Speedo picking up engine revs?????

[Darren C]

I've done a few modifications on the car including new leads (now routed below the exhausts), plus a Mallory electronic ignition conversion.

However, the speedo now shows 60mph when the car is ticking over!!!

I have a ford 302 with a Renault box; at present, the mechanical drive from the gearbox feeds into an electronic converter, which in turn sends a single wire supply to the speedo.

Prior to the doing the above modifications the speedo worked perfectly. The cable from the gearbox to the speedo is now closer to the HT leads, but is still about 100mm away at it's closest point.

I am confused.... Can anybody suggest a cause or solution???

Thanks in anticipation!


Darren

 

[Seymour Snerd]

However, the speedo now shows 60mph when the car is ticking over!!!..., the mechanical drive from the gearbox feeds into an electronic converter, which in turn sends a single wire supply to the speedo.

One basic question: does the speedo's indicated speed vary when you rev the engine?

General thoughts... As you suggest this could be a case of the electronic speedo picking up signal from other wires via radiation. However, I think it's more likely that there is "noise" in the grouding (earthing?) system (that is, the ground at the speedo is not instantaneously at the same voltage as the ground everywhere else eg the battery negative post or the transaxle box). This could be a result of a marginal grounding situation made worse by the higher alternating currents associated with the new ignition system. The experiments I suggest are to make sure the ignition box is well grounded (meaning a short, relatively thick wire to the frame via clean bare metal) and the same at the speedometer and at the transaxle box.

If that doesn't help an experiment would be to temporarily connect the speedo to ground only via a single wire directly to the battery negative post. Same idea with the speedo's gearbox unit and the ignition box. This can be difficult because automotive things tend to be grounded via their mountings. Particularly in the case of the transaxle box, it may be impossible to electrically isolate it since it has to be mechanically connected to the transmission.

To explore the radiation idea you could try connecting the speedo to the transaxle box with shielded cable where the shield is grounded at only one end and left open at the other.

It might be helpful to know exactly how each part of this "three-way" problem is grounded (speedometer, it's box at the transaxle, and the ignition box).

 

[Darren C]

Thanks Alan

The speedo seems to move both as I rev the engine, and as I get faster, so I guess the speedo is picking up rogue signals from somewhere.

If the weather is good enough tomorrow, I'll whiz the car around the block and kill the ignition to see if the speedo then reads corectly. At least that would show that the ignition system is definately causing the problem.

I'll then check the earthing as you suggest.

Thanks again!

Darren

 

[Seymour Snerd]

...I'll whiz the car around the block and kill the ignition to see if the speedo then reads corectly.

Great idea!

 

[Darren C]

Great idea!

I can't claim that as my own idea! I found that suggestion on a Jaguar forum! Seems so obvious, I can't beleive I didn't think of it though.....

 

[Rich E.]

Not sure if this will help your issue but my 94 LS7 powered RX7 had/has the same problem when stopped. Turns out the speedo is picking up TDC from cylinder #1 and converting it to a speed reading when stopped. As soon as I get past 10 mph. it goes away. Might want to try and isolate the wire from RFI as that's what my problem was.