Lights interfere with Speedo
03-25-2010, 06:59 AM
#1
Racer
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Location: Austin, TX
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Lights interfere with Speedo
I'm having a little problem with my speedometer on my '87 targa when I turn my headlights on.
The speedo reads fine when the lights are off but if I turn the headlights on the speed reads much slower than I am going.
Anyone have any suggestions where to look before I start tearing the car apart this weekend?
Paul
The speedo reads fine when the lights are off but if I turn the headlights on the speed reads much slower than I am going.
Anyone have any suggestions where to look before I start tearing the car apart this weekend?
Paul
03-25-2010, 09:06 AM
#3
Team Owner
What about if you just put the marker lights on does it still do it ? if you don't have a voltmeter connected to the car ( and personally i think everyone should have one ) stick a cigarette light adapter in with two wires and measure the 12v when you put the headlights on .. if the 12v drops significantly you have bigger issues than your speedo. This is a general health check of your 12v globally . you may want to measure the voltages your speedo uses and monitor them while putting the lights on although that may be tougher.
Also check your wiring to make sure someone hasn't modified anything. Did this start suddenly or has it always been this way ? that may be a clue ..
Also check your wiring to make sure someone hasn't modified anything. Did this start suddenly or has it always been this way ? that may be a clue ..
03-25-2010, 09:52 AM
#4
Racer
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What about if you just put the marker lights on does it still do it ? if you don't have a voltmeter connected to the car ( and personally i think everyone should have one ) stick a cigarette light adapter in with two wires and measure the 12v when you put the headlights on .. if the 12v drops significantly you have bigger issues than your speedo. This is a general health check of your 12v globally . you may want to measure the voltages your speedo uses and monitor them while putting the lights on although that may be tougher.
Also check your wiring to make sure someone hasn't modified anything. Did this start suddenly or has it always been this way ? that may be a clue ..
Also check your wiring to make sure someone hasn't modified anything. Did this start suddenly or has it always been this way ? that may be a clue ..
There have been some mods done to this car. I've been taking care of a few other items and this one finally bubbled to the top of the list.
03-25-2010, 11:55 AM
#5
Team Owner
i think your 12v is suspect and the lights are the biggest draw. get a bently and see if you can see what voltages and circuit the speedo is on .. if it is not the grounds you could have an alternator/ regulator issue.
03-26-2010, 02:54 PM
#6
Rennlist Member
I've just got the same problem. Headlights off - no problem. Headlights on and the speedo hand drops to 0 and the illumination of the speedo goes out. Rest of gauges work just fine. ??started when I put a fuse back in for AC/Blower motors - but cannot find an obvious intersection of those two circuits in Bentley. Be interested in what you find out. First step will be to pull that fuse [and eventually all the AC eqpt] out and to check voltage with lights on.
[Update] though it took a bit of fiddling, what with all the extra wires going to the heating circuit, cruise control and all, but finally figured out the one of two things were wrong- the grounding wires were badly grounded elsewhere and/or the case of the speedo needed to be grounded as well. Once I fixed those two things it works fine.
[Update] though it took a bit of fiddling, what with all the extra wires going to the heating circuit, cruise control and all, but finally figured out the one of two things were wrong- the grounding wires were badly grounded elsewhere and/or the case of the speedo needed to be grounded as well. Once I fixed those two things it works fine.
Last edited by RShepHorse; 10-01-2010 at 02:03 AM.
