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Discussion Starter · #1 ·
Hello All, new the the boards and first time post.

I finally got my cherry 87 Spider Veloce back on the road after a 5 year 'time out' in the garage. After a good tune up etc, it's running great except for some electrical gremlins specifically with the reverse (back up lights). Here's how far I've gotten and checked thus far.

- All new fuses (blade type)
- New 1156 (21w) bulbs in L/R Break Lights, center break light, L/R Reverse Lights, L/R Fog Lights.
- New smaller round globe (5w) bulbs (105/67/97 forgot which) in L/R rear parking lights.
- Checked and cleaned ALL the grounds, connections and bulb sockets.
- Checked the back up switch in the bell housing and wires coming from. Seem fine, the switch operates 'on/off' when put into reverse and the wires do not show sign of short.

Now hear's where it gets wacky. All lights, directionals, hazards, etc in the back work fine except for the reverse lights.

When I take both reverse light bulbs out of the sockets, turn the key all the way on and check with VOM it shows 12v on both left and right. I plug in a (working) bulb and nothing. Leaving the bulbs in I check with VOM and no voltage. I take out the bulbs, check with VOM and 12 volts are back! After this back n forth I took out both of the sockets and again cleaned and checked them thoroughly thinking there was a short but they are fine.

The only other electrical issue (that I'm aware) is a malfunctioning clock but I would not think the two are related.

Is there a 'load' issue that can be causing the strange behavior in the reverse lights or any other thoughts would be appreciated. Thank you

Billy
 

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not sure when you say you checked the back up switch in the bellhousing, and it was OK?:
did you remove the 2 wire connection from the back up switch on the side of the tranny and join them together (ign. on)?

might have misunderstood you, but that is what I did as mine also do not work and my lights lit up......meaning, switch is bad & I need to remove the tranny to replace a 20$ switch!!
 

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Well, start by making sure you have a good ground from the socket.

If that's not it, it's quite possible you've got a high resistance in the circuit somewhere else. If you can get to the connector to the reverse switch, try measuring the ohms across that with the switch closed.

You can also check the whole circuit (note that I'm assuming the power feed is +12 through the switch to the socket like in my '63, here). Disconnect the negative battery cable to be safe. Then put the car in reverse to close the switch and also turn the key to "on". Measure resistance from the positive terminal in the socket to the positive battery terminal: should have continuity and <1 ohm or so.

Be sure to turn the key back to off before reconnecting the battery.
 

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You are likely experiencing the Heisenberg uncertainty principle in action which says “we can never really know anything” because when we try to measure it we change it.

Anyway the input impedance of a DMM is extremely high several megohms (millions of ohms) while the resistance of a bulb is relatively low a few ohms when actually producing light and basically zero until the filament gets hot enough to light up.

With the meter, what ever loose connection is causing the problem has very little voltage drop because there is basically no current flowing, as soon as you put the bulb in current flows and the voltage drops across the bad connection.

Check the grounds and you may want to rig a bulb on on a long piece of wire to ground or +12v ( sorry haven't looked at the diagram for the switch) so that you can make sure the switch is actually making a good enough connection for the current to flow.

Lest you think I'm completely talking out my butt, I spent most of my life as an electronics technician in the U.S. Navy.
 

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Most likely a faulty ground. A voltmeter is not a particularly appropriate tool for testing lamp circuits such as this, a more useful tool is a test light which will verify the integrity of the circuit with a load on it. Even so you don't want to go from the positive terminal of the socket directly to ground as you are then bypassing the actual ground connection of the lamp. You want to test from the positive terminal at the base of the socket to the socket barrel, which is where the ground lead is connected.
 

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Make sure you didn't push the ground terminal tang out of the socket when you change to bulb. The bulb sockets are open back so if the bulb catches the ground tang thats on the side of the socket it will push it out the bottom.
 

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Trained (ex)Professional, , 1953-2018 RIP
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Leaving the bulbs in I check with VOM and no voltage. I take out the bulbs, check with VOM and 12 volts are back!
Yep. Classic example of high circuit resistance/faulty connection as Gubi said. The dead giveaway is no-load volts are good, loaded volts are not.
 

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My personal poor-man's ground tester is a single filament turn-signal bulb with a couple of very long pieces of wire soldered to it; one (I like red) to the positive tip of the bulb and the other (any color but red) to the side of the bulb base. Wrap a little electrical tape around it and go. This lets me check the ground against a known-good ground point, and alternately check the positive against a known good source. Alligator clips are a nice addition to the tool.

Yes, I have a genuine Harbor Freight Voltmeter or two, but for this kind of stuff the 'Bright, Dim, Nuthin' test is good enough. I save the Voltmeter for searching for 11.9 volts et al.
 

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Discussion Starter · #11 ·
A big thank you all for the insight and advice. The 'Voltage drop' scenario describes my issue to the tee. I will test that and keep hunting the 'iffy' grounds.

And, I will make a test bulb/light and stop falling for the misleading meter numbers. It's also good advice about sliding the bulb into the socket and not crimping the ground tabs but I was careful with those.

Thank you again,

Billy
87 Spider Veloce
 

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From a lot of past experience with faulty front and rear lamps I always go for the "secure ground" solution first. Without a really good ground you can get all kinds of "impossible" meter readiings and quirky light problems. I am never above adding additional ground wires to problem lighting circuits. After all, ground is ground and ground is good. You can have too little ground but never too much ground.

Robert
 

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I can tell you of a recent experience I had where my '87 had a faulty brake light. Son was driving and when he stepped on the brake the clock would go off and left hand brake light would not work. We chased everything,changed bulbs checked voltage and fuses. Then decided to review papajam's schematic it was invaluable. Turned out that the common black wire that runs from socket to socket within the loom had only a few strands of wire left where it was crimped to the brake light socket. by wiggling each wire along the circuit we found the light would flicker. It broke as we were checking it and after stripping and soldering it all is well, we did have to replace #9 fuse as I believe I grounded one of the sockets as I was swapping bulbs. The convoluted electrical path and grounds of our cars can drive you nuts. Just review the schematic understand the circuit your dealing with and pick a starting point and work through it you will figure it out.
 

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Once you've eliminated all the grounding issues my experience was that the switch in the bell housing had high resistance and there wasn't enough current to light the bulbs. I wired in a relay so that the feed from the reverse switch energized a NO relay and with clean 12V through the relay to the bulbs everything works nicely.
 

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The 'Voltage drop' scenario describes my issue to the tee. I will test that and keep hunting the 'iffy' grounds.
Diagnose first.
Repair second.
No need to hunt for iffy grounds if it isn't the problem. Test for it. Voltmeter probes to the + and - terminals in the bulb socket. Load the circuit (install a bulb in the second socket) to confirm voltage drop. Now move the voltmeter ground probe to a good chassis ground and load the circuit again. If the voltage drops, the problem is the power circuit. If it doesn't, it's the ground circuit.
 

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Discussion Starter · #17 ·
Again I thank you all for your advice and encouragement, you guys ROC ! 'I'm going in',, and hunting for the bad connection/voltage drop. I'll let you know where the issue was and hopefully it can and will help someone else. Cheers

Billy
87' Spider Veloce
 
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