89 400SE Coil / Tach wiring.

[B@man]

Anyone know if this diagram is correct for an 89 400SE, this seems to imply that the Tacho is Current driven rather than pulse driven.



I thought the current driven tacho's went out of production a long while before 1989.

I would have expected a 12v feed to the coil and the ECU and tacho to be connected to the other side of the coil sampling the pulses.

Drawing is marked 400SE, 450SE and 350i and is dated 25/11/88

Edited by B@man on Saturday 20th February 17:43

[2mad]

Check out the ignition module/amplifier on the side of the distributor if your having problems with your tachometer they are known for breaking down use the correct lucas unit not some after market job as they dont last.

[B@man]

No It's not broken, The distributor will soon by employed only to drive the oil pump so I need to make sure I can drive the Tacho from a pulsed source.

Looks like the age old take it apart and see what happens method will be employed

[rev-erend]

Many people converting to aftermarket ECU's then need a tacho driver

http://www.emeraldm3d.com/em_pricing.html

[Wedg1e]

On the 390 ('87) the tacho gets 12v and ground, plus a signal from the ignition amp (white/black) that is the same feed used to trigger the ECU... as you say, pulse driven.
The ignition amp has two wires: white from coil '+' (ie 12v) and white/black (coil '-', tacho, ECU).
Could be your diagram is from TVR

Incidentally, if you have a dead coil then the injection won't fire either because it's not 12v pulses from the ignition amp the ECU looks for, it's the high-voltage back-emf spikes of the coil.

[B@man]

Cheers Ian,

Your description is exactly as I expected, the Tach picks up the ignition pulses the same was as the Fueling "computer" does. The bible also includes this configuration in it's V8 schematic so finding the TVR diagram was a bit of a curve ball as I'm about to loose the distributor (well the top half anyway). If it does turn out to be a RVI tach I'll just have to get new guts put in it !

Dash needs to come out to fit one of Stephens new veneers so I can pop the tach out and check the wiring.

[SEvans]

B@man said:

Cheers Ian,

If it does turn out to be a RVI tach I'll just have to get new guts put in it !

Had the same problem when I converted mine from a standard Pinto with Carburettor to a Cosworth with ECU. Sent the Tach off to Speedy cables and they replaced the inners.

[B@man]

Today I can confirm that the TVR wiring diagram is CORRECT, my 400SE Stewart Warner RVI type tacho.



Said tacho is now in pieces all over my desk and it actually looks pretty straight forward to convert it to pulse operation, fingers crossed !

Edited by B@man on Saturday 6th March 16:49