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kenewagner
10-15-2008, 05:33 PM
When I drove my car to another garage to store, I had the tach drop to zero after a few blocks. After about 7 or 8 miles the tach started working. I thought I remembered somewhere that is a sign of a DIS going out. Im not worried about driving the car now for obvious reasons but need to know if I should be looking for a new DIS module

Ken

QuickMustang
10-15-2008, 06:05 PM
That's more likely your cam sensor...

JimmyMan
10-15-2008, 06:26 PM
i second the cam sensor - same symptoms on my 89, changed cam sensor and all has been well for 2+ years

S_Mazza
10-15-2008, 06:37 PM
If the car was running correctly when the tach was at 0, then it's probably not the DIS. My DIS failed and it left like the car was running on 4 or 5 cylinders. Then it would snap back and be fine.

kenewagner
10-15-2008, 07:35 PM
Car ran fine with the tach at zero. Came back on line and no hickups. Does the cam sensor go bad slowly?

Ken

Duffy Floyd
10-15-2008, 07:40 PM
Like the crank sensor, which is also a semiconductor hall effect sensor, they often go intermittent before failing entirely.

KMT
10-15-2008, 08:07 PM
>Does the cam sensor go bad slowly?

The EEC will jump in and substitute a signal/value if the camshaft sensor fails to supply one...intermittently or completely. I consider that it is usually a combination of poor circuits (wiring/connectors) and marginal ground paths that contribute, rather than simply a black & white issue at the sensor alone, that frequently work in concert to produce what appears externally as an 'intermittent' issue. Factor in heat, both local and ambient, along with humidity - the gremlins rule.

Mike8675309
10-15-2008, 09:04 PM
Cam sensor, and DIS module can both cause that type of behavior. Cam sensor is much cheaper, so you can start there.

96SCbird
10-16-2008, 03:28 AM
How does the cam sensor affect tachometer operation when the crank sensor and EDIS module are responsible for delivering engine speed information to the computer, which then outputs it to the tach?

kenewagner
10-16-2008, 09:22 AM
Like the crank sensor, which is also a semiconductor hall effect sensor, they often go intermittent before failing entirely.

Is there way to test a cam sensor? I have several lying around?

Ken

perm102
10-16-2008, 09:51 AM
Is there way to test a cam sensor? I have several lying around?

Ken

I am not sure how to test it but if you have one around, I would just swap it. Mine did the same thing. It would not move from 0 and then jump to fine and then back to 0 after a few miles. Also, it gave a no to hard start problem sometimes too. Replaced the cam sensor and all was good.

KMT
10-16-2008, 12:01 PM
>How does the cam sensor affect tachometer operation when the crank sensor and EDIS module are responsible for delivering engine speed information to the computer, which then outputs it to the tach?


Crankshaft sensor is responsible for PIP (profile ignition pickup).

Camshaft sensor sends CID (cylinder identification) to DIS.

DIS delivers signal to EEC.


A signal goes from the EEC to the VMM (Vehicle Maintenance/Measurement/Management Module) and then to the tach.

-=-=-=-
When a valid CID signal is lost from the camshaft sensor, the DIS and EEC both switch to FMEM (Failure Management Effects Mode).

During FMEM, the DIS handles injector firing and the EEC handles CID, each 'guessing' as to which cylinder needs fuel and spark. FMEM cuts output to the VMM and thus, no tach. Note that the VMM will go dark after 15 minutes without a signal. The tach 'bounces' when the signal is suddenly dropped/activated as FMEM, in this case, comes and goes based on the EEC's attempts to reestablish a connection with the camshaft sensor.

KMT
10-16-2008, 12:07 PM
>Is there way to test a cam sensor?

As suggested, the easiest method is to replace it.

>I have several lying around?

Valid only if the problem is cured (used sensor is good). If the problem remains, the used/replacement cam sensor could also be bad, meaning nothing is learned, prolonging the diagnosis - ask how I know :) Try more than one spare if you have them.

The price for a new cam sensor currently ranges from $37 ~ $70.

kenewagner
10-16-2008, 01:42 PM
>Is there way to test a cam sensor?

As suggested, the easiest method is to replace it.

>I have several lying around?

Valid only if the problem is cured (used sensor is good). If the problem remains, the used/replacement cam sensor could also be bad, meaning nothing is learned, prolonging the diagnosis - ask how I know :) Try more than one spare if you have them.

The price for a new cam sensor currently ranges from $37 ~ $70.

Guess a new one is in order.

Ken

kenewagner
10-16-2008, 01:51 PM
>Does the cam sensor go bad slowly?

The EEC will jump in and substitute a signal/value if the camshaft sensor fails to supply one...intermittently or completely. I consider that it is usually a combination of poor circuits (wiring/connectors) and marginal ground paths that contribute, rather than simply a black & white issue at the sensor alone, that frequently work in concert to produce what appears externally as an 'intermittent' issue. Factor in heat, both local and ambient, along with humidity - the gremlins rule.


Several times, while tuning on the dyno, My tuner has felt that there might be a grounding problem. I think I should sometime in the near future inspect my neg ground off the battery. The battery is in the trunk. The car has never had good numbers for the modifacations done.

KMT
10-16-2008, 02:01 PM
>The battery is in the trunk.

Ordinarily, I emphasize checking/checking/checking the stock ground paths, but in your case, you might want to consider adding additional grounds, even if only for testing. Don't overlook (freshly?) painted brackets, etc. as points of resistance - example: the (early) DIS grounds via the mounting plate, top of A/C bracket, through the engine, etc., then to the body.

Good luck w/your issue(s) :)

Ken

96SCbird
10-16-2008, 03:00 PM
Crankshaft sensor is responsible for PIP (profile ignition pickup).

Camshaft sensor sends CID (cylinder identification) to DIS.

DIS delivers signal to EEC.


A signal goes from the EEC to the VMM (Vehicle Maintenance/Measurement/Management Module) and then to the tach.

-=-=-=-
When a valid CID signal is lost from the camshaft sensor, the DIS and EEC both switch to FMEM (Failure Management Effects Mode).

During FMEM, the DIS handles injector firing and the EEC handles CID, each 'guessing' as to which cylinder needs fuel and spark. FMEM cuts output to the VMM and thus, no tach. Note that the VMM will go dark after 15 minutes without a signal. The tach 'bounces' when the signal is suddenly dropped/activated as FMEM, in this case, comes and goes based on the EEC's attempts to reestablish a connection with the camshaft sensor.
The cam sensor does not go the EDIS, and the EVTM verifies this, at least not on the 94/95 SCs.

KMT
10-16-2008, 03:11 PM
>at least not on the 94/95 SCs.

Looks like it goes to the PCM for those models, instead...

In KW's case, I believe we are discussing a '93, but that is just an assumption on my part :)

http://www.sccoa.com/member/images/wagner5.jpg

watsonlk
10-17-2008, 10:08 AM
My DIS failed and it left like the car was running on 4 or 5 cylinders. Then it would snap back and be fine.

Exact same thing here, new DIS no problems... Until I snapped my new crankshaft last weekend, but I have yet to determine if they're related.