Instrument wiring questions

[John2kx]

After receiving two gages which were modified for use in a v8 conversion, I have a couple of problems wiring tach and fuel gage. Tach was converted to work with 8 cylinders and also had oil pressure (0-80 psi) reading installed. Fuel gage had top red brake light removed and replaced by water temperature gage. Note: PO had a aftermarket tach installed with shift light. This unit had 4 wires (red, black, white, and green) which I've unlanded and am having problems determining what goes where.

Tach: oil pressure worked after landing new wire from sender to lug marked by repair shop. The white 4 prong connector on back of tach is marked as follows (left to right as viewed from back of tach:

( + ) I think black/purple from coil goes here
(+6) This one is used for new oil pressure reading and works
( I ) Nothing connected to this one
( - ) Ground I assume and has brown wire connected

I do have a factory red/white wire that went to aftermarket tach which is not connected to anything........should it land on ( I ) above? Thought I'd ask help before screwing this thing up. I've located in on electrical diagrams but can't tell for sure which lug it lands on.

Water temp.: this gage never registered anything with engine at operating temperature. The section of fuel gage which was modified with water temp. reading has two lugs on back of gage....one marked ( + ) and the other (G). Tried connecting wire from water sender to both of these with no luck. Thinking the "G" might be a ground, I installed a ground wire from chassis to this lug with sender wire connected to (+). Wire/bulb which used to connect to top portion of gage is not connected to anything and assume it will no longer be used.

My hair is on fire to take this thing for a ride but don't feel comfortable without the water temp. working. Any help would be appreciated.

John

[Dave at Pelican Parts]

The stock VDO tach has three connections (that matter to the tach part of it). One is the red/white wire, which is a switched +12V. One is the brown wire, which is a ground. The third is the signal from the points, which is the black/purple wire.

A VDO tach would have numbers by the terminals. 1, 15, and 31. 31 is the ground. 15 is the +12V. 1 is the signal.

I have no idea what kind of tach you have--if VDO, the connector may have been modified as well. But if I had to guess, I'd guess that you want to leave the brown wire hooked up to where it is, hook the red/white wire to the (+) terminal, and hook the black/purple wire to the (I) terminal.


The temp gauge is probably reading a simple thermistor circuit, like most do. Hook the (+) terminal up to a red/white wire (+12V), and the (G) terminal up to the wire from the sender.

--DD

[SummerSledSix]

While we're on the subject of gauges, I have a 930 tach that I had modified by NHS so that the boost gauge is now my fuel gauge. Just recently, the tach started doing this thing where it doesn't work until the car has been driven for awhile. I could be guessing, but it seems to be temperature related - it "wakes up" sooner when the ambient temp is warmer.

Anyone seen this kind of thing? I checked my connections, clean and solid all the way back to the brain.

[John2kx]

Dave,

You were right on the money! Tach lead was not landed on the right lug and there was no red/white power supply for water temp.. I jumpered from another red/white on fuel gage and everything is working.

Funny, the oil pressure was working before I got started with this repair. Maybe something came loose while pushing tach back in place.

Thanks again, now I can drive with confidence. Did manage to finish cv's and shifting linkage today.......did a test run in driveway but I need to make some shifter adjustments before hitting the road.

Regards,

John