My 71 Cutlass Convertible is suddenly blowing the 20amp fuse for "turn signals and back-up lights". Im not even using them and it blows just as soon as you turn the ignition switch on. Any idea on where to start on this? TIA.

 

Do you own the 1971 CSM & do you have access to the color wiring diagram in the 1971 CSM? Begin by reviewing the wiring diagram to gain an understanding of the wiring for the 20A fuse. That's the 1st place to begin because you're going to need to trace the wiring.

1971 Original GM Oldsmobile CSM Wiring Diagram A Body "V-8"

 

You might (also) begin by reviewing the troubleshooting procedure in the 1971 CSM in the electrical (lighting) section. There is one or several charts. It's a good starting point to remove your bulb/lamp lens covers on each of your turn signal markers (front & rear) for any tell tale signs of blown up bulbs/lamps which could provide insight.

 

thanks gotta start somewhere

 

It shouldn't be too much of a big deal to diagnose/troubleshoot. It never hurts to ask this question though: What was the last thing you did w/ the car? Anything involving electrical? Change bulbs, change any wiring, horn relay distribution block, install anything new. Easy to bump something sometimes & create a ground short.

 

Did you do any engine work on the car recently? I had a 70 years ago that did the same thing. I had changed the pass side valve cover gasket, and when I put it back on, I got a wire in the engine compartment, under the cover and cut the insulation, causing a dead short. Not sure why there was a wire for the lites there, but it was, taped up wire and no problem.

 

If &/or when you get to the point of having to trace wires the easiest/quickest method is to use a circuit light tester.

 

Actually kind of yes. I installed a new set of under dash gages. The oil line and thermostat wire had to be run up to the front of the engine. I retaped the wiring harness somewhat in the process. I should undo that mess and make sure nothing is touching.

 

Well some success. Unbundled the wiring harness going across the engine and made sure nothing was exposed and touching metal. Fuse held, did not blow. So it must be something about those wires that is grounding out.

 

One of those wires that you had bundled together could have been chaffed to ground. Check at the firewall where they go through a hole in the sheet metal. It'd be a good idea to use wiring loom on them to prevent chaffing in the future.

 

I am skeptical that switched wire would have anything to do with the engine harness, it could only be possibly tied into the charging system. so take a good look at the wires going to the voltage regulator/alternator. I don't have a diagram so take it for what its worth.


Edit- I didn't see the link for the diagram. Looks lIke the TSC is on that curcuit which runs under the hood. That is starting to add up with your findings..

 

Yes, the TCS (not TSC but I know of what you refer to) wiring is suspect. I'll post several pictures - give me several minutes. I think it's best we simply try try to attempt to keep this w/in the realms of a (hopefully) simple electrical issue w/o burdening ourselves with the function of the TCS.
To the OP, this is an area where it would pay off for you to own the 1971 Chassis Service Manual (CSM). A used paperback OEM 1971 CSM is the bible for your car - check eBay for original used paperback editions.

 

Which engine do you have - 350 or 455? With or without AC?

 

The TCS (Transmission Control Solenoid) received several varying (different) wiring scenarios based upon engine (350 vs. 455), model year & installed component(s) e.g. A/C. The 1971 model year is modestly unique in its wiring schematic (from other years). Below is an image from a 1971 455 TCS w/ its ensuing wiring. Note there is a resistor on top of the TCS plug. It is this wire which 66Jetstar has mentioned might be suspect. You'll find (1) owners who maintain the TCS and (2) owners who have disconnected or deleted the TCS (entirely). Remaining w/in the context of a simple electrical issue, we'll just focus on the wiring itself. You can disconnect the wiring by removing the plug & resistor and there will be no ill effects associated w/ engine performance. However, since we can't validate what any PO has previously done with these wires (note there are two wires), we'll just move forward with "suspecting" this could be the culprit. Yes, it is tied to the 20A turn signal and backup lamps fuse. Albeit, it's a switched wire which becomes energized as soon as you turn the IGN to ON.

 

Its a 350 with AC. I do have the CSM although I will admit the wiring harness diagram is confusing to me, Im no electrician. And I see theres some discussion about TCS or TSC? Im not sure whats being referenced there,,,,,,

 

To remove the TCS from the electrical circuit requires nothing more than disconnecting the plug; but, it is for obvious reasons vitally important the resistor & plug is properly "wrapped" & isolated from any metal anywhere on the vehicle. Most simply remove the TCS wiring from the wiring loom, roll it up and attach it to the firewall (out of the way). Again, it should be obvious the resistor/plug is wrapped and isolated. Something to think about as well. No one knows the frustration which may have ensued with previous owners who may have tugged, pulled, yanked on these wires in vain attempts.

I guess the second order of business is to identify if you have the TCS installed on the vehicle or is it removed or the wires disconnected? Please address the previous question - 350 or 455 with or without A/C?

The TCS valve assembly is often referred to as a 'Christmas Tree':

 

OK, good. TY.

 

Historically it began as the TCS, it evolved into the DVCS as Oldsmobile engineers &/or writers began to describe the convoluted band-aid (Christmas Tree). I'll puke up a couple diagrams for you but again let's keep this w/in the context of hopefully a simple electrical issue rather a discussion of the TCS/DVCS (and get you back on the road).

 

These are the images you'll find in your 1971 CSM.

 

You have the exact same routing as I had on my 1971 CS convertible w/ AC. I completely eliminated the TCS/DVCS. But again, let's remain w/in the electrical issue. I'll post a wiring blow up for you, one second.

 

OK, this wire is no longer used and is simply taped off and tucked into the wiring harness. I did notice today the tape around the resistor had gotten brittle and had come off. It is entirely possible the resistor might have been contacting the intake manifold. IDK if that would have caused the problem but Im going to get some new split wire loom tubing and redo that whole section of wires going over the valve cover/intake manifold.

 

Note location and color of wiring for the TCS/DVCS is tied to the 20A fuse. It is energized at the fuse box when the IGN is switched to ON. Suspect (as 66Jetstar suggested) there may be an issue w/ these wires which lead to the TCS/DVCS. The wires are energized at the fuse panel>TCS/DVCS and then down to the transmission where there exists a plug which plugs into the side of the transmission case. Follow this wire along its path. It may be the wire causing your issue. You can also disconnect the wire from the transmission with no ill effects.

 

Good. Well, it appears you've at least "located" the suspected "area" of concern. Next, it's simple isolation of the wires themselves. There was some original fabric tape used by Oldsmobile on this resistor to help isolate it when it was connected/installed onto the top of the Christmas Tree (TCS/DVCS). In any & all cases nothing can be touching any metal (either the resistor or the wires themselves) if the TCS/DVCS wiring has been removed and no longer in a functional state. Just remember, that/those wires are "energized" at the fuse panel and must be isolated properly.

 

Yes, thank for the insight. Im going to double down on making sure these wires are isolated or removed.

 

Good luck to you! Many of us are rather fastidious regarding wiring, employing new wiring, isolating wiring, types of wire connectors, heat shrink rubber insulation, etc. Perform good repairs to build confidence you tackled the issue appropriately.