I’m adding three gauges (not doing a center console and not using it’s wiring) and want to confirm the connections for the Bosch relays I plan to use each, and figure out the grounds. Gauges will be Voltmeter, CHT, and Air/Fuel if it matters.

can someone confirm:
#30 gets “+” from the battery
#86 gets switched power from the fuse box
#87 gets “+” wire from the gauge
#87a goes un-used

#85 seems to call for a “-“ connection directly to the battery - but I’d rather not add a ground wire all the way back to the battery - I was wondering if the “-“ from the added gauge could go to #85, then loop it on to a body ground point?

Thanks.

Here's a Bosch relay pin out. That said most of the items you mention are low draw and usually just fused not relayed.

ok, thanks - that confirms what I'd documented above regarding inputs.. and also points to the need to ground the relay separately, and to a chassis ground instead of having to go all the way back to the battery.

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I put a relay on my volt meter. I have it running directly to the battery. Anywhere else and it is worthless.
Click to view attachment

In theory, all electrical grounds are identical. Sometimes in practice they aren't. The culprit is usually the 914's favorite R-word, RUST.

The voltmeter is a little tricky. The stock one is fed from a circuit with a fair amount of load on it, and the reading will drop when various lights or other systems on that circuit are on. This can be handy for noticing a light out, once you get a feel for how the gauge usually behaves, but it is less ideal for checking the health of the car's charging system.

Using the relay's output for the voltmeter power/signal wire will take the other loads out of the circuit that drives the meter.

Also note that most of the popular aftermarket voltmeters have heavily damped movements, so they do not react that quickly to changing voltage. (E.g., if you hook one up to the stock wiring it can take a second or two to respond when you turn on the headlights.)

--DD

Yeah, the whole reason for adding relays is because of the voltmeter. I’d read that a direct wire to the battery is best, but that a relay was recommended so that it would not become a parasitic draw - so that’s what I’m doing.

I was planning a relay for the other two gauges (CHT & A/F), are you guys saying that it’s not needed for those? (Just run those to switched power off the fuse block?)

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Not needed at all for the others. Just switched is fine. The only pain is getting the wiring to the lights to them. The dd cht has an automatic dimmer, which you will want.

Good, that simplifies things a bit
Regarding the lights/dimmer (yes I have the dd CHT), for the three extra gauges I was planning to just daisy chain off the stock gauge’s black/blue wiring .. that should work, right ?

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Why should the voltmeter be a parasitic drain on the battery if it is directly switched off when the car isn't running? Besides, a relay is just a remote controlled switch which is typically only needed for high current applications. Yes? Measure the internal resistance of the voltmeter & all the meters for that matter.

FWIW, if I were to do what you're doing, I'd run both the hot & ground directly from the battery & put all the gauges on a relay. Everything is obviously wired in parallel so there will by a slight voltage drop. Measure the voltage at the battery (with a voltmeter & the relay off) vs. what the new voltmeter is reading (with the relay on) & note the difference. It shouldn't be much. What's most important is if there's a sudden or short term drop in the new voltmeter's reading? Yes? And daisy chaining the gauge lights should work fine.

CTTOI, if what you mean by daisy chaining is connecting the gauge lights in series, then I wouldn't do this. If one light blows out, then they all go out. Cheers.

You can tap them parallel for the dimming effect. When that prong sees 12v it dims the LEDs, which are always on of course, regardless of the lights.

The reason you don't just run dash power to the voltmeter is because you get completely worthless readings from it. Nothing like watching it drop to 10 when you turn on the headlights and hit the brake. Or flop back and forth with the turn signal or wipers.

The reason for the relay is so it isn't on all the time.

Not wanting to start a new debate about it, I understand that some will say it’s not necessary but my research saw several recommendations for it. I like the idea of eliminating it as a possible problem, after all it’s simple, cheap and easy enough to do while I’m working on my electrical system anyway. Just erring on the side of caution.

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Go for it!!! Watch out for rusty ground connections.

Ha, I have more of an issue with painted grounds! I taped off grounding studs for the most part, but for some reason forgot ( ) to tape off the multi-ground fixtures at the headlight buckets. Stripping them would be hell! But yeah, I hear ya..

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I did that exact thing. Sucked getting down there with Dremel and little wire wheel. It happens.

If you run the wire directly from the battery, the gauge never switches off. If you use switched power from the stock location, you get the gauge moving when you turn the lights on and off.

The goal here seems to be to have the voltmeter reading battery power directly (pretty much) but still switch off when the key is off.





Look at the way the stock lights are done. This is daisy-chained. It's all in parallel; the "daisy chain" refers to how the positive wires are chained one to the next. The grounds are all separate, or may be themselves daisy-chained together.

--DD

Yes, that’s it.

Yep, that’s what I meant by the term daisy-chained.


Ok, let me ask this:
It’s my understanding that lots of people pull the relay for the open door buzzer (‘cause it gets annoying) - could that stock round relay be repurposed to work for the voltmeter if wired up to it instead? (I don’t know how different the stock round relays are to the square Bosch ones)

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I got one off Amazon. It made me feel all technical and stuff installing it. I even put an inline fuse on the line to the battery. Made me feel all warm inside.

You can even buy cheap relays that are Bluetooth. All kinds of possibilities.

Sorry I wasn’t clear..I have several extra Bosch square relays, so getting one is not the issue. However, I was thinking that it would be convenient to repurpose the door buzzer relay (or whatever it is, I have one empty round relay receiver on my stock fuse panel. 2 GY/BR, 1 RD wires , ‘72) Mainly because it’s already there taking up space and not being used.

I did notice tho, that this empty relay receiver has only 3 wires. Still it could possible work for the voltmeter IF grounding off the gauge itself is sufficient… no?


DAPO in training
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That's exactly why I didn't run all my gauge insanity into the cars wiring. I don't mess with the original stuff. My relay is outside of all normal car wiring. If a next owner wants to return it to the three gauge console, I have done no strange things.... just pull my wires and you are good. Don't be a dapo.

https://inspectapedia.com/electric/Electric...vs-Parallel.php

Hmm...

https://inspectapedia.com/electric/Electric...vs-Parallel.php

Yeah on the one hand it’s kind of a clever move, to make use of this ignored/hated spot on the fuse board. Especially since this car will never be going back to stock configurations. But you’re right .. a future owner would likely be cussing me at some point! it’ll stay as is, thanks.

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