electrical issues

[steve1200]

i have an outstanding issue with the coach heater on our 1997 Chevy HR.

there aircon / demister keeps blowing the 20Amp fuse. i have discovered that when i replace the fuse and turn on the ignition the headlights come on and the fuse will blow in a few minutes without even using the aircon.
the headlights are exactly the same brightness and also will light if the beam is on high or low.

i have looked through the wiring diagram to try and find a common area to start my investigation without success.

have any of you heard or had this problem in the past ?

any help much appreciated

[Robi.1014]

Has this rig been previously owned, and its wiring altered by a previous owner? Based on this speculation, perhaps I can offer some ancillary thoughts ... and this might guide you during troubleshooting.

Headlamps:
Due to safety issues, headlamps are usually "fused" by a resetting circuit breaker whose source of power comes from battery positive. Why? Well, if the headlight circuit overloads, the breaker will trip, but after a short cool-down period (5 seconds or so), the breaker will power the lights again. Imagine driving down the road at night at 60 mph and losing the headlights. Ouch.

The 20A fuse that you are replacing: What is the description label on the fuse panel? When you write that "...the fuse will blow in a few minutes without even using the aircon" (and I'm assuming you mean the dash A/C), do you know that the A/C is fully "off". Example: There may be settings on your dash air where the compressor clutch (powered by 12VDC) might be powered ON even tho you think the A/C is off. Try removing the black 2-wire connector that attaches to the A/C compressor....and seeing if the fuse is OK or continues to blow. I've got other thoughts, but need some feedback from you.

[steve1200]

thanks for the reply.
I am learning slowly about the problem, heres the problems and what does and doesn't happen

the fuse is marked "gauges" on the fuse panel and according to the wiring diagram there are three customer connections.

when i replace the fuse its ok while the ignition is off.
when i switch the ignition to on but not start the motor the following works.

aerial warning light glows if the aerial is up.
air con fan will operate three speeds.
the headlights come on just as if the headlight switch has been turned on.

i understand the current draw for the headlights and running lights are more than 20 amps and the fuse will blow.

what i think i am looking for is something that is short circuiting the headlight switch when i put the fuse in and ignition on.

without the 20 amp fuse the head lights work fine using the park and headlight switch.

it would not surprise me if the previous owner had something changed (or DIY !!). some of the wiring i have already resolved were down to DIY wiring.

the steering column wiring is a mess and i need a few hours to try to tidy and check there are no faults there

i have read the manuals and read about the canadian daytime running lights system, so i am not sure if it could be that causing the problem

while i have been looking i have discovered the wipers are now only full speed the delay/intermittent wipe has stopped working !

thanks for any help finding this annoying little problem

[EngineerMike]

1) I'll assume this problem is new, and that you have owned the rig for a time w/out the problem.
2) sounds like short in the dash some place.
3) could be short between headlight switch & the 20A fused circuit, could be elsewhere in the dash. Dash shorts are a beeyatch to diagnose for the ordinary DIY'er (well, this DIY'er anyway) due to interconnectivity of grounds that can backfeed into other circuits.
4) headlights should NOT consume 20A, but headlights in connection w/other stuff could, easily if (as I suspect) you have a high resistance short to ground.

IIWMI'd start by unplugging the headlight circuit (sounds like this circuit runs correctly otherwise?) and see if it cures the fault. If so, the headlight circuit is probably central to the problem. If no cure, or if symptoms change, post your results for more feedback.

[Robi.1014]

Given that you may have a DIY accidental cross-connection on your hands, the following popped into my head as to how to use this condition to perform troubleshooting:

We know that your gauges and dash A/C should only have power while the ignition is on. We also know that the headlights are independent from ignition switch (you can turn headlights ON w/o the ignition switch being ON). Soooooo.... Due to the accidental cross-connection, you've got a condition where turning the headlights ON will also power the gauges (and any other device connected to the fuse labeled "Gauges").

So:
Remove the "Gauges" fuse from its socket. Attach the + lead of a voltmeter to this fuse socket and the negative lead to a known-good ground. [A paper clip may be used to poke into the fuse socket to make the connection.] Now, turn the ignition ON, and if you read 12VDC on the meter, I want you to move the + lead to the other end of the fuse socket....where you should read zero volts with the ignition ON (because the fuse has been removed). Turn the ignition OFF; but now turn the headlight switch ON. You'll read 12VDC now because of the cross-connection. When you find the cross-connection, the volt meter will stop reading 12VDC. "Finding" the cross-connection can include disconnecting sockets and/or home-brew (DIY) wiring. Via a process of elimination, the volt meter will guide you to the cross-connect.

[JimM68]

Hmmm, it kinda sounda like "daytime running ligghts" might have something to do with this...

[ahicks]

Originally Posted by JimM68

Hmmm, it kinda sounda like "daytime running ligghts" might have something to do with this...

If it's like mine, it does have day time running lights (unless they've been defeated for one reason or another?).

What doesn't work when the fuse blows? Just the AC fan?

[steve1200]

thanks for all the advice people, when i have some time to check and use these points pointed out i will get back to you