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Factory tach

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Dipstik86

Factory tach

So my truck is a factory 6cyl truck and the previous owner installed a cluster with a tach. I didn't trust that it was accurate and I installed another one inline with the factory one and sure enough the oem one reads 500rpm low. Is this normal or could I have a v8 tach in it and if so is there a way to make it read closer to correct?

1986 f150 4x4 reg cab 300/6 np435

mat in tn

Re: Factory tach

Actually there is a way. On the back of the cluster there is a raised portion with four connections from the tach to the printed circuit film. Power, ground, tach signal from ignition and a fourth being “ 8cyl ground” the 8cyl ground needs to not be connected. Odds are that a cluster from a v8 was installed without this unddstanding. The 6cyl printed circuit does not have the same tach circuit

Dipstik86

Re: Factory tach

But wouldn't the fact that it's a factory 6cyl truck mean it doesn't have that wire in the harness? I'll check tonight.

1986 f150 4x4 reg cab 300/6 np435

Atlas75

Re: Factory tach

This may help. You are looking for a wire in location 12.

Carl

1980 F-350 4X4; 400 C6; Dana 61 rear, Dana 50 TTB front
1984 F-250 4X4; 6.9L T19; Dana 70 rear, Dana 44 TTB front

mat in tn

Re: Factory tach

Thank you atlas75. That is going to be helpful. Im in thd feild this week looking for another truck and wont be in the shop for another week at least. Both 6 and eight cyl trucks were wired for a tach . At least the ones i have done have been and i have done base models and xlt’s. Maybe i have been lucky. Most of the wiring exclusions i have had to deal with have been tank selector or not. Ac or not. Efi or carb . Years may vary also

Gary Lewis

Re: Factory tach

Administrator

On the page at Documentation/Electrical/Tachometers you'll find the schematic and statement below.

I should point out that the stock gas tachometers work on both 6 & 8 cylinder engines. There is a terminal on the tach that when left open puts the tach in 6 cylinder mode, but when grounded puts it in 8 cylinder mode, as shown in this diagram. The 8 cylinder engine wiring harness grounds it, but a 6 cylinder harness doesn't. (You can find the diagram on Page 27 of the 1985 EVTM. Or, click on the image to have it open larger.)

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

ratdude747

Re: Factory tach

Agreed... I put a factory tach in my 300 and AFAIK it's accurate... but I haven't verified it (and don't have a good way to, maybe high speed video of a timing light?)

Question for the OP: how sure are you that the aftermarket tach is accurate? Could it be in 4 cylinder mode? Usually there will be a setting for 4, 6 or 8 cylinders if memory serves.

1984 F150: 300 L6, AOD, RWD. EEC IV / TFI, Feedback Carter YFA Carb. Stock everything but radio (for now).

Dipstik86

Re: Factory tach

Tried 3 different aftermarket tachs and all were on 6cyl mode, also tested it with a snap on timing light which displays engine rpm. All of them say it's 500rpm slow.

1986 f150 4x4 reg cab 300/6 np435

ratdude747

Re: Factory tach

Fair enough. The only other thing that comes to mind is the needle got moved on the shaft somehow... botched gauge restoration?

1984 F150: 300 L6, AOD, RWD. EEC IV / TFI, Feedback Carter YFA Carb. Stock everything but radio (for now).

Gary Lewis

Re: Factory tach

Administrator

In reply to this post by Dipstik86

Is it 500 RPM slow at all engine speeds? That doesn't sound like 6 vs 8 cylinder mode. In 6 cylinder mode the tach will show 3/4's of what the engine is turning, so at 800 RPM it'll show 600, and at 4000 RPM it'll show 3000.

Have you checked for a ground on the black/light green wire going into the plug on the instrument cluster? That is Circuit 60 and is Pin 12 of the connector. If that isn't grounded then the tach will be in 6 cylinder mode. And since the ground comes from the distributor and the circuit goes through several connectors it is possible the circuit is broken.

And, by the way, all engines use the same printed circuit on the back of the instruments. By that I mean the same printed circuit is used for 6 cylinder and 8 cylinder engines. The only way the tach knows which mode to go into is via Ckt 60. So if it isn't grounded you are in 6 cylinder mode.

Gary, AKA "Gary fellow": Profile

Gary Lewis

Re: Factory tach

Administrator

In reply to this post by ratdude747

ratdude747 wrote

Fair enough. The only other thing that comes to mind is the needle got moved on the shaft somehow... botched gauge restoration?

That's very possible.

Gary, AKA "Gary fellow": Profile

ratdude747

Re: Factory tach

*Puts on Electrical Engineer hat* I was thinking maybe a bad connection, but as tachometers (in terms of the wiring to the cluster) are frequency driven rather than voltage/current level driven, I'd think such would result in erratic readings or no reading. Technically there could be a fault in the cluster's circuit board (I don't recall if there's any circuitry between the tach input and the gauge itself)... but otherwise, I'd lean towards a poorly zeroed needle if the shift is consistent at multiple values (fast idle, normal idle, etc.). If not, possibly a faulty gauge (or cluster if there is circuitry on such). *Takes off Electrical Engineer hat*

1984 F150: 300 L6, AOD, RWD. EEC IV / TFI, Feedback Carter YFA Carb. Stock everything but radio (for now).

Gary Lewis

Re: Factory tach

Administrator

The wire I'm talking about is just the ground that tells the tach to go into 8-cylinder mode. So I'd think it was either in 6 or 8 mode.

But I think you are onto something about the needle. If it is consistently 500 R's off then it isn't that ground. In fact, it is hard to imagine what it could be other than the needle being offset.

I've pulled the needles off tachs and put them back in, and it can be done fairly easily. I think I'd note where it is, pull it off, and put it back on 500 R's higher.

Gary, AKA "Gary fellow": Profile

Dipstik86

Re: Factory tach

Now just to throw this into the mix, the truck has a DUI HEI distributor on it. Would that be the cause of the incorrect readings?

1986 f150 4x4 reg cab 300/6 np435

Gary Lewis

Re: Factory tach

Administrator

I doubt it. Usually it either works or doesn't. But I can't imagine it would subtract 500 R's. I'm staying with the needle being on wrong.

Gary, AKA "Gary fellow": Profile

Dipstik86

Re: Factory tach

Well I popped the cover off the dash and attempted to move the needle. Not sure what was holding it on but I didn't even get a chance to grab it and it just fell off. There is like a black portion of the needle that holds it to the pin and that was mostly broken off. I'm attempting to epoxy the needle back in place so we'll see if that was the issue the whole time. Now the question is, do I set it to 0 with the engine off or below 0?

1986 f150 4x4 reg cab 300/6 np435

Gary Lewis

Re: Factory tach

Administrator

Mine points at 0 with the engine off. I can't say it is perfect, but it seems to be about right.

Gary, AKA "Gary fellow": Profile

Dipstik86

Re: Factory tach

Well I ended up having to make a new needle out of an old speedo needle cut down and with a shortened stem to fit properly. Seems to be accurate now when compared to the aftermarket tachs.

1986 f150 4x4 reg cab 300/6 np435

Gary Lewis

Re: Factory tach

Administrator

Gary, AKA "Gary fellow": Profile