Rev counter... aargh! Still!

[tony_ack]

I've been unsuccessfully been trying to get to the bottom of the problem of my rev counter not working. I've done a lot of research on here, searched through countless threads on the subject, and even come up with a troubleshooting process which I've shared on the maintenance board.

Before I thought through what the problem might be, I tried the fuse 21 pull out trick, I've reconnected the bonnet earth strap, and cleaned the rocker cover earth connections, and I've also replaced the clocks with a VDO set. The rev counter still refuses to work.

The MFA MPG reading is showing 99.9, which now leads me to believe that the clocks are not getting any signal for the rev counter (I'm guessing that a bad earth wouldn't affect the digital rev counter reading, which I presume the MPG is calculated off).

I've had a look at the coil for the infamous red/black wire with white plug which supplies the rev counter signal. I found one to the l/hand side of the coil (looking from the front of the car), slightly underneath it. I looked in Mr Haynes's manual, which shows a similar plug connecting to terminal 1 (-ve) on the coil. I disconnected it, and sure enough, the car wouldn't start.

Is this the correct wire which gives the signal to the clocks? Could someone get me a photo of the wire, or draw me a diagram showing all the different connections/cable on the coil, and what they're for (I don't trust Haynes for being complete).

The car is a 90 spec 8v Digi.

Many Thanks!

[rubjonny]

there should be 2 wires to the terminal 1 side of the coil. one comes from the TCI-H unit which tells the coil to spark, usually its a green wire on digi and 16v cars. the other wire runs direct to the fusebox and is what feeds the rev counter, and is usually red/black.  Sounds to me that this wire is broken somewhere along its length, I'd follow it along as much as possibe, if it looks good get behind the fusebox and check there. 

The rev counter wire runs direct to Plug G1 pin 12, and then the clocks get the feed from plug U1 pin 6. check my fusebox FAQ for more info and piccies of the back o the fusebox

[tony_ack]

Thanks - useful info about the rotuing of the cable. Do both the TCI-H unit wire and rev counter signal wire attach using the same white plug?

Should be a nice fiddly job to keep me busy this weekend 

[rubjonny]

yep, the 2 pin white plug on 1 side o the coil

[tony_ack]

It was the one I thought it was. That's where I'll start - thanks for the help!

[tony_ack]

Tried to get to the back of the fusebox last night but the damn thing just will not come out. I've removed the white plastic clips from the left and right hand side of the fusebox, and even removed the driver's lower dash tray for better access, but it I'm still having no luck. It's definitely loose now, but I can't seem to lift it out of the right-hand catch as it will not lift high enough. Obviously I don't want to force it, as I imagine it's pretty delicate in there.

Am I doing something wrong?

[rubjonny]

on the left side there is a locking peg on the holder, what you do is push the left hook sideways to release the peg, tilt the top of the fusebox towards you, then it will lift out

[tony_ack]

Got a chance to look at this finally today - wish I hadn't have bothered!

Worked out why the fusebox wouldn't tilt forward- there was a set of short wires connected to it which turned out to connect the alarm to the central locking (giving me remote central locking). Problem was that I only realised this after I put everything back together again. So it looks like I'll need another forage down there to put that right, and I don't have a clue where this plugs in as it's probably non-standard (red plug, two red wires, two brown wires)

The problem with the rev counter must be somewhere on the clocks. I checked for voltage at the coil, at G1 on the fusebox, U1 at the fusbox, and at the dash plug - all checked out okay. I even checked for voltage off the bolt at the back of the rev counter, and this gave a signal. I tried grounding the clocks to the banking plate I was using to ground for testing - nothing from the rev counter. I moved the spinde by hands and it moves freely.

I can't beleive that this is the second set of busted clocks I have. The voltage regulator and the green vacuum bit are off the original clocks - is there any way that these could be the culprits.

To add insult to injury, I seem to have damaged the speedo cable because it's a pig to get off, so it's all wobbly between 20 and 30mph!

[tony_ack]

Tried ANOTHER set of clocks today. These went one better - the rev counter needle moves to just above 0 when I switch on the ignition. But that's about all it does. At least it's getting power (but I already knew that).

I think I'm missing something else. I have run a wire from the nut on the back of the rev counter, and connected it to the metal dash support next to the left hand side of the colcks, via a multimeter. When the ignition is (but no engine) I get a reading of 0.06 volts (may explain the slight movement in the needle). When I turn on the engine, it goes up to 0.31 volts. I opened the throttle to what I'd have guessed was 2000rpm, but the reading remains a steady 0.31 volts. I would have thought the reading would have increased?
Does this point me back to the signal being the cause, rather than the clocks? What sort of voltage should I be getting at idle and at 2000rpm? And if the signal is the problem, where do I investigate next?

[tony_ack]

Forgot to mention... I didn't reconnect the speedo cable in my last round of testing - surely this wouldn't have made a difference?