I found a voltage drop of 2 Volts due to the rear defogger relay, I have a replacement on the way.

I have traced the other 10V to the clock, at least when I remove it from the system the drain disappears.

Now get this, it is a 47000 mile car and the clock works really well, and all the dials look in good condition.

So the question is Any ideas why the clock would cause such a large voltage drain?

Any similar problems and solutions or useful clock fixers in Texas?

Clock part number is 214/13/2 manufactured May 1976

Matt

[Matt Wild]

Am i being sensible here or just stupid? The clock is always going to draw current as it is hard wired through an 8 amp fuse.

At present it seems to draw 0.1 Amps and obviously a high resistance combines to give a voltage drop of about 10 Volts..

Any smart sparkies out there?

matt

[aus911]

Matt, I don't understand where you are measuring the 10 volt drop...??? please explain more

Ray

[Jim Sims]

Vehicle electrical components are usually not in series but in parallel so the voltage drop across any given component should nominally be 12 volts. Look for a direct short (with some contact resistance) in the clock circuit (not in the clock itself): dome light wiring in the roof or perhaps the front trunk light switch as the cause of the current drain if the drain has been traced and isolated to the circuit containing the clock.

[Matt Wild]

Aus911, Jim,

Thanks for the response, this isn't my strong point by any means.

I disconnected the earth cable to the battery and hooked up my volt meter between the clamp and the earth terminal, the voltage drop across my meter was close to 12volts. The car would discharge the battery overnight at this point.

I started pulling fuses to see which circuit was responsible. Pulling the 25Amp fuse for the redefogger accounted for 2V and this has been attributed to a sticking relay allowing a drain thorugh the rear window defogger. The other 10V seems to be accounted for by the clock/interior lighting circuit. I have disconnected the various interior/glovebox/trunk lights with no effect. So I pulled the clock and disconnected that. The voltage drain of 10V then disappears.

I presently have the clock wired through an 8 Amp fuse to the battery with the rest of the cars electrics disconnected. If this drains the battery then I know it is something to do with the clock. I hope!

opinions, suggestions ? Matt

[Early_S_Man]

Matt,

What year and model car are you dealing with?

I doubt that the clock is the problem, unless there is a short in the 'multiple' wiring harness that distributes +12 Volts to the instruments!

Is the rear defogger relay a two-stage or single-stage version? The problem may be in the socket for the relay, such as corrosion or melting of the socket body causing a short between terminals.

Your desciption of the problem as a Voltage drop problem with two independent circuits seems inappropriate. You can only measure Voltage drops around the individual circuit loops ... when you have multiple, independent circuits, they have to be treated separately, and thus Voltage drops in one circuit can't be combined or compared with the other circuit.

I assume you have a digital multimeter. What you should do is measure the total current drain inline with the ground or positive terminal of the battery with the ignition switch off. Total drains less than 0.060 - 0.100 Amp [60 - 100 mA] don't cause a battery to drain over a week or more, much less overnight!

[Matt Wild]

Warren

It is a 76 912E, 47000 miles from new but has sat for the past two years for some time outside.

The defogger switch/relay is two stage and I could measure a voltage on both the input and output terminals of the first stage. There should be no output until the switch is turned on. The socket and connectors seem in good bright condition.

After identifying that, I disconnected the +ve terminal on the battery and connected the earth. I then looked for a voltage draw on each of the 5 +ve power cables. i.e starter lead etc. Only one showed a reading, the one that powers the clock and interior lights etc. I now have only this +ve lead connected to the battery and have been measuring voltage draw between earth terminal and earth strap as described previously, in other words hopefully dealing with one circuit.

I discounted the various interior lighting circuits in turn by removing them from the circuit. Only disconnecting the clock caused the voltage drop to go away.

Does that make sense? Matt

[Wil Ferch]

Voltage is nothing more than the "potential" energy applied....it should e 12.6 volts for a fully charged battery everywhere you measure independent, parallel circuits.

Now amps..or milli-amps ( thousandths of an amp) is another matter. That is "current flow" and it is unusually high amp draw that will drain a battery. Saying you found 2 volts here and 10 volts there , some of which were "lost" or "found" as you did ...makes no sense at all.

The voltage is the potential and should be the same everyehere.

BTW...the clock will draw some "standby" amps as it runs all the time. For reference, the spec standby draw for a mid -80's Carrera is 16mA..and the standby draw for the heavily electrified 964 series is about 64 mA( milli-amps).

- Wil

[Early_S_Man]

Matt,

When measuring the drain inline with the positive battery terminal, the meter is set to a DC current range such as 10 Amps, right? So, let's refer to the values in Amps or mA, not Volts, OK? The only way I would accept a 10 Volt reading at the clock as a valid Voltage drop reading is if you measured 10.0 Volts from the clock + terminal to ground and 2.xx Volts from the clock positive terminal to the battery + terminal! All of the Voltage drops in a single series circuit are additive, and MUST add up to the source Voltage at the battery.

Here are two threads with details of the two-stage defroster relay circuit:

2 Stage Defogger Switch - Wiring

Rear Defroster Relay Repair

[dad911]

As stated above, you need to measure current draw, not voltage.

It may be simpler for you to disconnect the circuits you feel are causing the problem, and see if the battery still drains overnight.