Odd electrical issue

'89, 4.0, AW4, 110K. Today, a strange thing started happening. I noticed when I have my turn signal on, the dash clock backlight would turn off and on, in opposite to the signal light. As in, when signal light on, clock off and vice versa. The turn signals are lighting up on all four corners, and flashing at the normal rate.

Also noticed the clock backlight turning off when I'd hit the brakes - backlight would come on when I let off the brakes. When the brakes would "cause" the clock to turn off, I could hear a distinct click sound from somewhere near the pedals when pushing the pushing the brake pedal down. No click sound if the clock was unaffected. Brake lights also functioning normally.

Didn't happen every time during my trips today, maybe half of the turn signal/brake uses would cause the clock light to go off and on. Volt gauge stayed at a steady 13 or so. No issues with radio or fan, and no change in how the engine runs.

Any ideas on where to start looking for the issue?

This smells a lot like a ground issue. Know that RENIX (87-90) are extremely susceptible to ground issues. Here is more on it. REFRESH your grounds!

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Grounds can be the root cause of many electrical gremlins. Refreshing grounds is NEVER a bad idea!

You can't just check or look at these grounds. You must remove, scrape, clean until shiny the cable/wire ends and whatever they bolt to. The one on the back corner of the head, and where it attaches to the firewall. Scrape the paint from the firewall. Best to replace that woven cable with a #4 or #2 gauge cable. You can attach the one end to the intake manifold if you would like.

Next go over to the engine dipstick tube stud. Remove the nut and clean the wire ends and scrape the block until shiny at the stud. Reattach tightly.

If you are so inclined, add at least a #6 cable from the negative terminal of your battery to one of the bolts on your radiator support. Scrape the paint off first.

First off, do what Birchlake advises. Refresh all your grounds in the engine bay. It needs that done anyway. Secondly, up under the driver side dash, attached to a black metal brace with a screw, is the ground for your instrument panel lights. Remove screw, scrape, and reattach. I add another ground wire from under that screw to the driver side kick panel when I do it.

check the grounds in the sockets themselves. When the running lights don't ground properly somehow they end up grounding through the cluster.
Mine would light up both turn signal indicators solid whenever the parking lamps were on. Cleaned the sockets/grounds in the front turn signals and fixed.

87manche said:

check the grounds in the sockets themselves. When the running lights don't ground properly somehow they end up grounding through the cluster.
Mine would light up both turn signal indicators solid whenever the parking lamps were on. Cleaned the sockets/grounds in the front turn signals and fixed.

Click to expand...

Yeah, do that, too.

Thanks for the tips, I'll get to work checking/cleaning up the various grounds you all mentioned.