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Most electrics only work reliably with engine running (& lights off)!?


jlb
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To set up the scene, my m20 swapped '74tii has lots of electrical quirks, which i'm steadily trying to work through. Some of the quirks are due to old/worn out components and switches - such as inop. turn signals and dash cluster lights, and others are due to the previous owner's "unique" approach to the m20 swap, which is a source of constant frustration so far. 

 

Anyway, a consistent problem i've had during my ownership (only 6 months or so) is that the electrical system operation is either extremely weak or non-functioning unless the engine is on and running, at which point things work much better. For example, my turn signals will either not function at all, or blink once or twice and then quit while the ignition key is in the 'on' position, but will come to life just fine once the engine is running. However, once the engine is running and the lights are turned on (lows or highs), the turn signals will once again refuse to operate. There are other examples as well, but this is a particularly annoying one, since I can either have lights or indicators, but not both. Luckily I don't drive often in the dark, but i'd like to solve the problem. Another example is that the dash cluster lights will not illuminate with the engine off, but will come on just fine with the engine running and lights on.

 

The engine-off issues suggest battery charging issues, but the battery is known to be good, and reads right around 12V when the engine is off. It reads somewhere above 13V when the car is running, suggesting that the alternator is doing its job at least partially.

 

Any ideas where to start troubleshooting?

 

Thanks!

 

 

 

 

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12 hours ago, jlb said:

The engine-off issues suggest battery charging issues, but the battery is known to be good, and reads right around 12V when the engine is off. It reads somewhere above 13V when the car is running, suggesting that the alternator is doing its job at least partially.

 

Any ideas where to start troubleshooting?

 

Thanks!

How is the battery known to be good? Has it been load tested? How old is it? Does it ever read below 12 volts while just sitting? If it does it's not charged fully or NG. If it's new it's probably okay, but not necessarily. I've seen batteries defective right off the shelf. Can't recall how many times I've had clients come in with wierd electrical problems and exclaim it can't be the battery and then I find the problem is the battery. I would start there, then go looking for poor grounds. Alternator output should be 13.5 - 14 volts. If you have a decent battery charger there is an easy way to tell if it's the battery. Simply connect it and see if the problems go away with it still on, engine off of course.

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13 hours ago, jlb said:

the battery is known to be good, and reads right around 12V

 

 

58 minutes ago, torquewrench80 said:

Has it been load tested? How old is it? Does it ever read below 12 volts while just sitting?

 

 

12V is low.  Should be more like 12.6 and it does make a big difference.

 

Here's a generic battery discharged state diagram.

Battery Discharged States.png

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  • 4 weeks later...

Thanks all for the help thus far!

 

I spent some time this weekend tracking down all the grounds and cleaning them up. I do have a ground strap from the engine block to the frame, and the rest appeared to be in reasonable places and not corroding. The caveat to all this is that it's an M20 swap car, and soome of the grounds have been moved or replaced as a result, like the main ground from the battery that's grounded to metal on the inner rear fender as a result of the battery relocation.

 

The PO installed several gauges, one of which is battery voltage. That gauge reads just under 12V resting, and right around 14V when the engine is on. I will plug in a battery tender and see if that 12v raises up at all.

 

In the meantime, as an unfortunate side effect of cleaning up all the grounds (and adding one to the instrument cluster), fuse #12 now blows out as soon as I start the engine. So, same electrical gremlins remain plus the added 'perk' if having zero instrument cluster function at all. Ugh.

 

I just ordered a bunch of spare fuses to help troubleshoot. Is there any way you isolate the shorted component without blowing a bunch of fuses along the way, i.e. using a multimeter to measure across the fuse #12 while no fuse is installed? There's a lot of stuff on that circuit, and even disconnecting one item at a time is likely to result in a while bunch of blown fuses before I find the culprit. There must be a more elegant way!?

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On 10/16/2017 at 6:48 AM, halboyles said:

12V is low.  Should be more like 12.6 and it does make a big difference.

 

 

I've had my battery connected to a charger for a little while, and it's reading 12.7v after charging up. Problems persist after doing so, unfortunately. Also, the battery is practically brand new, as the PO purchased it specifically to help combat these electric issues that i'm describing in the original post. So I think the battery itself is fine. But, i'll try a different battery at some point as well.

 

Interestingly enough, the voltage gauge the PO installed reads ~11.7v even when the battery itself reads 12.7v. So that's bizarre. Also, the voltage reading on the gauge drops from 11.7v to about 11.3v when the ignition is in accessory mode and the lights are turned on. 

 

Any ideas why there would be such a voltage drop between the battery and the voltage gauge? I removed, cleaned and re-installed the ground screw connecting to those gauges, and no change. 

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The volt drop can be caused by inadequate gauge wiring between the volt meter and where ever it is measuring the voltage from. It could also be inadequate wiring between the battery and wherever the voltage is taken from. A poor connection anywhere in circuit can also do this. 

 

The volt meter can can also be out of calibration as well. 

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