Factory tach

[Dipstik86]

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?

[mat in tn]

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]

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

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.

[Atlas75]

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.

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

[mat in tn]

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

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]

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

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.)

[ratdude747]

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.)

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.

[Dipstik86]

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.

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.

[ratdude747]

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.

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

[Gary Lewis]

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.

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.