Gauge Electrical II - HELP!!!

SeaDweller · 07-11-2002, 04:41 AM

Ok, here's a picture of the original tach. Sorry, both pictures are a little blurry, well actually, quite blurry!

There are spade connectors at 3,2 and 1, with #3 having a double brown wire, # 2 having a black wire w/ purple stripe, and #1 a double red w/ black stripe.

This is the new tach, and it has spade connectors at 2, 5 and 6. What I need to know is which wires go to the respective connectors.

304065 · 07:01 AM

Wiring Diagram

SeaDweller · 07-11-2002, 07:29 AM

Thanks John, but unfortunately, I can't make heads or tails of the wiring diagrams. When it comes to electrical stuff, I'm a complete dunce!

From the diagram, how can I determine which wire goes to the three connections available on the back of the tach? In other words, what indicates that the black wire with purple stripe would go to the #5 connection, for example? Or the brown wire to the #6 connection, etc., etc?

304065 · 07-11-2002, 09:56 AM

A fine question. Is there any nomenclature on the white plastic female plug? Maybe a very subtle "+" or "TD" as in the diagram? How about a DIN code, like a "15?" Probably not that lucky.

The outline of the shell around the connections looks like it accomodates some kind of a cannon plug that can only be fitted one way. Do you know what year the replacement tach is from, could it be that along the way they changed the harness to accomodate this plug? If you could find someone with a later SC and note the relative positions of the wires in the "male" plug on the car this would do it. (probably why you posted this thread in the first place!

)

If it were me, I would get a 9-volt calculator battery and a couple pieces of wire, and try combinations of pins until one of the lights turns on. There are about 15 combinations but eventually one will light something up. You then know that one of those pins is the ground for the lights, and the other one is the "hot" that corresponds to the bulb that lit. So once that happens, keep one wire on one of the pins, and make connection with the other five, to see if another bulb lights. If not, then put the wire on the OTHER pin from the first combo, and do it again. One of the other lights will turn on. Now you've identified the ground. With the wire still attached to the ground, identify which pin makes the three lights turn on- instrument case left, instrument case right, and high-beam indicator. You've now nailed down four of the six. The other two are a guess. I don't know whether the tach has a diode in it that will be blown up if it's hooked up backwards. You might ask Warren a/k/a/ early_S_man on that one.

Or you could cheat and call the nice folks at North Hollywood Speedometer. . .

Good luck!

Early_S_Man · 07-11-2002, 10:53 AM

OK ... this is getting a bit confusing with multiple threads on the same question, so here is what I posted in the original thread:

SeaDweller · 07-11-2002, 01:47 PM

Thanks Warren,

Unfortunately, I still can't tell from the diagram what goes where.

My old tach had a "+12," "TD," and "-" over the 3 spade connectors. The new tach has no such markings. There are only 3 spade connectors on both tachs, so I'm assuming the new tach will work with my current wiring harness. There's just nothing on the new tach to give me a clue as to what wire should go where.

In the picture of the new tach (above), there are only spade connectors under the numbers 2, 5 and 6, but I can't even determine which is the positive and negative (ground) terminals. Three wires away from getting the car back together, and I'm completely stumped.

I'm afraid to just go plugging away, because I can already envision smoke coming out of the dash!

304065 · 02:42 PM

SeaDwelller:

I don't think it is going to be possible for you to confidently use this new tach. The reason for this is, the Tacho drive signal is coming from 2 pins on the DME box-- pins #21 and #11 according to Warren's helpful diagram. But here's the thing-- rather than connect pin #2 on the back of the tach to your black/violet tach wire and the other to ground and go for it, I think the tach signal may be driven by a different voltage from the DME- not pulses from the ignition system like in the earlier cars. I would guess, best case, it won't work, worst case, it will burn.

Just for fun, here's the correlation between Warren's diagram and the pins from the other post:

top of tach, back side facing you

1 (black-to DME #11 tacho signal) 2(Blk/violet- to DME #21 tacho) 3 (Red- to speedo and pickup)

4 (grey- to exerior lights) 5(red/bk to ign. switch) 6(brown-ground)

If you want to hook the black/violet to pin #2 (middle of top row)
and the red/black to pin #5 (middle of bottom row) and the brown to #6 (right side, bottom row) this is entirely at your discretion. Not trying to discourage you but I don't want you to cook the new tachometer and not be able to return/resell it.

SeaDweller · 07-11-2002, 02:53 PM

Thanks John!

Believe it or not, that is how I had figured the connections might be! There's an "H+" along the bottom row, and I figured they might follow the same pattern as on the old tach, with the tach signal wire on the top spade.

I didn't pay much for the guages, so I think I'll give it a try. I'll still have 4 other gauges to sell if I burn this one up......

304065 · 07-11-2002, 03:05 PM

Like they say in Club Racing, please be sure the pin is pulled on the halon extinguisher. . .

Good luck!

Early_S_Man · 07-11-2002, 03:23 PM

Michael,

I have to assume that the numbers on the Bentley diagram correspond to the terminal numbers on the tach. That combined with Joe's list makes it a fairly good supposition ... as it seems to be very close

You can forget the #11 Black wire from the DME for now ... as it is only for the 'Upshift Light' that many want to disable, anyway!

Her are the three wires that should make your tach. operational:

Brown is ground, and #6.

Red/Black is +14 Volts and #5.

Black/Violet is tach. trigger from DME #21, and #2.

Good luck!

john walker's workshop · 07-11-2002, 04:07 PM

being it's a DME run tach, it won't work on a CD system.

SeaDweller · 07-11-2002, 06:38 PM

Well guys, I finally threw in the towel, and threw my old tach back in the car. The other gauges made a big difference, as they're slightly more modern looking and much brighter. I never had an issue with illumination on the tach, so the gauge lighting is well balanced.

Much appreciate all the help everyone!!