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Headlights on dims instruments lights


Lee McDowell
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When my headlights are turned on my instruments change there readings.  Example are the voltmeter will go from 13.75 volts to 11.75 volts, temperature will go from normal to abound 220 degrees and the other instruments will change also.  I have changed the headlight switched.  The instruments go brighter along with the warning lights.  I suspect  a ground problem and would like to correct it.  I have had the dash cover of while addressing an other issue (loose outside driver side mirror) so removing the dash cover is not a problem if I have to do so.  I would like get some help/direction on correcting the issue. I have not been able to locate an electrical drawing for the coach.

Lee McDowell

1998 Diplomat

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I can't help with an electrical drawing but can help you identify the issue weather it is low voltage or a bad ground.

You need a voltmeter and a known good ground. Oh sure you say, that may be my problem. So you must get scratch a good ground on some metal place. A dremel with a sanding drum works well. I have actually places some connectors in a couple of places in my RV and have a long wire to plug into those ground spots. Once you have a nice shiny spot and can put a metal screw into a predrilled hole or use  self tapper you can attach a scrap piece of wire to it and then strip and wrap that around your negative voltmeter lead.

The goal here is to avoid getting fooled or in taking faulty measurements:: With this settled you can move on to taking only two measurements at first.

Find one of the guages or devices that is going wanky and figure out how to measure the hot wire going into it and the ground side.

Sharpening your positive meter probe on a bench grinder to a needle point is a good way to let blood out of your fingers but sure ensures you will get a good measurement probing wires. Tape up most of the positive probe so only a small tip shows. You don't want to shorting things as you work.

Only two outcomes from these measurements. IF you have close to battery voltage on the HOT side of the instrument or bulb you are measuring and the device is reading wanky then I would bet you will find some voltage on the ground side of the device.  This means you have a bad ground.

If the voltage is low on the input side then time to chase that. 

If your ground is the culprit you can use the nice temporary ground you just made to prove it. Skin the insulation back on the device being tested (ground side), and tie your jumper to that. 

Conversely you can substitute your own hot lead but must take great care doing so. I have a wire with an inline fuse to do just that. I can put a fuse in that is just enough to do the job. Don't want to cram a 40 amp fuse in something that is only going to draw a couple of amps. 

I hope this is clearer than mud.

First post from me on this new forum platform.

Myron T.

08 Endeavor

 

 

 

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3 hours ago, dl_racing427 said:

Myron,  

As a longtime electronics tech, I can confirm that that's a very logical and effective troubleshooting method. 

Appreciate it. Over 50 yrs as a tech and managed and owned electronic service centers. Senior Certified Electronic Tech. FCC 1st class ....

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This is a known problem that has never been solved. I have a 2000 Diplomat that does the same thing since it was new.  i ignore it as these are false readings probably caused by low voltage caused by the load that the lights put o the 12 volts. You could rewire the headlights using a relay and pick up 12 v somewhere else. I chose to ignore.

Bob U  2000 Dip

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If I understand right you have the same issue.  How long do you operate while driving with the headlights on.  I have been concerned about overheating the circuit and tripping a breaker or blowing a fuse.  Also, have you had the problem looked at by a rv tech?  Another issued concerns me is draining the battery while driving.  Thanks. 

Lee M

’98 Diplomat 

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This is a well known problem that existed 20 years or more ago. When the voltage drops it will affect the gauges on the dash. I have not ever heard of any problems of blowing a fuse. These circuits use 12v from the chassis batteries which are charged from the alternator . If you check the 12v going to the gauges you will see a drop when you turn on headlights. Rather than rewiring the headlights via a relay using another source for 12v chassis I chose to ignore it. No problems in 20 years. If you do rewire it the headlights will be brighter.

 

Bob U

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Thank you for your reply.  I just did not want to damage any electrical  wiring from drawing to much current.  Worst yet would be to loose my lights while driving at night or inclement weather.  I think at this I am going to do as you do.  I may try driving with lights on during the daylight hours as an experiment. 

Lee McDowell

98 Diplomat 38A

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