Electrical problem with 87 300SDL
 

Have shorted circuit on fuse 15. This is one that has antenna, dome lights, and courtesy lights. Thought problem was with alarm system. Unplugged module with no change. Only clues to this mystery is the seatbelt warning light flashes continually when power is on. Have found light flashing with power off and doors closed. The switch for the map light/courtesy light doesn't work correctly. In one position the light comes on. Another it is off. in still another the passenger side light comes on. Have also unplugged antenna motor thinking there maybe a short there. No change. Have manual on CD. There is a anti thief module in the circuit that that appears to effect the circuit. Have no idea where it is located or how to test. Any suggestions?

Been there, done that. The problem is a burnt trace in the dome light circuit. You can either disassemble the dome light and repair the trace, or replace the unit.

I hope the repair is that simple. I can check that tomorrow. What is trick to remove the light and switch without ruining the headliner?

I'm not following your test across fuse #15? Naturally, the fuse will always test at battery voltage.......whether you have a current draw from the device......or not.

What you need to check is the current draw across that circuit with the light installed........and then with it removed. If all items in the circuit are off, the current draw should be zero.

With the fuse removed, The red probe (+) of the meter touches the bottom fuse contact point. The black probe (-) contacting the upper (spring) contact point, The meter reading was about 13.25 volts. The battery tested same voltage at cable post.

In retrospect I should have checked the continuity and resistance between the spring contact and a good ground for a more accurate reading.

I think you may have found your problem.

In the scenario you describe, you would read full battery voltage only if the upper contact point is at ground potential. So that means that when you insert the fuse there is a direct path to ground (a short). Pop goes the fuse!

One of the circuits that this fuse feeds has a short to ground.

Yes. That was my thoughts. I didn't put a fuse in to watch it go poof but, that would have been my prediction. I did carefully use a probe to momentary make contact between the fuse contact points. I got a small arc.

Although you clearly have a device that consumes some power downstream of the fuse holder, and hence the voltage potential, the amount is the question.

Is is a dead short where the amount is infinite, or is it a clock or a radio that consumes next to nothing?

A check with an ammeter would tell the tale. Don't keep it in place very long..........

If you still suspect a dead short, it's safer to blow another fuse rather than damage the ammeter.

Well, we know this much: The current draw is in excess of the fuse rating.

Go back to the trunk and disconnect the antenna from the power supply.

Check for the voltage again at the fuse or install another fuse for a test.

I'll do that tomorrow and check resistance to ground at the spring contact point to confirm the result. I have been using an analog meter so far. Tomorrow I bring out the digital meter for a more accurate reading. Thanks for the suggestion. Robert

I've never used a digital meter. If you're not very familiar with them, they'll typically confuse the hell out of you. This is especially true if they are not in the proper range.

Others have suffered the fate of the digital meter many times on here. Use it only if you're positive of it's displays.

My main diagnostic tool is the $10. pocket meter from Radio Shack. It's over 10 years old and has never failed to confirm or deny a theory. About the only thing it can't do is to test the glow plug resistance accurately. Then, I drag out the $50. analog meter.

Digital meters can be confusing. My analog meter i was using was an old inaccurate Amprobe my father gave me 20 years ago. He was Mercedes mechanic in his day. This getting off the subject but, driving his Mercedes convinced me I must have one. A will confirm the result with the analog meter.