I was driving highway for about an hour (70-75, pretty constant), got to where I was going, stopped for half hour or so, started the truck and the tachometer got erratic. It would drop really low (below 500 rpm) the shoot high as the engine revved, then drop to normal for a few seconds, then back down to 500 or below. The engine speed wasnt following the tach. I drove a little while then it appeared normal, stopped, started got erratic again (it bounced around pretty stiffly, that is, it didnt smoothly go to different speeds, very jerky). Did the highway return trip, in the driveway it didnt act erratic, but I was doing 2200 RPM or so all the way home. Anyone have a similar problem, know a cause or fix?? Thanks.

 

Welcome to FTE!!!

What year and engine. I would recommend pulling the codes.

 

Sorry, its a 1995, 4.0, what codes would I be looking for?? The engine isnt loping or idling badly along with the tach, they are definitely independent. Thanks.

 

The 95 is the OBD-II system which requires a scanner, some parts stores will do it for free. The tach runs off a signal from the PCM so it might be an indication of a senser problem that the PCM would pickup.

 

Thanks - I have a scanner. I guess I dont understand the tach that well, but what sensor(s) might be the problem?? Just trying to understand.

 

The crankshaft position senser, but you could try the following procedure from the Ford manual.

PINPOINT TEST L: TACHOMETER NOT REFLECTING CORRECT ENGINE SPEED

L1 CHECK POWER SUPPLY TO GAUGE
Key OFF.
Partially remove instrument cluster panel to gain access to the cluster connectors.
Turn ignition switch to the RUN position.
Check voltage at Circuit 640 (R/Y) at connector.
Is battery voltage present?
Yes No
GO to L2 . SERVICE Circuit 640 (R/Y) for open. RESTORE vehicle. RETEST system.

L2 CHECK GAUGE GROUND
Measure resistance between a known good ground and Circuit 398 (BK/Y) for 4.0L engines, Circuit 397 (BK/W) for 5.0L engines and Circuit 570 for all engines.
Are resistances 5 ohms or less?
Yes No
GO to L3 . SERVICE Circuit 398 (BK/Y), Circuit 397 (BK/W) and 570 (BK/W) for open. RESTORE vehicle. RETEST system.

L3 CHECK TACHOMETER INPUT CIRCUIT
Key OFF.
Disconnect instrument cluster connector C1.
Disconnect ignition coil, or DIS.
Install Rotunda EEC-V 104 Pin Breakout Box 014-00950.
Measure resistance of Circuit 11 (T/Y) from Breakout Box Pin 30 to instrument cluster connector C1.
Is resistance 1.8 ohms or less?
Yes No
GO to L4 . SERVICE Circuit 11 (T/Y) for open. RESTORE vehicle. RETEST system.

L4 CHECK CLUSTER CLIP TO CLUSTER CIRCUIT CONNECTION
Reconnect instrument cluster connector C1.
Check for continuity from the tachometer connector clips to instrument cluster printed circuit.
Is there continuity?
Yes No
GO to L5 . REPLACE connector clips. RESTORE vehicle. RETEST system.

L5 CHECK PRINTED CIRCUIT
Check each circuit listed below for continuity from tachometer connector clips to cluster connector.

Connector Pin Circuit Function
C3 8 398 (BK/Y), 4.0L Ground Through 4.32 k Ohms Flex Circuit Resistor
C1 16 397 (BK/W), 5.0L Ground
C1 13 640 (R/Y) Battery Power (Hot in Start or Run)
C1 15 11 (T/Y) Tachometer Input
C2 2 570 (BK/W), All Engines Ground


Is there correct resistance at all circuits?
Yes No
REPLACE speedometer/tachometer assembly. RESTORE vehicle. RETEST system. REPLACE instrument cluster printed circuit. RESTORE vehicle. RETEST system.

 

Ken - thanks, that makes a lot of sense, havent pulled codes, but was wondering if a bad CKP would set the MIL - that hasnt happened (yet), but maybe becuase its intermittent. Thanks Again for your help, to be honest was hoping to try something other than pulling the dash.

 

No codes in the PCM.

 

It should set a code, I guess the L pinpoint tests are the best thing to do.

 

I think I am going to try the CKP, drove all today without problems - is it possible that the problem is 'too' intermittent to set a code (I did rev the engine a bit when I noticed the tach drop)? Does it matter if the engine was running or was pullling codes with just key on enough?? One thing that makes me suspicious is that the wandering tach happened after I drove the truck for over an hour, have heard that the CKP sensors get 'heat' sensitive while dying. I stopped at the DMV, didnt see the problem when I restarted the truck and drove home. Maybe it was heat soak or something, pretty hot day. Thanks again (still dont want to take the dash apart).

 

It is possible to have a sensor to be going but still working within the tolerances. I think that it would have set a code if you saw it in the tach but I could be wrong.