Strange Voltage Drop

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Strange Voltage Drop

Postby Simon_P » Wed 12.06.2019, 12:36

Hi All,

I'm tackling the electrical issues (and the alarm!) and interesting wiring on the Blue Car. Following up on the post regarding the voltage stabiliser. I checked the output and it was a steady 10v. Thanks to Geoff who gave me a spare VR at CC just in case.

So here is the problem.
The Volt gauge reads just under 14v - within the bounds of "normal" and I'm not worried as when I checked with elanscan it showed 14.2v.
I can switch the headlights on and there is no difference.
If I turn the interior fan to position 1 the oil pressure gauge flicks slightly and the voltage gauge drops noticeably, position 2 sees an additional drop, position 3 and it is showing under 13v, but when I switch to position 4 it goes back up to normal.

Any ideas?

From the wiring diagram the interior fan speed is switched ground connected to 2. the pedalbox which appears to be secure. I'm going to take the fascia out again so will be able to check in more detail later.
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Re: Strange Voltage Drop

Postby Bern » Wed 12.06.2019, 14:25

In order for the fan to run at lower speeds I'd imagine there are resistors, when in position 4 you are giving it the full 12V without any resistors.

Perhaps?
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Re: Strange Voltage Drop

Postby Brit-Car-Nut » Wed 12.06.2019, 14:34

Simon,

If I am reading the wiring diagrams correctly, the dash gauges AND heater get their grounding at position #2 (at the pedal box) but the pedal box gets its ground from a cable/wire at the "braid" connection at the RH Longeron. I believe if you are getting a fluctuation, the heater is not getting a good ground and the gauges are showing it is probably around ground point #2 or the connecting wire/cable.
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Re: Strange Voltage Drop

Postby GeoffSmith » Wed 12.06.2019, 14:41

Connections to the pedal box ground (2) would seem to be the culprit - I suspect that a large current elevates the ground connection to the voltmeter so you read less volts with more load. The fan full speed relay (position 4) may be separate from the fan switch / instrument pack connection and not therefore not be associated with the voltmeter. Windows draw a big current (especially on start up) and are also grounded via (2) so check them out first as a quick check.

[EDIT] Common connection is IPHC pin C4.
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Re: Strange Voltage Drop

Postby Simon_P » Wed 12.06.2019, 17:03

Bern, Perhaps, but the load increases as the resistance reduces so position 4 shouldn't get better.

John, I had assumed wrongly, while upside down with my head in the footwell that the big black cable was to the battery - will check in the engine bay - the earth strap to the engine was loose so it wouldn't surprise me if this is also.

Geoff, I had to read that twice! will get the meter out and check. I noticed (because I'm doing a fan override switch) that there is a ground connection by the relays in the binnacle which isn't shown could be for the fan relay switched ground.

I did try various loads but it was only the fan that made a difference to the volt meter. I will try the window switch again, not least because some of the interesting wiring is to the window switch.

It's cheered up outside so I'm off to have a play!
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Re: Strange Voltage Drop

Postby Bern » Wed 12.06.2019, 20:17

It's been quite a few years since I did any electronics, but.....

The load, in this case, is resistance?

So add more resistance you increase the load. That's assuming that the resistors are in series with the motor.

Not sure that I'm thinking about that properly though.......
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Re: Strange Voltage Drop

Postby Brit-Car-Nut » Wed 12.06.2019, 21:24

This is that portion of the circuit ----- The resistors are in series

Heater Circuit.jpg
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Re: Strange Voltage Drop

Postby Simon_P » Thu 13.06.2019, 20:22

Bern the load is a resistor and the fan in series. As the switch is moved the resistance is lower.

John, the #2 ground cable checked out OK.

I had the fascia out and checked the wiring, discovering that the ground for the stereo is attached under the wingnut at the side ie nicely insulated - that explains why the radio only works when the aerial is connected. I checked the wiring from the fan switch and Instrument cluster which share the same ground wire at IPHC4 (as Geoff mentioned), scratched the connector and plugged it back in - Much better.

The connector is accessible from the top so I could have avoided a few hours of dismantling and reassembly, but I was able to check the wiring and change a light bulb.

The fan switch now has an insignificant affect on the voltage gauge - one less problem - Thanks all.
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