Hobbs Meter Wiring

[kupcar]

Hi Guys, Was wondering what would be the best way to wire in a hour meter. I have a faceplate over the radio so I was thinking of mounting the guage there. The car is 1977 911. Where would be best to tap into the power, etc. The power needs to turn on when the ignition is turned on? Looking for a easy way to wire without hacking into the factory wiring. Any ideas? Thanks foe the help.

[cgarr]

Pretty much into any power that's on with the key in the run position. I stuck one right on my engine and tapped right off the coil wire which is hot only when the key is on.

[BGCarrera32]

I did this exact same thing and tapped the power at the DME relay under the seat. The hour meter is only on when the fuel pump is running. This way I only log hours on the meter when the engine is in use, as opposed to when playing the stereo in the car in the garage, etc.

[kupcar]

Thanks for the ideas fellas. I'm thinking maybe one of terminals in the front fuse box? The coil ideas could work but my guage would be in the dash rather than in the engine compartment. No DME box in this dinosaur. I guess I could find a circuit that is hot only when the engine is on and connect it to that. Fuel pump? Ignition? As you can tell, electrical is not my strong suit. Just don't want to burn this thing down by creating a electrical issue. Thanks again.

[BGCarrera32]

Yeah right on...no DME. I missed that you have a '77. You could just tap off any circuit that is powered up when the ignition is in the on position, such as the radio, to get power to the meter. The hour meter draws so little current I wouldn't worry about messing anything up.

[Walt Fricke]

Use the fuel pump circuit for this. The fuel pump on your car only runs when the engine is running (unless you have disabled the interlock). You want the 12V power out circuit from the fuel pump relay. Without looking anything up, one place to get that is at the fuel pump itself - just add another ring connector to the positive post there. The black wire should be the fuel pump positive, but that's easy to check. You have to take the gravel guard off to get at the pump, though.

Looking at a wiring diagram, that's terminal 30 of the fuel pump relay, but the relay holder has its wires molded into it, and the diagram shows that wire going unbroken from the relay socket to the pump. However, a red wire with white banding has a connection up front before it heads back to the engine. It is listed as being "behind the fuse panel," so some work is involved perhaps in getting to it. These single connectors are a hollow rectangular piece of plastic designed to cover up where two wires connect. You can figure out how to tap into this in some sensible way I am sure. And how to feed your add-on wire through the firewall.

Most of us put this back in the engine compartment, as there is little reason to look at it often.

[Mahler9th]

Mine is connected to +12vdc through a Summit Racing RPM switch. It only sees +12 VDC when the engine is running past 3k rpm.

I found no need to have this meter in the car's interior, so it is mounted to the top of a custom fuse panel I made that resides in the same location as a stock '75 911 fuse panel.

[kupcar]

Thanks much for everyone's input. This should steer me in the right direction. I had every intention to put this guage in the radio faceplate. Now I'm questioning my decision. Maybe it is better in one of the truck areas. The pilot in me likes the cool factor, but it is far from a critical gauge. Maybe a AFR guage would be better suited for that spot. Anyway, I appreciate all the ideas.
Thanks, Jeff

[Walt Fricke]

Mike's idea of having the meter count only when the RPM exceeds some value is another way to skin the "what if I have my ignition on for this or that purpose, but the engine isn't running" issue. Fuel pump in a stock car does that just fine, but an RPM switch will pretty much do that also - and not count time spent idling in the paddock/grid or elsewhere. Fact is, at 3K you would not record most street driving either, so that figure would work for race only engines.

A similar method would be an oil pressure switch. You can buy one which closes when the pressure reaches some (low) value. These are often used as a safety feature for fuel pumps. The starter switch circuit bypasses this, but when the key is released to the run position, the fuel pump (or the ignition) current (or relay current) passes through the switch. A few pounds of pressure, and all is well. You crash and stall the engine, the fuel pump cuts off. But it would work fine for an engine hour meter also, giving you running time. And, with a car with the fuel pump up front where it belongs (like a '77 911), you can have this switch and the meter both back in the engine compartment and don't have to run any new wires back and forth. It isn't hard to rig up a T fitting for the stock oil pressure sender (or the idiot light sender, though that one is hard to get to with engine in car).

With the use of a relay, you could just T off of the idiot light sender wire, though. It runs through the 14 pin fuse panel to engine connector, so that wire is easy to get to. Use a normally closed relay (most have a NO and a NC position), with its coil ground attached to the idiot light wire. When the idiot sender is closed, the relay coil gets current, the relay switch is open and the meter doesn't get current. When you get oil pressure (even idling pressure), the relay reverts to its closed position (as you have wired it), and the hour meter gets current.

[kupcar]

Thanks again for everyone's help. Just ordered the meter and will put this on the "to do" list. I feel much more confident now.
Cheers, Jeff

[dgmark]

Rpm switch is the way to go, no reason to count hours under 2000 RPM