Aux VDO Temp gauge wiring - what did I do wrong?

[jp02ti]

I have several sets of Aux gauges pulled form various VWs, 2 full sets of oil pressure, oil temp and voltage. My temp gauge has been giving me serious fits by climbing and dropping from red to blue rapidly, or parking itself 1 or 2 needle widths from the red. I thought I'd lust drop the temp gauge in to see if I could get a second opinion.

There are three tabs on the back for wires:

+ where I have attached 12V

- which I grounded

upside down T where I ran the line to the temp post where the radiator hose goes into the head. (We all know the one) I spliced my connection into the exiting line so that the dash gauge would still read, hopefully so I can compare. I know now that I need a second temp post since with both lines connected to the single post the dash gauge reads at about half power when the aux gauge is connected.

However, as soon as the aux gauge gets power the needle jumps to full and it stays there. I tried my other gauge and it does exactly the same thing. So, assuming that BOTH my gauges are not broken, what did I do wrong? Should I be sending less than 12V to the gauge?

Eventually I will tap the oil filter assembly and install an oil temp post there, but in the meantime, I need to be sure the gauge itself is not broken.

If anybody can help, having done this or knowing about it please let me know!

Regards,

Jon

[ifayeoh]

your temp post(sender) needs to match your gauge. if gauge goes up to 300 sender needs to be a "300 degree sender", understand?

[jp02ti]

yes, I agree, but they are matched - the needle in the aux gauge is pegged at over 150 C, even with a cold motor.

I'm thinking it must be something to do with voltage.

Jon

[SteveJ]

Leave the +12 and the ground connected as is. Disconnect the sender wire and then turn the ignition on for the car. When powered up, what does the guage do? Are you sure your sending unit is OK?

Usually, when the guages are powered up, the needle should go from a full bottom delflection up to the zero mark. Then as the input rises as the car warms, the needle should increase as well. Some guages react differently to disconnecting the sender. Some will drop to a zero point(but not to full negative deflection), while others will peg to full positive deflection when the sender is disconnected. With the ignition on, try connecting and disconnecting the sender to see what effect it has.

Are these lighted guages?

[visionaut]

Is your used VDO Temp Gauge a water temp or oil temp gauge? (Sounds like you're trying to use it to measure coolant temp, so asuming its a coolant temp gauge).

I've installed a set of VDO gauges in the center console of my 76 2002. (Lighted Oil Temp, Oil Pressure and Voltmeter gauges in 2 1/16th diam with spinlocks). I got an Oil Temp sender (mounts to the oil pan drain plug) and an Oil Pressure sender with adapter (replaces OEM sender and has connectors for the warning light wiring plus the gauge). I'm still using the stock built-in coolant temp gauge in the dash.

My VDO gauge installation instructions called for most of the wiring to be 14-gauge, but for the wiring going to the factory warning lights it called for 16-gauge. Also it indicated that the gauges measure ohm resistance created by the sender - thus the ohm range of the sender and gauge must match. Also it stated the positive power source must be switched and protected by a fuse.

My gauges peg full left-hand when key is off. The test for the temp wiring is: key on & pull the sender wire off the sender, the gauge needle goes full left -- and key on & ground the sender wire to the block, the gauge needle goes full right. The recommended resistance readings on the Oil Temp sender for proper operation were: cold = 700ohms, hot (180degress) = 68ohms. I realize you're working with an OEM water temp sender, so I don't know what the hot and cold readings should be, but I'd expect them to vary with temp also.

My guess is your problems are likely resistance match related - maybe based on the gauge of wiring you're using, maybe related to a ohm mismatch between your VDO guage and the OEM sender. Or utimately, just a bad sender (maybe why your OEM gauge was reading funny to start with.)

HTH!

[jp02ti]

I'm going to rewire with 14 gauge wire I think. when I put on the power, only power and ground connected (sender wire unplugged) the needle goes full right. maybe a bad gauge, but I'll try it will the correct wire and see what I get.

It is actually an oil temp gauge, and the connection to the water temp post was really just a test. I plan on either tapping the oil filter housing or getting a drain plug sender unit. Any suggestions either way?

thanks again,

Jon

[visionaut]

The drain plug sender is the only I've seen offered by VDO. It's a nice compact unit - replaces the OEM drain plug. Don't know much about the Oil Filter assembly option (I'm assuming there's a mod to install one to it, but there's no 'default' threaded location there for a sender I can see.