Dave J
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| posted on 7/7/10 at 08:10 AM |
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Weird speedo problem
Hi all,
Up untill recently, I have been running with a Mallory dual point distributor on my V8. My instruments are ETB electronic dials...speedo, tacho
etc.
At the weekend I ditched the points in favour of a Mallory E-Spark electronic ignition conversion module which sits in the distributor in place of the
points (infra red beam with a chopper plate).
Now here is the problem;
On acceleration my speedo shoots up to 90mph+ (actual speed accelerating through 25-30 mph). Once I have gone through 2000rpm or not accelerating any
more at a steady cruise, the speedo settles down and reacts normally, until I slow right down or stop then accelerate through the gears again, then
once again the speedo goes crazy.
The electronic tacho behaves normally. The car is running very well on the new system.
The Tacho gets it's pulses from the coil. The speedo gets its pulses from magnets fitted to the prop.
I tried covering the leads from the electronic module to the coil in baco foil and then earthed it just in case excessive RF is being emitted and is
having an effect on the speedo. this didn't do anything
I've been in touch with ETB to see if they have any idea what the problem might be. Whilst being extremely helpful, there was lots of head
scratching going on, so no joy there.
The speedo was behaving perfectly well prior to the conversion, so there is obviously some sort of feedback getting to the speedo head, possibly coil
related??. The electronic module is connected to the coil through the ballast resistor as per instructions.
Can anybody shed any light on this?
Many thanks.
Dave
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02GF74
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| posted on 7/7/10 at 08:50 AM |
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strange.
1. does the speedo go lulu when staionary and revving engine?
2. when coasting down a hill with low rpm, does speedo read correctly?
3. what sort of cable do you have from the speedo pick up?
4. you have checked the magnets and speedo pick up are secure?
5. if you move speedo pickup away from the magnets, does the speedo go mad?
6. I'm sure you are but you are using resistive ignition leads?
7. is speedo pick up indictive or hall effect type? (2 or 3 wires?)
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RazMan
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| posted on 7/7/10 at 09:12 AM |
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Sounds like some interference is causing crosstalk somewhere. Have you used shielded cable or 'twisted pair' for the speedo sensor and any
other input related stuff? I had chronic misfire problems until I shielded my crank sensor wire.
[Edited on 7-7-10 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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Dave J
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| posted on 7/7/10 at 01:31 PM |
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Thanks for the replies. here are some answers to your questions;
02GF74 asked:
quote:
1. does the speedo go lulu when staionary and revving engine?
No it remais on zero.
2. when coasting down a hill with low rpm, does speedo read correctly?
When coasting at low speed around 1500 revs, say in 3rd gear, it does tend to behave irratically as described.
3. what sort of cable do you have from the speedo pick up?
The cable is part of the loom supplied by Luego so I assume it's fit for purpose (no probs untill I fitted the Mallory conversion)
From memory there are two wires from the sensor ,multi strand, about 1 mm dia.
4. you have checked the magnets and speedo pick up are secure?
Yes, all ok
5. if you move speedo pickup away from the magnets, does the speedo go mad?
Not tried this.
6. I'm sure you are but you are using resistive ignition leads?
Yes, Magnacor HT Leads
7. is speedo pick up indictive or hall effect type? (2 or 3 wires?)
It is inductive (2 wires...red and black)
Thanks
Razman wrote:
quote:
Sounds like some interference is causing crosstalk somewhere. Have you used shielded cable or 'twisted pair' for the speedo sensor and any
other input related stuff? I had chronic misfire problems until I shielded my crank sensor wire.
All the wiring is pretty standard stuff, no shielding. Twisted pair I suppose as there are only two wires from the speedo sensor.
I tried shielding the wires coming out of the distributor electronic module to the coil, but this didn't have any effect.
The speedo sensor is toward the rear of the car by the prop, so I would be amazed if that was picking up any interference (who knows??).
Hope the answers are some use.
Thanks again guys.
All the best
Dave
[Edited on 7/7/10 by Dave J]
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Canada EH!
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| posted on 7/7/10 at 01:39 PM |
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Sounds like electrical interference. The 60's Corvettes had shielded distributors and wires on models equiped with radios. do you have a GRP
Scuttle? I had to shield the ignition on my fathers GRP boat for the same reason.
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Dave J
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| posted on 8/7/10 at 07:27 AM |
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Canada EH! wrote:
quote:
Sounds like electrical interference. The 60's Corvettes had shielded distributors and wires on models equiped with radios. do you have a GRP
Scuttle? I had to shield the ignition on my fathers GRP boat for the same reason.
Thanks for your thoughts. I do indeed have a grp scuttle. It has crossed my mind to put a metal shield in front to see if that will shield any
interference, but this will involve some serious work, so I was hoping somebody might have had a similar experience with a simpler fix.
I'll probably finish up shielding everything in sight.
I'm wondering if there is a coil issue, as somebody mentioned that a different coil is needed for electronic ignition over points type.
I've never come across this, anybody know if this is the case??
Cheers
Dave
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