Back of the Speedo

[TomoHawk]

I picked up a l'ike-new' speedo off eBay and I saw a little box on the back with wires coming out of it. What's that for?

thx

[Fun_in_my_z]

if im not mistaking thats the control thangy for the speedo. it has gears in it to drive the needle. i believe. but i could be wrong.

[TomoHawk]

I think there's a cable connecting it to the inside, but it has wires on it too. I think it might be the things that controls the BCDD.

[2ManyZs]

It's connected to the ECM is some way. I know what it does, but don't know exactly where it ties into the FI system. At one time I knew what it was called, but it's name escapes me at the moment.

My 75 280 had a problem crop up whenever I pulled up to a stoplight, or when shifting the idle wouldn't drop. I'd pull up to a stoplight, and the car would idle at 1200RPM's. One of the techs at the Nissan dealer told me there was a sensor in the back of the speedo that told the ECM what speed the car was running.

I pulled the speedo, replaced it with another used one, and to this day the car is fine.

[TomoHawk]

I knew there had to be a speed sensor in there someplace. It probably does connect to the F.I. and the BCDD.

O-L.... I thought I could simplify things.

[Zedrally]

In the ozzie 260's [read 280 with flat top carbs], this was part of the emmission control system.

From memory part of the anti back fire system, as to whether you can chop it in a FI system I don't know, but you can in the ozzie 260.

HIH

[Mr Camouflage]

I asked this exact same question. even took photos of the little bugger, no one answered.

My 260 doesnt have EFI, so it must be for something else, unless the wires go to the plug box, and nowhere from there.

I did a speedo swap with a 240z to a 260z speedo so i could have KMH instead of MPH, and was wondering what the box was for.

The wire goes into the speedo and connects to the bit that goes round and round, so i guess its some sort of pulse type revolution counter that counts how fast the speedo cable drive is rotating.

I left it tucked away in the 240z casing, as i didnt have a use for it.

I guess it could be intergrated in a cruise control system.

[Zedrally]

Me thinks you have mis-read what I wrote.

Being based in the land of Aus, I deliberately prefaced that this refered to our 260's, which came equipped with those notorious carbies, up to '78.

These late models had all different pollution control systems and the wires coming from the rear of the speedo was part of the anti-pop or anti-backfire system.

As to what they do on 280's fitted with EFI, I have no idea.

For all that I know they may not connect with anything!

Sorry to have missed your earlier question on this.

MOM

[Mr Camouflage]

My previous question, with photos:

http://www.classiczcars.com/forums/showthread.php?s=&threadid=12096

[Alan Pugh]

As Zedrally said it is part of the anti backfire system on our Australian delivered 260's with flat tops.

It is actually an amplifier conected to the speed switch inside the speedo, which operates the throttle opener solenoid on manual transmission cars only.

The throttle opener is there to reduce HC emissions during coasting situations ( high manifold vacuum ), by opening the throttle slightly to let more fuel in to the combustion chamber.

The switch/amplifier in question on the back of the speedo operates the solenoid above 10 KPH.

I have no idea what it does on a fuel injected car.

Hope this helps.

Alan P.