marlinspike

Well the wiring to the relay comes straight off the battery, but the shop that installed these used seriously narrow wire going from the battery to the relay. Going off memory, something like 20ga.

Apparently each one draws 15amps.

And yes, honking the horns will dim the lights. I think if I hold it longer the lights come back up, but at least for a moment the lights dim.

Also, I think the relay might say 30 amps on it....'

The wire from the battery to the relay isn't that long...basically 2/3 of the length of the fender.

subaruboy

If honking the horn dims your lights, I think your answer is there... regardless whether or not the wire is proper or not, your alternator/battery configuration isn't capable of handling the load from the compressors. When you're pushing your horn and the relay flips, the draw is causing the voltage to droop. You need a stouter battery, or a capacitor like the stereo guys run, to handle the load.

The reason the lights dim and come back up is there is an instantaneous drop, then it recovers, to put in laymen terms. A capacitor would get rid of that instantaneous drop. But even then, the AC puts such a load on the system, a capacitor might not even do the trick there.

To clarify: the reason your lights dim is because the the draw of the compressors in addition to everything else is more than your alternator can handle, the battery makes up the difference, but you're pushing the upper limits of the combined alternator + battery.

When your AC is running, the current output of the alternator is lower than when when the AC is off, so the draw of the compressors is > the current capacity of the lower alternator output + the battery

Best chance of fixing it easily/on the cheap: increase wire gauge to something more substantial, and adding a stouter battery. If that doesn't solve the problem, I would then go capcitor and higher current alternator, in that order.

marlinspike

Hmmm...the battery is actually pretty big already. Far more powerful than was specified for the car (Mercedes has Narva make them 2 sizes of battery, a big and a small, as time has gone on each battery has gotten stronger, but they're sold as simply the big and the small, even though the small is now as powerful as the big was when my car was being sold - my car originally came with the small, but when I went to the larger engine I changed it over to the big). I think it's like 880CCA.

As far as alternator goes, I'll have to pull the specs at home, but part of going to the bigger engine involved upgrading to a bigger alternator - the bigger engine was only available in later years when the cars had external amps, more speakers, heated seats, airbags, all sorts of stuff that my car doesn't have).

Fabrik8

Using that small of a wire is what is causing the voltage drop between the battery and the compressors. Copper has a certain resistance, and that resistance changes (decreases) as you add cross-sectional area. The voltage across a resistance can be described as V=I*R, where R is the resistance of the wire and I is the current going through the wire. It's easy to see that as you decrease wire resistance by making the wire larger, the voltage drop is lower for a given current through the wire, or conversely you could pass more current through the wire for the same given voltage drop. Something like 20AWG wire is too small for the rated current of the compressors, and there is the additional consideration of the compressor's inductance, which causes a surge of current when power is first applied (and depending on inductance can sometimes be 2 to 5 times the steady-state current draw). That's the same with all inductive loads like motors and lamps. That's the reason that the lights dim, but then don't dim as much once the horns are on for a second or so.

Anyway, at idle when the voltage is slightly lower, it takes more current for the compressors to spool, because they need a certain amount of power (P=I*V) so the lower the voltage the higher the current. It would be worthwhile to add an extra relay because you really don't have much (if any) headroom over what the compressors can handle. A slightly larger ground might help with local voltage drop back to the battery also.

marlinspike

Alright, so I guess I'll start with the easiest thing and swap out the 20ga wire for some 8ga and go from there.

Fabrik8

8 gauge isn't needed. 8 ga is a giant pain in the ass to work with and isn't necessary at all based on the length of wire you have. It's must easier to use additional relays with individual wiring for each. The largest wire you can usually use on commonly available relay crimp terminals is 10 gauge if it's TXL wire.

So how many of these little compressors do you have in total? I got confused early on. Do you have 2 pairs of 2 compressors (4 total) or just 2 total?

marlinspike

3 compressors total. It might be a 50 amp relay on there, I'll have to look. But it's 3 compressors and theyre 15amp at 12V each.

What gauge wire would you use for all 3? that way I can just buy it at the hardware store on my way home and try it out tonight. I'll probably do the relays and all eventually, but later.

Killerzracing92

8ga wire isnt big at all, its not that big a pain to run. I have that size running my radiator fans. I have a run of 4ga for my 4 channel amp, and Im running 2/0ga to my big amp which is .4" wide. That'd be a pain.

If it were me Id run a 4 gauge wire with a D block or homemade bus bar going to each compressor.

Oh and capacitors are worthless piles of crap that do nothing but mask the problem.

marlinspike

I'd run 3 relays with 3 14ga wires before I ran 1 4ga.

marlinspike

So my memory was faulty. There was only one skinny wire, it's black, and I'm guessing it's a ground. The rest of the wires are decently sized (prob like 12ga). However, the relay is only 30 amp. Does that mean the relay is definitely holding me back? I hate working with wiring, so I'll probably just pay someone, but I want to be able to tell them what to do because it seems to me most shops don't know shit.