Headlights causing false Tachometer indication

Anyone ever have their tachometer jump up to a higher constant (erroneous) rpm reading at idle once headlights are turned on?!!!
My dash instrumentation lighting is very dim as well with no known rheostat to adjust to brighter. Sounds like I have a poor ground somewhere. Any known history with any of this? Thanks

 

happens in my ‘89 as well. It affect oil pressure gauge also

Have not found an answer. Maybe someone will let us know?..[emoji4]


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Consider it part of the Italian charm? Sorry, not too helpful, but my '85 QV Cab does the headlight thing as well. I really only ever drive it during the day so it isn't an issue for me.

 

+1 -- but what is the magnitude? Your use of the word "jump" implies it's quite large. I'd say it's "normal" for the gauges to move up maybe 1/2~1 needle width higher when the parking/headlight are turned on (but I haven't noted any tach effect). Do your gauges also move up just slightly or a lot when you turn on the lights?

 

You don’t have the little pole sticking out of the tach to adjust brightness?

 

Dinner adjustment is there. Just makes gauges dimmer but did not affect the jumping.


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With headlights to on, my Tach jumps +1000rpm and stays there +/- 1-2mm movement. The car does not change idle to 2000rpm so its clearly some sort of short. I haven’t noticed any gauge movement but will check when able.

 

I do have the pole but never messed with it, assuming it was the odometer trip reset like in other cars! Thanks

 

Same on mine.

 

Just checked mine again today, and get a small movement of the gauges (maybe 1/2~1 needle width) but no change in the tach at idle at all. Can you be more specific about what model, year, and version Mondial that you have? (I'll have a peek at the corresponding wiring diagram.)

 

Sure Thanks

1989 T Cabriolet US version
Last 6 digits vin: 0082519

 

I can tell you what to check from looking at the schematic as a likely cause for this tach problem, but, unfortunately, I can't tell you where it is physically located (most of the useful spatial information that used to exist on the schematics up to the mid-80s has been lost on the later schematics). The electrical architecture of the tach and its illuminating bulbs strongly suggests that it is a ground problem so first things I would check would be:

1. The connection of the N 0.8 wire at the "B" connector of the 70092 common ground block, and

2. The ground connection between the 70092 common ground block and the chassis.


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Based on the location of 70092 on the schematic, I would guess that it might be located on the floor of the passenger footwell, but could be in the forward floor section of the central tunnel or even on the Driver's side or anywhere . Good Hunting!

 

Alright then! Thank you I’ll begin the hunt this evening and report any positive findings.

 

My 90 T does a similar thing when the lights are on, adjusting the dash brightness will change some of the readings - it's on my list of things to do, but my guess is that its a grounding issue in the dash itself.
The 12v from the lights is going through the instruments to ground which gives the false readings.
I had to add a ground to my oil temperature sensor to get that to work as there wasn’t one fitted, this may be a similar thing that it’s not a bad ground, you just need to fit an extra ground wire to the lighting circuit in the dash

 

Not a Mondial, but my 84 308 QV exhibited similar needle jumps when turning on the park or headlights - about 2 needle widths on fuel, coolant temp, and oil pressure. I would gain 1/8 tank, 20*F, and 20 psi! I just did a cleanup project last week to address this, and now have zero movement of the gauge needles when I turn on the lights.

I removed the instrument panel, and with emery board or sandpaper cleaned the whitish corrosion off of every single male blade connector in the 3 main multi-connectors for the panel harness; in the separate speedo and tach multi-connectors, on each of the wires on the back of each of the gauges; and on each of the light bulb blade connectors. After cleaning, I applied a very light coat of Stabilant-22A electrical contact enhancer (https://www.amazon.com/Standard-Motor-Products-SL5-Stabilant/dp/B000O3N0U0/ref=sr_1_1?dchild=1&keywords=stabilant+22a&qid=1603981146&sr=8-1) to every wire connection point. The little 15ml bottle of 22a costs around $80, but it goes a long way, and it's very effective - I'd dare to suggest it's like a budget version of Dave Helms' gold contact kits, and as such it's a genuine bargain!

I also bypassed the dimmer (just needed a wire with two male blade connectors to jump between the two female harness connectors that connect on the dimmer), and replaced all the lights with LEDs - that brightened up the panel significantly so it's actually visible and not just like a glow worm.

Along with cleaning and Stabilant-22a on all the connections, I also added a second ground wire - all the gauge and lighting ground wires are on a single chain through one wire into one of the 3 main panel connectors, so I tapped one of the wires at the opposite end of the chain, and ran a 14 ga wire to an unused bolt under the dashboard.

End result, the instrument panel is now readable at night, and the gauge needles don't move at all when turning the lights on.

 

Wow. Great explanation! I’ve got the S-22a so going in headfirst.

 

I own a mondial T from 91, I had the exact same problem. The solution add a new ground from tach to chassis. SOLVED,!! Now all readings perfect! Look for bad ground to dash, or in my case better add a new reinforcement ground to tach ground!,

 

Best is to put an extra thick black wire that goes from every ground connection direct to minus battery.
See = https://www.ferrarichat.com/forum/threads/sticky-mondial-8-qv-3-2-t-repair-thread.209843 #3
Chassis is a poor conductor...

 

My T is currently in for its Major service & the technician suspects a grounding issue with the headlights causing my Tachometer issue as the fog lights come on when the headlights are on (I hadn’t noticed that)

As i said in an earlier post all my dash instruments would change readings with the lights on, there has been a lot of work done to the electrical system on the car (not good work either)

I’m fairly sure that it’s a bad ground somewhere & because the headlights seem to be the main problem that’s where I will start

 

Since the all the instruments and the light switch are daisy-chain grounded - wouldn't it be logical to isolate the light switch ground from the others and see if that cures the issue?