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Tach

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Matt Wood

Tach

So, installed a new cluster late last year with a tach, worked great. now after sitting over the winter the tach has stopped working... i have a bench power supply so have means of testing it (i think) but cant find any info on what exactly the parameters would be, i don't want to blow anything up as being in the uk they are basically gold dust!

Any suggestions or diagrams would be greatly appreciated

Thanks

Matt

'Shelby' - 1985 bronco XLT, 351W C6, Limited slip 9 inch

Gary Lewis

Re: Tach

Administrator

Matt - It isn't unusual for the tach not to work in the cold. I've had them do that and then start working when warm. But I don't know why that happens.

As for how to test, about the minimum is 600 RPM = 10 Rev/Sec. And at 4 firings/rev you need a minimum of 40 pulses/sec.

I don't know what it takes to trigger the tach, but the signal it sees is from the primary side of the coil. It goes to battery voltage and then is pulled down almost to ground. But there is a lot of ringing and the voltage goes to something like 40 volts when the signal is allowed to go back "high".

If you have an extra distributor, coil, and ignition module you could spin the dizzy with a drill. But if not, do you have a signal generator?

How about an extra vehicle speed sensor from a speed control? That might work as I've seen 15 volts peak/peak on them.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

Matt Wood

Re: Tach

oddly enough the temperature may be the culprit, i remember starting it up just after Christmas and the tach took a second to 'jump' into life, i've taken the cluster out for bench testing but i may wait as there are too many unknowns for me to feel comfortable trying to emulate the coil. I do have access to a signal generator but again im not confident enough to want to start the guessing game on what could be the right voltages and pulse frequencies. I do actually have a Dakota Digital tach controller set up to take signal form the coil and output to my phone (as well as all other sensors) as you would expect from any new OBDII reader. I did this when i was having issues and no 'real' data from the gauges. I could probably emulate the coil from that and get the tach to work? I am going to try the cluster first thing in the morning and see if the temperature has made a difference.

Thanks for your thoughts! ill post back in the morning with my results (Hopefully positive!)

'Shelby' - 1985 bronco XLT, 351W C6, Limited slip 9 inch

Gary Lewis

Re: Tach

Administrator

As usual, Matt, it is already morning there.

On the signal generator, if you dial in 12v peak/peak in sine wave mode and 40 pps you should see a reading of about 600 RPM. Oh wait! That assumes you ground the terminal that says GND. Otherwise you'll be in 6 cyl mode and it should read 800 RPM.

Gary, AKA "Gary fellow": Profile

Matt Wood

Re: Tach

While I can burn the candle at both ends I absolutely will! One day it will catch up to me and maybe ill go to sleep at a reasonable time!

I will give it a go if the tach doesn't work in the morning, really hope nothing goes wrong! Tach clusters are hard enough to get... let alone over here 😂😂 this one cost me well over $180 once it was all shipped....

'Shelby' - 1985 bronco XLT, 351W C6, Limited slip 9 inch

Gary Lewis

Re: Tach

Administrator

Check the circuit board. If I remember correctly some have found "yuk" on their boards and gotten the tach to work again by cleaning the board with isopropyl alcohol and a cotton bud.

We do have a schematic on the tachs on the page at Documentation/Electrical/Tachometers.

Gary, AKA "Gary fellow": Profile

Lima Delta

Re: Tach

I just cleaned the "yuk" from a tach that I pulled out of one of my parts trucks (one of the closed-back type tachs). After reading about it somewhere I had a look and certainly there was enough of it on the circuit board that if it was at all conductive, it would be causing problems. Cleaned up easily enough though.
I'd like to install it soon to see how it works, but waiting for the weather to warm up first since I don't like messing with the plastic parts and wiring in the frigid cold - my kingdom for a heated garage!

Lucas

Lucas
"The truck" - 1985 regular cab F250 4x4 - 351W HO, C6
"Beige Beast" (project) - 1981 regular cab F250 4x4 - 300 straight six, T18

Matt Wood

Re: Tach

In reply to this post by Gary Lewis

thanks guys, after a night in the warm i plugged the cluster back in and as if like magic...

seems like it doesnt like the cold lol, really strange as to why... if it keeps up i will look into it further and try and find a solution for everyone

'Shelby' - 1985 bronco XLT, 351W C6, Limited slip 9 inch

Gary Lewis

Re: Tach

Administrator

Glad it works, Matt. But if you pull it again see if there's "yuk" on the PCB. I've read that it can cause problems.

Gary, AKA "Gary fellow": Profile

Matt Wood

Re: Tach

i will have a look when i next have it out, how much do you think could be back there? do you think it has low conductivity high resistance and its enough to change the voltage drop on the input? (to below 6.2v?) must be some effective 'yuk' to achieve this lol

'Shelby' - 1985 bronco XLT, 351W C6, Limited slip 9 inch

Gary Lewis

Re: Tach

Administrator

It looks to be rosin from the wave soldering, from what I've seen. But it wouldn't have to drop the input down. There are other circuits in that system that might stop it.

Gary, AKA "Gary fellow": Profile