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tachometer wiring

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layaj2 Andy Lay
Bisley, Surrey, UK   GBR
Dear All

On a previous discussion I mentioned that my tach was running at about twice the correct RPM. I decided to have a look at it during the recent bad weather and found a weird set up with the tach wiring far from standard. The car is +ve earth. I sent the tach away to a local specialist recommended by the owners club and he said the innards were for electronic ignition which my car doesn't have, so he replaced it at a very decent price, (pic with yellow wires with loop which fro other diagrams I have looks correct).

What’s really thrown me is that a single black wire that came out of the rev counter went to the CB or +ve side of the coil. This black is clearly non-standard to the car as I can see it’s been run outside of the loom and isn’t on the wiring diagram at all. (pic with black wire and sticker from a previous re-conditioner)

I found a white wire behind the rev counter that a) has a loop in it around a plastic/nylon pick up on it and had a soldered join in it. (see pic)

When I unwrapped the black insulating tape, de-soldered the join and got my meter and buzzer out, I found one end of the wire went to the ignition switch. When I connected the meter (-ve lead) to the wire and the other side (+ve lead) to earth I got 12v after I turned on the ignition so I assume that’s the “hot” side of the switch.

The other end of the white wire went to the –ve side of the coil. According to the wiring diagram for the BJ8, that must be the original wiring from the coil to the ignition switch via the rev counter loop.

My understanding for a +ve earth car is that CB is the +ve side (my coil says + and -). From what I can tell, if the car is +ve earth, then the wire to the switch via the rev counter loop comes from the –ve or SW side of the coil.

I checked the green wire which goes to the spade connector on the back of the rev counter and I get 12v there OK as well.

Can I just wire it up as per the factory wiring diagram, and if so does it matter which way round the looped wires are connected - any words of wisdom please as my main fear is blowing it up!

thanks

Andy

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RAC68 Avatar
RAC68 Raymond Carbone
NJ, Jersey Shore, USA   USA
Hi Andy,

Based upon the pictured differences to a standard installation of the electronic tachometer within the BJ8, I can not even speculate what should be changed to bring your installation back into original compliance. I would suggest you go to John Sim's Healey6 web site ( http://www.healey6.com/technical ) under the Gauge subsection, where there are a number of documents on the BJ8 Tachometer.

Keep in mind that the coiled direction, through the plastic fixture, of the original white wire which extends from ignition key to hot side of the ignition is dependent upon the polarity of the car. The coils purpose is to present a firing signal for the tachometer to sense for calculation and presentation of RPM. Additionally, gauge case grounding is usually supplied by a ground wire connected on one of the mounting studs and secured by the mounting hand-screws.

Again, not knowing what exactly was done to the tachometer leaves too much open for nothing more then grossly speculate and your review of the the documents at Healey6 provide a better probability of you discovering resolutions to your issues.

All the best,
Ray(64BJ8P1)

layaj2 Andy Lay
Bisley, Surrey, UK   GBR
Hi Ray

Thanks very much for your input. I had a go the other night, followed the factory wiring and tried it - nothing. Then I reversed the connections on the yellow wires - bingo!

I haven’t taken the car out yet but it certainly seems more like it should at idle - it's showing <1000 rpm not 2000 rpm which is a breakthrough indeed. A few revs look far more normal too. I'll get a dwell/tach on it some time to see how accurate it actually is.

I have used Healey6 in the past too - very useful thanks.

When the chap who replaced my tach tried the old one he said it was accurate to 2 RPM, so I'll never know whether re-wiring would have done it (he said it was configured for electronic ignition), but as you say, given the state of the wiring on the old tach I'm happy with what I have. Also he charged just of a third of the price of any of the main AH suppliers.

regards

Andy

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RAC68 Avatar
RAC68 Raymond Carbone
NJ, Jersey Shore, USA   USA
Hi Andy,

Glad you have gotten the tachometer to work. When my tachometer seemed to be registering high, I was considering the conversion to a neg. ground electrical conversion. During the change, I followed the document that described the condition and replaced the faulty 0.2 micro fared capacitor. The document also described the presence of an adjustable pot (potentiometer) on the tach's circuit board that can be accessed by making a hole in the back of the case and adjusting. I would follow the document and adjust to reflect the reading of a hand held tachometer. That was in the mid 1990s and my tachometer has been reasonably accurate ever since (Keep in mind I use my tachometer to provide a perspective and need not be accurate to the revolution).

Taken from the document:
"If this turns out to be the problem and the tach then appears to work it must then be calibrated. There is an adjustable pot on the back of the circuit board with a small screw driver slot facing the inside of the back of the tach case. I drill a 1/2" hole in the tach case at that location so I can access the adjustment with the unit fully assembled. Reconnect in the car then just compare it to a known good tach/dwell meter and slowly adjust the pot to read the same at 1000, 2000 and 3000rpm. Should be good everywhere after that. Tape over the hole when done to keep out dust and moisture."

All the best,
Ray(64BJ8P1)

pkoot Avatar
pkoot Peter K
Alkmaar, NH, Netherlands   NLD
I once received this document from Smith. Maybe it can help you.

Peter


Attachments:
Smith Tachs.pdf    358 KB

layaj2 Andy Lay
Bisley, Surrey, UK   GBR
Hi Peter

Thanks for this too. I have that article from somewhere and I agree it's very useful. I used it to check wiring with the person who replaced my tach.

regards

Andy

RAC68 Avatar
RAC68 Raymond Carbone
NJ, Jersey Shore, USA   USA
Hi All,

From what I understand, the calibration screw in is only Peter's drawing is for their later versions. Our tachometer does not have this adjuster on the outside of the case. Per my last post, the rear of the case must be removed and the pot with a screwdriver slit identified and a whole drilled through the back in line with the pot. This will allow you to do everything described in Peter's document. A piece of tape can be used to seal the hole once the adjustment is made.

Ray(64BJ8P1)

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