Temp Gauge & Tach Grounding

[Stutzdriver]

Strange electrical problem.....I replaced the non-functioning tachometer with a good unit purchased on eBay, should be a straight forward, easy replacement. The replacement tach had no ground lug on the backside as there was on the non-functioning original so I soldered one to the housing in the approximate location of the original. Hooked up everything, tach works great but as soon as you turn on the ignition the temp gage pegs at full HOT (all the way to the right) and stays there. Do some research on this forum and discover I might have a shakey ground connection (a known problem), Pull it apart again add an aux ground still the same problem. Pull the ground wire off of the tach and the temp guage and the tach work great but now I don't have any light in the tach at night! Why can't I ground the tach? Looks like I can have the functioning temp gauge or the Tach lights?? Any ideas??

Car is an '84 944 5-speed

[Stutzdriver]

Nobody has any input???

[JivenJim]

I don't believe that Vdo Gauge housing had a ground Tab on them? I'm confused. So if I understand, you soldered a lug and then connected that with a ground wire. Is this a new wire or did you use a floating ground wire? If new, where did you ground connect??

The Instrument lights are Daisy Chained (Wired in Parallel) They are thin two tone Gray and I think Blue strips. I believe that the Led Bulb holder with attached wires has a grounding contact that spring contacts the inner feed thru light hole/gauge housing) So the light bulb grounding/return would be through the long Daisy chain wires.

Does your Tach Light Bulbs have two wire or just one?

Also remove the light Bulbs from Tach and Test that both wires for Pin 2 (Ac Pulse wave of Ignition coil - green) and pin 4 (Blue/Yellow) go to grounds.

I think, that the Pin#4 on the Tach is Ground and Feeds the ground of the Temp Gauge, so by altering the Ground path you may have introduced some stray potential Voltage.. thus effecting your Temp reading.

So If the gauges both work with out the Light bulb attach attached. You could install a light bulb that does not make metal contact with gauge might patch this. (maybe put tape on ground case lightbulb contact)

[Stutzdriver]

The tach I removed had a ground lug spot welded the the back of the tach housing and a pair of brown (ground) wires attached to the lug. Since the replacement had no lug I cleaned th back of the housing a silver soldered a spade lug as was there on the old tach in the same spot of the housing. Now that I think about it might the replacement tach been form an older (newer) 944 and it is internally grounded? The old one might have not had the housing internally grounded?

I'll test the old tach as to the ground as you describe and let you know what I find.

[Stutzdriver]

I forgot to say the light bulbs have 1 wire and ground to the housing.

[Arne2]

I know this is an ancient thread, but it gave me a clue on my similar issue, and I thought I'd post what I ended up with to help others.

The issue is that the tachs for manual and automatic equipped cars are different. The manual transmission cars have a CASIS upshift light, the auto-equipped cars do not.

The tachs with CASIS have an extra ground spade on the back, sometimes soldered, often just attached with one of the backside screws.

When you get a replacement tach for your manual car, often you end up with one from an automatic. And if you connect it all as normal (adding the ground spade, without which the tach doesn't function), the temp gauge page anytime the ignition is on. Unplug the extra ground and the temp gauge works again (somewhat), but the tach is wonky.

The hint from this thread was that the temp gauge input also goes to the CASIS bit in the tach, via terminal #4.

When I looked at the wiring going to the tach, it was easy to see - terminal #4 has a wire from the sender, and it also forks to the temp gauge.

My suspicion was that if I disconnected #4 from the tach, the temp gauge would work, but I'd probably lose the econo-meter.

This was exactly the case. Once I pulled that female spade out of the 6-gang connector, the econo-meter was dead, but both the tach and temp worked normally. I used some shrink-fit insulation to prevent the terminal from shorting out and killing the temp gauge later, and put it all back together.

All is good now, as I couldn't care less about the econo-meter.