Tach Install Question??

[Zilverado]

So I just finished installing my tach (first time). Seemed simple enough, but I have a few questions.

At idle, the tach jumps around between 1000 and 2000 rpm. As soon as the throttle is pressed and the truck starts moving, the needle drops to approximately 1000 and begins to slowly rise with the acceleration. All seems well until I take my foot off the gas and the engine again is not under load. The needle again starts to jump. What could this be? I thought my wiring was solid and the ground was good. Any thoughts?

Next, I just want to verify my engine speed. At approximately 60mph, the needle shows around 1500 rpm. This is with a:
305
700R4 tranny (in OD)
3.07 rearend
LT235/75 R15 rubber

Does 1500 rpm sound right for that set-up. It would explain my good gas mileage .

BTW the tach has a 4-6-8 switch. I have it set on 8 (obviously).

Any thoughts would be greatly appreciated. Thanks.

[76bonanza]

If you have an hei mny tach need a filter to remove the jumping caused by the hei.

[Blue Beard]

Sounds like it might need a better ground.

What is your idle suppose to be at in park?

[Zilverado]

Thanks for the responses Guys.

76bonanza
What kind of filter are you talking about and is it installed in the the tach wire?

blue beard
I haven't checked my idle speed to confirm, but I think it's supposed to be around 650 or so.

Anything else?

[Zilverado]

Bump ;-)

Any other ideas?

Thanks

[PHOENIX]

I also say check the ground along with all other connections. Are you sure that you have the tach hooked up properly. Did you buy the tach brand new?
If you have any way of hooking these vehicle to another machine or something that can read rpms (like some timing guns), I would do that. See how that reads.
I have a tach hooked to my hei and I didnt use any filter or anything. I have an autometer tach so maybe it has something internal, but I doubt it because it didnt say anything about "for use with hei" or anything about hei period. I think its a wiring or internal tach problem. Are the connections at the distr. cap good? Take the cap off and check it.

Good luck!

[Zilverado]

Originally posted by PHOENIX
I also say check the ground along with all other connections. Are you sure that you have the tach hooked up properly. Did you buy the tach brand new?....

Thanks Phoenix.

Yes, I'm pretty sure I have everything hooked up correctly. Only 4 wires (tach, power (ignition), light, ground) - pretty straight forward I thought.

The tach is new. It's not an Autometer though. It just seems strange how it would be so irratic only at idle, and smooth the rest of the time.

As far as verifying the rpm, I have a tune-up kit including a meter that measures rpm. I just have not had a chance to hook it up yet. Just thought I'd go straight to the "on line help" .

Any further thoughts?

[TIMSPEED]

I have that same problem kinda, it's just my IDLE goes along with it. I think it's cuz of my cam and my idle is too low for the cam...either that, or I need to get a better ground on my HEI Cap, because it's just kinda jammed in there. I need to fix that soon, as well as drop my starter and put a shim in it...

[Zilverado]

Originally posted by TIMSPEED
I have that same problem kinda, it's just my IDLE goes along with it. I think it's cuz of my cam and my idle is too low for the cam...either that, or I need to get a better ground on my HEI Cap, because it's just kinda jammed in there. I need to fix that soon, as well as drop my starter and put a shim in it...

Are you saying that the idle is too low for the tach to register it properly? That seems a bit odd. Might explain my problem though. Now If I could only fix it...

[PHOENIX]

I never heard of that, so I really dont know. I would have to say if your wiring is good and everything is good at the distr., then its your tach. Is your tach adjustable from 4,6, & 8 cyl. engines? Alot of them are. You might want to check tht setting too.

[76bonanza]

My Tach was a cheapo and it came with a filter with instructions to use on hei or ford hi ouput distributers. Mine did just like you say and then with filter on turn little dial untill it stopped bouncing.

[apstguy]

I bought a cheap tach and has no such filter and rins perfectly fine. It is an air core so when you shut the truck off, it bounces everywhere. Probably a wiring problem.

[Zilverado]

Thanks guys,

Phoenix:
Yes, the tach has 4-6-8 switch. It is set to 8.

76Bonanza:
This tach is also not a very expensive one but I like the size and look. I guess I'll have to do some further investigation into this filter thing.

I verified the engine speed this afternoon with my tuneup meter. at higher rpm (1200+) my meter and the tach are bang on. Once it drops to idle (650 to 750) the tach starts bouncing.

Anyone have more ideas or know where I could get a filter? Is it the same as what you put in your stereo line to eliminate ignition noise???

[76bonanza]

Is your tach new? Mine came with it and is also small like 2 1/8 I think.

[Zilverado]

Originally posted by 76bonanza
Is your tach new? Mine came with it and is also small like 2 1/8 I think.

Yes, the tach is brand new.

So, assuming it's an inline filter, which wire is it on - the "power/ignition" or the "tach" wire?

[PHOENIX]

If it was on anything I would think i'd would be on the tach wire.
Call the company and ask them if you should have something there. If not explain the problem to them and see what they say.
Whats the brand by the way?

[76bonanza]

It is on the tach wire and has a small dial on it.

[Zilverado]

Thanks Guys,

Phoenix
The tach is actually a cheapy I picked up at a local surplus tool/farm supply store. The brand, "Power Fist" is the store's own in-house brand. So, I'm not sure who actually manufactured the unit. I will be heading back to the store in the next day or so, and I intend to talk to them about this problem. It may just be a bad tach.

76Bonanza
Thanks for the info. I'll be calling a local speed shop today to see if they have any ideas. Wish me luck...

[raceman6135]

I would just take it back to Princess Auto and get another one. As you know, they have stores all over the place (not just in Manitoba), so they'll be able to get a replacement for you quickly if they don't have another in stock.

DJS

[Zilverado]

Originally posted by raceman6135
I would just take it back to Princess Auto and get another one....

Thanks Raceman - I think I might just do that.

Just one more point about the filter thing.

I spoke to someone at one of our local performance speed shops this morning. I asked about the potential for problems with lower end tachs (they sell Autometer) and HEI ignition, and about any available filters. He said that HEI does not cause a problem, and that only engines running "multiple point ignition" (or something like that - whatever that is), would be helped by the use of a filter.

He also said that many lower priced tachs have a problem with registering incorrectly at under 1000rpm. Oh did I mention that they sell Autometer...

He did mention that I might be getting interference if my tach wire is too close to another sparkplug wire. To be honest, I never checked this.

Anyway, the saga continues......

[PHOENIX]

Very interesting about the plug wire. Never crossed my mind. Just always make sure you keep all wires away from plug wires, they will f*ck anything up

[Zilverado]

Well, I just hope that is my problem - I'll find out tonight.

I'll let you know how it turns out. Stay tuned...