View Full Version : tachy story

tahoe healey
01-14-2003, 10:25 PM
Some people shouldn't be allowed to have a day off.The rev counter on my BJ8 had been entumbed in duct tape by the previous owner. I've been curious why since I got it cause ir seemed to work fine. So I removed it and took off the tape and when I put it back in I mistakenly connected the green wire where the black wire went. Smoke! Found the problem and the fried green wire. Fuse is ok. Now the tach doesn't work. I by-passed the green wire but still wouldn't work. Since the wires where not internal to the tach I assumed there could be no damage to the tach. Is there any why to test it? It has that white wire loop at the back. Can this wire be tested. Any thoughts? I hate it when this happens! graemlins/sad.gif

01-15-2003, 07:29 AM
Hello Tahoe,

Was the black wire connected to where the green wire was connected when you attached the green wire to the tacho, if so it probably has damaged some electronic component. That would have put a reverse polarity feed to the unit and electronics don't like that. (The white wire is the feed to the coil and the tachometer senses and measures those pulses to give a reading.)
You might as well open it up and have a look to see if it's repairable. (try a TV repair shop or the like)
It is worth double checking that you have a supply voltage on that green wire, with the ignition on.

Alec graemlins/cheers.gif

01-15-2003, 10:50 AM
Is you car converted to negative ground? If it is, the previous owner took the easy way to convert the tach. He isolated the case of the tach and swapped the wires externaly.

If you car is negative ground, the innards of the tach are most likely (99%) damaged. Fortunately, the innards are rather simple (in modern electronic terms)so this damage is usually repairable. The only real exception is the meter movement, if it is cooked, it is best to source a new one.

I have a schematic of the tach circuit if you are inclined to repair it yourself. If you would rather send it out, there are several reputable services that offer instrument repair services. Their names escape me at the moment, but I'm sure one of the other contributors could offer a recommendation.

tahoe healey
01-15-2003, 03:14 PM
Thanks for the replies, Alex and ThomP. Yes, I have negative ground. (What is the advantage to doing that?) I didn't put the duct tape and insulation together in my mind. It wouldn't have been my first choice since it wont even stay on ducts. The schematics would be very nice. Can the tach be converted to neg ground? Or should I convert back to positive ground so I can trust the factory information? Anyone out there know the names of some quality service places in case the repair/conversion is beyond me?

01-15-2003, 08:19 PM
The tach is "a piece of cake"* to convert. Two connections inside. You may as well do it while you're fixin it!

Negative ground has the advantage of being compatible with modern automotive accessories; Radios, CD players, 8-tracks, etc. To the best of my knowledge, this is the ONLY advantage.

I'll generate a PDF of the drawing and email it to you or, if you perfer, I can fax it.

* "piece of cake" your POC may not be the same as my POC


tahoe healey
01-16-2003, 04:01 AM
Thom, thanks. I received the schematic.It does look simple but I don't have the test equipement. I know people at Radio Shack who may be able to help me. If cost isn't prohibative, I might replace all components in question if available.
The power (black) and ground (green) can be re-wired so duct tape insulation won't be needed? That would be great.
Thanks again.

01-16-2003, 08:13 PM
Here is a phone number for a firm that repairs tachomenters: Nisonger Instrument Services (914) 381 1952 EST. I thought this might help you decide which way to go with your tach. Give them a call and see how much it will cost and compare it to the effort you'll have to put in to get it back in good shape.


tahoe healey
01-18-2003, 09:41 PM
Thank you, again. Today I took the tach apart and found everything looked good. No burns, no discoloration, no singed parts and needle moves freely. I guess I'd better check the electrical as the green wire looks suspicious.
I am having trouble figuring out how to re-wire the tach so the case is neg (I don't have to insulate the case with tape again). The tab is easy but the case appears to be wired via the screws that hold the mech in place??? images/icons/confused.gif

Gary Lloyd
01-18-2003, 10:31 PM
Having never seen a Healey tach, but converting a couple of MGB tach, which I would assume are the same, it is simple!! Just go to Moss's website and it is right there!!! You just cut the white wire and reverse it and a couple of other simple things!!

tahoe healey
01-18-2003, 11:07 PM
Thanks Gary. All the stuff has been changed except the case wires. I couldn't get anything on tachs at Moss under Austin Healeys or MGB. Where do I find it? I do have other instructions of switching to neg ground but It doesn't discuss the case.

Gary Lloyd
01-18-2003, 11:16 PM
In the tech tip section!! If you want, I will see if I can find an old catalogue around here and fax it to you, if you want!! I have noticed that most Healey and MGB gauges are the same, just in a fancier case images/icons/wink.gif

tahoe healey
01-19-2003, 03:15 AM
Thanks Gary, that would be great. Any help would be appreciated. I could't get anywhere on the Moss sight. Tachs were not an option.

01-19-2003, 04:46 AM
I recently did this on the bugeye electric tach. Assuming its similar, you remove the 2 brass screws from the back that do not have "cut outs" in the case housing, then remove the chrome ring from the front ( slow going, use a little screwdriver to pry it loose all around the edge) once you have the body out you see that the spade terminal in the back of the housing has a resistor soldered to it. You unsolder this, then unsolder the wire that is on the terminal directly near it. Put the wire to the spade terminal, put the resistor to where the wire was. Presto! Have to switch the white wire passing through the back but since i had no idea what way it originally was (out of car no harness) I just put bullet connectors on each end, that way if its backwards, switch it in a minute to correct. Good luck -Josh

PS you dont "convert" the case to - ground, you convert the tach to + in, the case then grounds through the mount brackets/screws or a tied ground to them. you only need 2 wires +/- because it was isolated, its own system, now it is part of the whole.

[ 01-19-2003: Message edited by: jha2297255 ]</p>

01-19-2003, 04:59 AM
ThomP - ground has another distinct advantage. It slows the formation of rust, or it does not promote it as much as + ground does. We had a long long talk about this on MM message board, AH mag originally posted it in a convert your tach article. -Josh

01-19-2003, 12:05 PM

Went to the MM "way back machine" I din't see the reference. Do you know the topic title or can you supply me with the link? I like to read the thresd.

And, how did you know I'd know what the MM Message board was?


tahoe healey
01-19-2003, 01:25 PM
I may be wrong but I think the case is charged. The mech has two screws that hold it in the case. The case has a black wire for the light. The spade has a green wire when + grounded. I believe these are switched when - grounded. This puts a dangerous potential on the case if not insulated. The chrome bezelon is still hot and opposite charge from the speedo's. I'm thinking I may go back to + ground if I cannot find a solution.
Thanks all

Gary Lloyd
01-19-2003, 01:50 PM
It sounds totally different than a 'B tach!! B's had mechanical tachs until late '64!! Sorry I couldn't help!

01-20-2003, 05:08 AM
i think the topic was originally pos to - ground switching. Its been awhile. I guessed you knew minimania.

01-20-2003, 05:21 AM
ah figured out how to post reply in a new window. nice. Tahoe healey-i recently build a from scratch wiring harness for the bugeye. I am sure I can help you figure this out.

the tach- has a light in it, a spade terminal, 4 brass screws in the back, a white wire pasing through a plastic screw on H type piece. What you are doing:
You are changing it so that + wire is going into the spade terminal. Originally the car body was + and - wire went to the spade terminal. The gage always grounds through the body, except that, the previous owner of yours isolated the gage. Isolated means that it needed to have both + and - wires going to it to work. The gage now will have only a + wire going into the spade terminal. If you take the gage apart, you will see that the spade terminal is isolated from the body. The electricy flows from the spade terminal, through the actuating mechanism for the rev indicator and finally grounds to the body. SImple eh? You are thinking that the spade terminal is direct wired to the body, but it is not. You need to have + power into the spade terminal. On my bugeye, I used the power in feed from the dash lamps, obviously, before the switch. The amp draw is low, so you can feed it off any ignition switched + wire. or wire directly from the ign switch. Draw a diagram to help you see. Now the light, the light will also have + in from the dash switch presumably. Your car may or may not have a chain of ground wires. Ground here means -. The chain would go from gage to gage, with ring terminals under the thumbscrews. I did this, but my dash was coated with POR. Your gages may ground without the chain of - wire. The - wire only provides a path back to the battery. If you want you could take the wires directly to the - battery terminal, it makes no difference. the metal of the dash simply acts like one giant wire to the - bat terminal. make it easy on yourself, only use black wire for "grounds", - feed. Buy some wire at jcwhitney or get type 3 marine wire if you are OCD like me. I will be more specific if you need help. Draw it out and you will see, dont worry about wire colors, you could wire a car in all pink wire if you wanted, they are only there to make identification easy later on. -Josh

If i figure out how ill post a diagram for you

01-20-2003, 05:37 AM
&lt;img src="https://therobinsnest.50g.com/car%20stuff/tach%20wiring.jpg" &gt;
this is very simplified. It has no switches, but once you understand you can put the switches in. Also , the blue wires might not really exist. They symbolize the metal dash. Let me know if you follow-Josh


[ 01-20-2003: Message edited by: jha2297255 ]

[ 01-20-2003: Message edited by: jha2297255 ]</p>

tahoe healey
01-21-2003, 12:07 PM
Thanek you Josh. You must be a teacher. What a great explaination. But this is the point at which the problem begins.
When the car has a + ground the spade has a - lead. When changed to - ground, a + lead on the spade. This reverses the flow to the electronics which is not good for them. Some instructions mention reversing the internal wiring but I cannot figure this part out because the case is screwed to the mechansim forming one of the contacts needed to be switched. Or am I worng in this and only the spade and ground wires needed to be switched externally? Thom, that's what I gathered from your schematics of the circuts you sent me.
All of you guys have been grest! I'm sorry to be so dense.
Rich graemlins/square.gif

01-21-2003, 06:46 PM
you have to take it apart and reverse the internal connections too. Its very straightforward. The hardest part is getting the chrome ring off, after 30-40 years of being held on. Once the glass is off and the black ring removed, the whole gage slips out of the metal case. You then unsolder the resistor that is on the back terminal and move it to the adjacent terminal. Switch the wire that was on terminal 2 to the spade connector terminal. Then presto! -Josh

tahoe healey
01-22-2003, 02:32 AM
This is the problem with the conversion. There is no post. It utilizes the internal frame which atches to the case. TI cannot beleive there are no others with this problem. It's the original tack for a 1964 BJ8.

01-22-2003, 04:38 AM
does the back of your tach look similar to this?-Josh

the green circled part is the only thing that needs switched inside. If you had/have wires coming from the 4 brass screws, the PO was using those as grounds, they are not necessary after you convert it.

[ 01-22-2003: Message edited by: jha2297255 ]</p>

tahoe healey
01-22-2003, 03:00 PM
It is,I beleive, the same as your picture (Thanks). Several sights I found mention this internal switching between spade to post. But "I got no stinkin" post. Secondly, this is DC. You are reversing the direction of flow of electricity to capacitors. Granted it is only 12 volts, but they fry and/or malfunction. Or am I wrong? I am supprised there haven't been failures.

tahoe healey
01-22-2003, 06:53 PM
But where's the fun and what wood we talk about.

01-22-2003, 09:05 PM
ah you are making it complicated in your mind. I did this switch from an old AH mag article. Its easy. I have my revinyled dash 90% assembled or i could take the tach apart and show you. Do you have a digital camera? Take a picture of the inside of the tach for me and i will show you what is swapped. Email me the pic if you get a chance. Pictures make it easy. -=Josh

PS capacitors, like most electrical things, are not load sensitive. i could get long winded here and go into why we have polarized plugs on lamps now, but seriously dont worry about it. It will work.

[ 01-22-2003: Message edited by: jha2297255 ]</p>

01-23-2003, 04:25 AM

If you get fed up fiddling with your Tach's you could always fit a more modern type, which will not waver and will be accurate, together with matching Speedometer.


www.cape-international.co.uk (https://www.cape-international.co.uk)

Cheers graemlins/savewave.gif graemlins/savewave.gif

01-26-2003, 04:33 AM
Tahoe healey

are you still working on this? I found a spare tach, If you are interested I can convert it to - ground and ship it, trade or? let me know. -Josh

tahoe healey
02-04-2003, 10:35 PM
Thank you all for your advise and imput. It helped me a lot. ThomP understood my concerns regarding the revers of charge to the electronics and devised a scheem that wo;rked perfectly. I now have a neg case and neg flow to the spade (as it is on positive ground). He isolated the mech from the case and supplied a tab direct to it to accept the positive lead(as the mech had with the original positive ground). Thom took his time to work this out and proformed the instalation on my tach. He went way beyond anything usual in helping me. This is what this forum is about. All he asked of me was to pass the favor on when the opportunity comes my way.
Thank you Thom and all of you how replaied.
Rich graemlins/cheers.gif graemlins/patriot.gif