TheSamba.com :: View topic - VDO Gauges

Mon Feb 22, 2021 12:55 pm

Installing VDO Cockpit gauges this spring. Specifically, tach, oil pressure and oil temp. gauges.

My question is what amp fuse do I use to protect the those gauges and what amp fuse to protect the gauge lights?

My thought is three separate fuses for each gauge and daisy chain the lights to single fuse of its own.

Obviously, if I had everything hooked up I could determine how much amperage these VDO gauges pull, but that won't happen to spring and I would like to be prepared.

Ideas?? Thoughts?? Answers??

What can I say I like to plan things out lol

Thank you in advance D74

Mon Feb 22, 2021 1:50 pm

denver_74 wrote: Installing VDO Cockpit gauges this spring. Specifically, tach, oil pressure and oil temp. gauges.

My question is what amp fuse do I use to protect the those gauges and what amp fuse to protect the gauge lights?

My thought is three separate fuses for each gauge and daisy chain the lights to single fuse of its own.

All you mentioned will pull very little amps. I've run tach, oil pressure and oil temp. gauges on my VW since back in 1977. I use one of the stock 8-amp fuse locations in the fuse box (switched power) for all three gauges.

The illumination circuit I tapped into the speedometer illumination circuit. Now the speedometer illumination circuit is un-fused, so might make sense to add an inline fuse to THAT circuit. A 3-amp fuse for that should be more than sufficient.

Mon Feb 22, 2021 2:26 pm

Yeah I am still a little foggy at where I am going to pull power from.

My last post on the subject left me thinking that I would pull main power for each gauge from the input side of a 16amp fuse then put a fuse between that source and the gauges (each gauge getting its now fuse). The lights would pull power from the light switch itself, specifically branching off 58b (gray/red wire) and again adding a fuse for the gauge lights.

D74

Mon Feb 22, 2021 3:25 pm

ok finally found tech data sheet for tachometer. power consumption without illumination <100mA

oil temp gauge without illumination 131mA

oil pressure gauge without illumination 82mA

Mon Feb 22, 2021 3:57 pm

denver_74 wrote: Yeah I am still a little foggy at where I am going to pull power from.

My last post on the subject left me thinking that I would pull main power for each gauge from the input side of a 16amp fuse then put a fuse between that source and the gauges (each gauge getting its now fuse).
When you use the INPUT side of the fuse box as a power source it doesn't matter what the fuse is rated for. For example, if you pull power from fuses #11 or #12 which are bridged together on their INPUT side the current flowing to your new wire never passes thru either fuse so you could connect the wire to either.

I suggest you get (+) power from either of these fuse's INPUT. Run an inline fuse for all three gauges. If your amperage specs are correct your fuse size can be small. Cusser's suggestion of a 3A fuse should be fine.

One thing to keep in mind... if you use one fuse for all three gauges, when the fuse blows (because one gauge shorts to ground) it will take out all three gauges. If you have one gauge that you want working even if the others are not you should power it from a separate fuse (or use one fuse per gauge so that none impact the others).

Ever wonder why VW used two fuses for parking lights and in some model years the left rear taillight was on its own fuse while both front and the right rear taillight + license plate light are all on the other fuse?? It was to make sure you didn't loose BOTH taillights at night while traveling down the road. So how many fuses you use is not solely based on load.

denver_74 wrote: The lights would pull power from the light switch itself, specifically branching off 58b (gray/red wire) and again adding a fuse for the gauge lights.
If you are adding an in-line fuse to the illumination lamps, install it immediately after the headlight switch. This way it protects ALL the illumination wires/bulbs. The bulbs themselves are 2W. That works out to around 0.15A (150ma) each. A 2A fuse could handle x10 2W bulbs.

Thu Feb 25, 2021 7:49 am

Does this work??? Not sure what size fuse to use for the individual gauges. The graphic does not illustrate the light circuit only gauge power.

Thu Feb 25, 2021 2:04 pm

Normally, you don't want fuses in line with each other. As long as the single line with the 2A fuse in your diagram is relatively short, it doesn't need to be protected by a fuse. Just place the aftermarket fuse box close to the source of power and no fuse is needed.

The other option is to just use the single 2A fuse and skip the need for the extra fuse box. You can use a distribution block instead.

Thu Feb 25, 2021 2:40 pm

ashman40 wrote: Normally, you don't want fuses in line with each other. As long as the single line with the 2A fuse in your diagram is relatively short, it doesn't need to be protected by a fuse. Just place the aftermarket fuse box close to the source of power and no fuse is needed.

The other option is to just use the single 2A fuse and skip the need for the extra fuse box. You can use a distribution block instead.

understood.

and for the record I think I miscalculated my fuse sizes lol

if the max amp of a 14 gauge wire is 15 amps and gauges draw a total ~.5 amps with a minimum fuse size of .75 amps (total amps of gauges x 1.5) then the inline fuse size should be ~8 amp (the average of 15 amps and .75 amps)

Thu Feb 25, 2021 4:08 pm

denver_74 wrote: and for the record I think I miscalculated my fuse sizes lol

if the max amp of a 14 gauge wire is 15 amps and gauges draw a total ~.5 amps with a minimum fuse size of .75 amps (total amps of gauges x 1.5) then the inline fuse size should be ~8 amp (the average of 15 amps and .75 amps)
That's not how that works.

The fuses are there to primarily protect the wires in case they short to ground somewhere along their path, or the end devices demand more current than the wire can support for the length. The amperage rating of different gauge wires depends on the wire gauge and the length of the run. Often, you also need to take into consideration if the wires are part of a bundle (harder for them to dissipate heat) or loose (easier to radiate heat). The fuse needs to protect the wire should the current load over the wire causes it to overheat. Using a fuse smaller than needed still protects the wiring (the fuse blows sooner), but using a fuse rated higher than the wire can safely carry could cause the wire to turn red hot; melt its insulation and start a fire all before the fuse melts/breaks.

VW was rather conservative with many of their fuses. Many wires used for 8A circuits can probably carry close to 16A max, but with an 8A fuse installed the wires are protected from ever overheating. It sends chills up my arms when I see pics of fuse boxes with blue 25A fuses installed. I know the stock wires cannot carry that load.

So, assuming your 14AWG wire lengths are kept below 4ft your max wire load amperage rating is around 30A. At these short lengths you would have no problem with an 8A, 16A or even a 20A fuse. But since you know the load should be no more than 0.413A (413mA) for all three gauges you could just install a 1A fuse. This gives you a bit of overhead to protect the gauges while easily protecting the wires should they ever short to ground. The fuse would blow before the 14AWG wires even got a chance to warm up. 8)

Fri Feb 26, 2021 4:49 am

ashman40 wrote:
So, assuming your 14AWG wire lengths are kept below 4ft your max wire load amperage rating is around 30A. At these short lengths you would have no problem with an 8A, 16A or even a 20A fuse. But since you know the load should be no more than 0.413A (413mA) for all three gauges you could just install a 1A fuse. This gives you a bit of overhead to protect the gauges while easily protecting the wires should they ever short to ground. The fuse would blow before the 14AWG wires even got a chance to warm up. 8)

thank you! i was over thinking it. i still have to figure out the fuse amperage for the lights but below is the corrected VDO-DBlock power setup

Fri Feb 26, 2021 6:24 am

Looks good.

Two tips:
1) As good practice, when you have multiple paths powered by a single source (wire from fuse box to distribution black), place the source wire in the middle so current flow for one path does not cross another path. In the above diagram, current for both the top and middle gauge must traverse the single red jumper between bottom incoming screw and the one above it. By moving the incoming wire to the second position the jumper wires will only carry the current for a single output.

2) Use the spare locations on your distribution block to do the same thing with the illumination bulb connections. Run a wire from the headlight switch 58b to the block with an inline fuse to protect the circuit. Power all the illumination paths (including the speedometer) from the distribution block.

Fri Feb 26, 2021 9:27 am

ok this is with lights. daisy chained the negatives for the lights.

Fri Feb 26, 2021 10:33 am

Looks great!

One last comment...
Daisy chaining the illumination grounds means that circuit has a single point of failure. If the ground wire comes loose you lose all three illumination lamps. Not really a big deal since it is only illumination.
On the power side you have individual power wires to the lamps. This is good for separation/resiliency so any one power wire failure doesn't impact the other lights. This is why you separated the power to the gauges themselves. You are doing the same for the illumination power, but are losing some of the resiliency advantage by using a daisy-chained ground. A break in the ground wire or if it comes loose and you lose all three gauge lights even through the gauges have separate power sources. You have a single point of failure.

My point is either go with separate power and separate grounds for all the illumination lamps, or go with a daisy chain on both power and ground. The separate wiring is obviously better/more reliable, but the daisy chain method is also acceptable for illumination lighting. It is how VW wired the two speedometer illumination bulbs from the factory.

Fri Feb 26, 2021 11:44 am

ashman40 wrote: Looks great!

One last comment...
Daisy chaining the illumination grounds means that circuit has a single point of failure. If the ground wire comes loose you lose all three illumination lamps.

Would it be wrong to have all the grounds on a single grounded bus bar??

Fri Feb 26, 2021 11:59 am

Denver_74, great diagram. I have the gauges to install in my '73 or '74 Super but can't figure out a good mount point for them.

Question to you and Ashman about wiring the gauge lights. Can't those be daisy-chained to the ouput side of the #1 or 2 fuses?

Fri Feb 26, 2021 4:18 pm

vamram wrote: Question to you and Ashman about wiring the gauge lights. Can't those be daisy-chained to the ouput side of the #1 or 2 fuses?

I am no authority but I am guessing that you should you use a distribution block like I did and not daisy chain.

The output side #1 and #2 sounds reasonable to me but again not the authority.

Both #1 and #2 are part of the lighting circuit so logically power coming out of the fuse block for lights would be ok. The power draw for lights on the gauges are so minimal that it could work.

My Illustration would be modified to show the wire going to #1 or #2 from the distribution block instead of the light 58b on the switch.

Hopefully ashman will chime in an give you/us further insight. You also try a search on the forum for ashman's comments on the subject of gauge wiring. Its all very informative.

Hope this helped D74

Sat Mar 06, 2021 8:50 am

vamram wrote: Denver_74, great diagram. I have the gauges to install in my '73 or '74 Super but can't figure out a good mount point for them.

Question to you and Ashman about wiring the gauge lights. Can't those be daisy-chained to the ouput side of the #1 or 2 fuses?

Just a side note, having reviewed the wiring diagrams for the gauges it instructs that the lighting power should be run from the switch.

Sat Mar 06, 2021 9:03 pm

The only downside of using the parking light fuses to power your gauge illumination lamps is you will have no control over the brightness. The gauge lights will come on full brightness anytime the parking lights are on.
The 58b circuit coming off the headlight switch is controlled by a dimmer (potentiometer) in the headlight switch. By turning/rotating the knob you adjust the illumination brightness. If this feature is not important to you then wire it to the parking light fuses. The small additional load is not a problem for the stock 8A fuses. If you have a Beetle where the left rear parking light on its own fuse separate from the rest of the lights go ahead and wire the gauge lamps to this fuse as it is under utilized.