Will a points tach work with GM HEI?

[Bonnewagon]

I knew a guy with a 1985 Grand prix. He put a Pontiac 455 in it and had a GTO hood tach on the hood too. But he ran the stock HEI ignition. I asked how he got the old style hood tach to work with a modern HEI but he did not know. A shop had rigged it up for him. Is this possible? I thought the points tach would NOT work with anything but points. I know there are aftermarket hood tachs that will work with anything. He claimed he paid a fortune for a genuine GTO hood tach so I am baffled. I know that when I had my '68 Firebird and I put an HEI in, the Cal Custom Hawk tach would not work. I eventually used an MSD box and even that needed the MSD adapter for the old tach to work. I am asking because I have a couple of original Firebird hood tachs doing nothing right now.

 

[Wageslave]

IIRC, you have to run a tach signal filter to get a points tach to reliably work with an HEI system.

 

[Bonnewagon]

IIRC, you have to run a tach signal filter to get a points tach to reliably work with an HEI system.

Wageslave is that all there is to it? I remember when pontiacgp sold me a set of GP dash gauges there was supposed to be a tach filter in the wire somewhere. My Bonnewagon was not equipped with a tach from the factory so I did not have the filter. His car did and he eventually found it buried under the dash. So if my guy's GP had a factory tach, with the filter, the old GTO tach would have worked?

 

[spidereyes455]

Mark, many years ago I had an issue where my 71 lemans wouldn't start one day. I messed with it for days and couldn't figure it out. It still had points btw. I eventually got disgusted and stabbed an HEI distributor in it and it roared to life. The point of the whole long winded story is it has an original in dash tach in it, it didn't leave the factory with it but someone installed an original long before reproductions were available. And it allways worked flawlessly with the hei even without changing anything else and it also had no tach filter. I've since changed it back over to points when I revived the car 2 or 3 years ago and it still works perfectly. Also the factory original in dash tach in my brothers 69 chevelle works perfectly with HEI and no modifications whatsoever. And that tach was plugged into an HEI all the way back to the early 80's when our dad used it as a daily driver and it allways worked then too.

 

[Bonnewagon]

Jim, you just made me totally confused again. Some guys say it needs a filter or it will bounce around. Some say none needed. Maybe it just does what it wants? I think I need to just test one and see. I am finally working on my 1968 Firebird project. I have a 400 hood that takes a hood tach. And my Bonnewagon would sure look cool with a hood tach too! I have been saving these original tachs forever.

 

[CopperNick]

My 85 came with a tach and there was/is a tach filter mounted on the firewall. The original timer was the CCC controlled version but the controller unit had long been deccommissioned so I swapped in an HEI and kept the filter. Tach worked just fine. Factory gauges are all analogue.



Nick

 

[spidereyes455]

It's not very difficult to hook one up to see if it works. I do recall that original 60's and 70's tachs supposedly will not work and can be damaged by the aftermarket electronic breaker less ignition systems like pertronix.

 

[Bonnewagon]

can be damaged by the aftermarket electronic breaker less ignition systems like pertronix.

And yet a guy I know in Witchita has the Pertronix HEI- and his 1968 tach works just fine. He had a dead light in the tach then figured out the LED bulb was in backwards. But he says the tach works fine. I am so confused!

 

[Wageslave]

It was my understanding that not all points era tachs needed the filter, but some wouldn't work without one due to there being some that got signal from the positive side of the coil and some getting signal from the negative side of the coil.

The easiest way to check is to just hook one up and see if it will play nice. If it doesn't work, they make modern tach filters that you can get on Amazon for $15.

 

[Bonnewagon]

If I told you what I paid for these original hood tachs back in the day, you would laugh me right out of town. But back then there were no re-pops, no knock-offs, only over-priced OEM stuff. So I am determined to get these working. Anyone know if the MSD tach adapters work on the same principal? I have the one you use with an MSD6.