Tach stuck at 2500rpm!

[Bill Jennings]

Hi Pelicans,
I have had an intermittent tach issue where I assumed I had an intermittent connection problem. The tach would work normally then not at all, then work normally again of it's own accord.

Today though it stuck at 2500 rpm and stayed there even after turning the car off.

This is new!

I just took a picture of the tach which was a couple of hours after turning the car off and the tach was now stuck at 250 rpm. So I thought I would see what turning the ignition on and off would do, no change, so then started the car which gave me the idle rpm and then back to zero when I turned the car off.

Two questions:
1. is the tach sticking a different problem than a poor connection?

2. where physically are the connections that I need to check to find the intermittent connection.



Cheers
Bill

[quattrorunner]

Was it wormer outside when it started doing this?

[Bill Jennings]

I've had the car nearly 1 year when I imported it from the UK to Australia and I didn't notice any problems with the tach in the UK and it was freezing there then. I only drove it twice over there one week apart it was very cold both times.
I think it is very possible the problem started last spring or early summer say maybe last september.

Today was the first time the tach stuck and it was about 25C today here.

We have had temps in the mid 40C range the past two months with a lot of rain so it is pretty humid here. The first time I drove with the new gauge was last weekend and it was a three hour drive each way in very heavy rain.

Not sure if it related in any way but I changed the oil temp/pressure gauge last week with a gauge I had just imported from California but that gauge and the fuel gauge fog up quite a bit where previously the old oil temp/pressure gauge only fogged up a little and the fuel gauge didn't.

The tach does NOT fog up though!

Thinking about the fogging I just checked the floor mats and all 4 mats were damp and the drivers was the wettest. I've now removed the mats.

It's not my usual practice to use this car in the rain, possibly 2 or 3 times since I've owned it.

I'll have to see if it's wet anywhere else.

Cheers

Bill

[darrenhall]

Hey Bill,

First off the signal for the tach can be found at two places in the engine compartment.

First--- In the engince compartment you have a large 14 pin connector that plugs in the the aluminum box on the very left hand rearword part of the engine compartment - IE the used to contain the relays on the earler 911s. The number 12 pin is for the tach. It is black and purple. Chech to see if this connection is clean.

second-- Underneith or next to the CDI box is a spade connector . One of the wires going to this connector is a Black and Purple wire. That is your Tach signal wire. Check this wire and make sure the connection is clean.

Also, a tach acting up can be a sign that the CDI box is going bad.

All the luck sorting my friend.

Cheers,

Darren E Hall
TEL: 406-471-5883

[Bob Kontak]

Originally Posted by darrenhall

First off the signal for the tach can be found at two places in the engine compartment.

Darren is right as to the CDI box. The color is violet and black, not purple and black. Sheesh!

Based on what I am seeing in the 77 USA diagram, the second feed (12V) comes from one of the front fuses (I believe front but don't bet your life on it) via a red with black stripe wire to a six pin connector "below the dash". It leaves the six pin connector with the same color wire and feeds the tach. However, the same Red with black stripe wire feeds a squillion other things after the tach so it may be easier to start at the tach and work backwards to the six pin connector. One of the directions will be the correct way to the connector.

I normally budget a good deal of frustration in these kinds of hunts.

My tach will freeze occasionally and I often tap the glass and it sets it free. It freezes more frequently when I "beat" the car. I suspect my issue is more than a simple connection, though.

I have 77 US diagrams already in an e-mail I can send you or others if you PM me your address.

[darrenhall]

Thanks for the correction Bod. I just couldnt remember how to spell Vylet, oops there I go again Violet, LOL

Cheers,

Darren

[quattrorunner]

Reason I asked about temprature was that I've had two older 911's (1979 and 1977) Both of which showed similar symtoms. And I hear it's not just me. The glass swells and pushes onto the neadle stopping it's movement when warmer outside and in the car. My fix is to remove the glass and fix it. Or send in for restoration. Good luck.