could some please tell me how to mod my Tach to work with a V8?? I searched the archives but only came up with vague references. I would like someone to take my hand and skip happily along spelling out every little detail. like capacitor or resistor values and where to put them and so forth. pictures would be great as well. BTW i am familiar with working on circuit boards and the tach is an '86 GT tach.

Thanks,
Tyler

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85 LT4 GT
Custom Everything
in progress

[This message has been edited by GT-X (edited 06-14-2003).]

i forgot what value it had to be but if you search the forum you will find it, maybe in the arcives... I havent seen the back of a Tach board in so long bot i belive it is in the lower middle, you will know it when you see it. anyways there are pictures and such somewhere on this forum, if not ask Archie. When I had mine done it took a long time for my local electronic place to do it because they had a ahrd time finding that exact value, they said it was a very odd one.

not that that helped you at all but its 1:50 im going to bed...
Kyle

You need to change the capacitor value to 0.049 uf for the V8. This is done by adding several capacitors together to get the proper value. You can use a 0.047 uf and two 0.001 uf capacitors to accomplish this. They get soldered where the original one is. Make sure you have them soldered in with the correct polarity. The stock capacitor is located between the circuit board and the faceplate, towards the top around the 6000 rpm mark. I got this information from The Big Guide to Fiero Kits & Parts book.

[This message has been edited by avengador1 (edited 06-14-2003).]

thats too wierd, i was just gonna ask that question thanks, avengador1 you dont know how much you just helped out

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Rich
* '86 se: , 2" drop, coil overs, KYB's., ported manifolds
* '86se: ,350 bored .030 over,World Products s/r Torquers - ported, Manley valves, edelbrock 750cfm, carb sanderson cc90's,4:10,Spec stage 3copper puck clutch , adj.Koni's, Eibach's,polly. and lots more!!
*'92 BMW 325i : borla exhaust, ST brace, cold air intake

I think the value for the new capacitor is .064, not .049.

Another method is to install a variable resistor.

You can read about it here: http://home.t-online.de/home/O_Scholz/tach.html

Actually, all of those values are incorrect.

The "precise" value you want is a capacitor with a value of 0.0064 uF (that's micro-farads for those of you who aren't familiar with electronics). It's a particularly small value, but as stated, you may have to combine a couple of capacitors together to obtain this value.

The capacitor you want to replace is at the very top of the tach circuit board. If the car was originally a 4 cylinder, the cap that is there now will have a value of 0.01 uF. If it was originally a V6, the value up the current cap will be 0.0082 uF.

Pull out the old capacitor, and solder in one (or a combination of caps) that has a value of 0.0064 uF.

I don't want to appear as if I'm nit-picking, but a couple of zeros DOES make a world of difference. The tach just simply wouldn't work correctly if you put in a capacitor with a vaule of 0.064 uF.

I already modified mine eons ago. If you would like photos, let me know...

yes photos please and thanks for the correct value you all get +'s also multiple capacitors are to go in series correct????

Tyler

Actually, caps are different than resistors - I believe they go in parallel. I'm off to work, but I'll post pics later today for you.

They go in parallel. I just posted the value that was written in the article on how to do it. If someone else has done this and he has the correct value, use that value.

the more pics, the better

I apologize for the delay, but I removed my working tach out of my car so I could take photos to benefit everyone who wants to know & see how it is accomplished.

As you'll see by the photos, I wired in 3 capacitors in parallel to get a value of 0.0064uF. The two large yellow caps in the photos have a value of 0.0027uF, and the green one has a value of 0.001uF. You can use any combination of capacitors you wish, just as long as they add up to 0.0064uF. This is a timing circuit, so it's critical you have the correct value in order to have an accurate tach.

Note: You'll see my caps don't have a specific polarity. Caps that have a polarity are of the wrong type (electrolytic capacitors are a good example of what NOT to use). You can get these at any electronics store (forget places like Radio Shack - I'm talking about "real" electronic supply stores). Anyone who knows how to solder can do this conversion in a manner of minutes.

Any questions, feel free to ask. Most things should be fairly evident by the photos. As you will see, I took a bunch of shots at varying angles.

i have an MSD 6AL , will that compensate for my tack bieng off, or do i still need to do this mod?

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Rich
* '86 se: , 2" drop, coil overs, KYB's., ported manifolds
* '86se: ,350 bored .030 over,World Products s/r Torquers - ported, Manley valves, edelbrock 750cfm, carb sanderson cc90's,4:10,Spec stage 3copper puck clutch , adj.Koni's, Eibach's,polly. and lots more!!
*'92 BMW 325i : borla exhaust, ST brace, cold air intake

Forgive my ignorance, but whats an MSD 6AL?

If you have a V8, you've got to modify the tach. I've seen three or four completely different methods on the net for this. One method involves cutting & hacking away at the circuit board to remove some traces, and also putting in a variable cap. The variable cap leaves room for error as far as I'm concerned. I think this method is a hack at best.

I've seen other methods that also involve cutting some traces, and adding in some resistors. A better solution, but I feel that by removing the original capacitor & replacing it with a specific value for the type of engine is by far the easiest, and least time consuming. Just my opinion....

When I found the method of just replacing the original capacitor with one of a different value to get the desired results, without having to cut anything, I knew I'd be happy with the results.

If this doesn't answer your question, just explain to me what an MSD 6AL is.

http://www.msdignition.com

go under automotive, and look it up

[This message has been edited by $Rich$ (edited 06-17-2003).]

Ok, it's an ignition enhancement. The tach is what "reads" the ignition pulses and translates that into crankshaft rotations that you read as RPMs on your gauge. Basically, it's a measuring device. You have an enhanced ignition system - it's not going to change what the tach reads.

You'll still need to perform the modification to your tach in order for it to read correctly.

so, i need to know what resistor(s) to replace for a tach. out of an 86 V6 is it the same modification as a 4 cyl??

Why dont you ask archie? I am sure he would do it. And "dont quote me" for like $30 bucks.

because im fully capeable of doing it myself for the $3-ish the resistors cost

I just ordered from Digi-key 10 sets of the capacitors necessary to do the mod (yeah, they have a minimum order). I'll have 9 sets left over after I do it. I'll mail them out for a couple of dollars a set through paypal if people want them. It's the same combination that Lambykin used: 2x0.0027 and 0.001. They are ceramic caps with very tight tolerances.

Rich - as was just stated, and as I stated originally when this thread began, you are replacing a capacitor, not a resistor.

Yes, the modification is the same regardless of what factory engine you have. If you have a tach from a 4 cylinder, and a tach from a V6, you'll see that the capacitor on each one is of a different value. Modifying either tach to a V8 means removing the original capacitor, and replacing it with the value stated earlier in the thread.

Will - I used the same combination of capacitors.

This conversion can be done for only a couple of bucks, and your own time. One of the members here asked me if I'd convert his tach for him - I'm waiting for it to arrive as we speak.

quote Originally posted by Lambykin: Rich - as was just stated, and as I stated originally when this thread began, you are replacing a capacitor, not a resistor. Yes, the modification is the same regardless of what factory engine you have. If you have a tach from a 4 cylinder, and a tach from a V6, you'll see that the capacitor on each one is of a different value. Modifying either tach to a V8 means removing the original capacitor, and replacing it with the value stated earlier in the thread. Will - I used the same combination of capacitors. This conversion can be done for only a couple of bucks, and your own time. One of the members here asked me if I'd convert his tach for him - I'm waiting for it to arrive as we speak.

Yes, capacitors, not resistors, i know, it was just a brain fart, thanks for the info, a + for all who helped

No problem - just wanted to be sure you knew it was a capacitor. Replacing the cap with a resistor would likely produce one funky tach.

quote Originally posted by Lambykin: I apologize for the delay, but I removed my working tach out of my car so I could take photos to benefit everyone who wants to know & see how it is accomplished. As you'll see by the photos, I wired in 3 capacitors in parallel to get a value of 0.0064uF. The two large yellow caps in the photos have a value of 0.0027uF, and the green one has a value of 0.001uF. You can use any combination of capacitors you wish, just as long as they add up to 0.0064uF. This is a timing circuit, so it's critical you have the correct value in order to have an accurate tach. Note: You'll see my caps don't have a specific polarity. Caps that have a polarity are of the wrong type (electrolytic capacitors are a good example of what NOT to use). You can get these at any electronics store (forget places like Radio Shack - I'm talking about "real" electronic supply stores). Anyone who knows how to solder can do this conversion in a manner of minutes.

Thanks Lambykin! You've REALLY covered this topic excellently!!!! Can you do the same thing (or something similar) to convert an 85 MPH Speedo to 120? I bought white faced overlays (which appear to be of VERY HIGH Quality) from ProCarparts.com.

Thanx,

Rob

I actually haven't looked into, or even researched, upgrading the speedo yet. Mine is the same as yours - an 85mph speedo (but I'm powered by a V8).

I really shouldn't get all the credit - I spent about a week researching the tach modification, and finally found the correct component, and the correct capacitor value to use. I found a lot of misinformation out there on the 'net, and MANY different ways to do the modification.

I'm sure each modification has it's merits, but while I have the skills and the equipment, I wanted to keep this as simple as possible. This is the simplest method, and doesn't ruin your circuit board.

Other methods involved adding components, and cutting traces. Some spoke of resistors, others involved adding a variable capacitor (which you'd have to adjust - unless you have the proper equipment, you'd never be able to get it 100% accurate).

I'm not about to knock these other methods - someone somewhere obviously sat down and studied the tach to come up with these other modifications. Since there were a bunch of choices out there, I had the luxury of choosing which one I liked best.

Anyhow, I'm glad the information helped you. Hopefully it'll help others in the future. Best thing about it is you don't require any electronics background. Anyone can learn to solder, and anyone can visit an electronics store to get assistance in purchasing the required capacitors.

If I do end up converting my speedo, I'll document that as well. Right now that's low on my list. I'm just trying to resolve some brake line issues.

Will, you have a PM

Wow, the caps haven't even gotten here yet and I already have requests! If you've contacted me, I'll get back in touch with you when the caps arrive.

COOL!

Thanks, man!

Caps got here today. I'll do the mod to my car sometime in the next few days and when I'm certain that I have the right caps, I'll get in touch with those people who contacted me.

I will take a set of caps if you still want to sell them. I'm doing a V8 conversion now. Motor should be in this week. Let me know were to send the bucks.

quote Originally posted by Rob Ernst: ... Can you do the same thing (or something similar) to convert an 85 MPH Speedo to 120? ... Thanx, Rob

The easiest way to convert from an 85 MPH to a 120 MPH is to buy one on eBay. It's a drop in replacement. I put on on my coupe and it works great.

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If you find my advice useful, then please give me a positive rating, thanks...

Firebird Interior Installation Thread

I finally had time to install the caps in my tach. It seems to work right at low RPM, but winds out to ~7100 while my rev limiter is set for 6400. I called Digi-key and there was some confusion with labeling on the website and what I actually got was +/-10% caps. If anybody still wants some I'll send them to you, but I found some radial packaged 2% caps which I'm ordering tonight.

I did the variable resistor mod as posted in the above link and it works like a charm. Very easy to do and you can "tune" the tach for accuracy.

Reguarding the resistor, I have found reference here that people have used a 210k ohm one, but it seems that it is out a bit. From my calculations, the resistor should be 187k ohms - now this is not verified by actually testing it, but that is what I came up with.

Most electronic parts you buy off the shelf are at 10% - also available are 5%, 2% and 1%.

Tim

I found some axial capacitors with a capacitance of 0.0068 uF, how far off will my tach be? Also, to modify your speedo from 85 to 120, you need to replace the crystal at the top of the board with a 4.43 Mhz crystal. Thanks!

I got the 2% caps and installed a set in my car. My tach reads 6.6-6.7 when I'm on my 6.4 rev limiter. I'm more or less satisfied with that, as that's the closest I can get the caps, and changing the resistor is more work for a pretty small benefit.

If those of you who contacted me are still interested in caps, shoot me an e-mail or PM. I'll probably use Paypal to the tune of a couple of dollars.