Flakey Tachometer

TDIFan1989

Veteran Member
Joined
Nov 28, 2013
Location
Toronto
TDI
2000 Mk4 Jetta TDI (Black)
I have a problem with my 2000 ALH TDI where the tachometer had been absent for about a year and the oil pressure light would come on and beep 3 times upon initial start up.

Other info suggested cleaning the Oil Pressure Sensor connector. I cleaned it, it was still flaky. I replaced the Oil Pressure Sensor. It's significantly better but still flaky.

The odd thing was that with the Cruise Control off, the revs would almost show up consistently (although it will cut out on occassion), but when I toggled the switch on, consistently the tachometer dropped to 0 and the oil light would come on again with the beeps.

I replaced the turn signal switch (which houses the cruise control switch). Same results.

Other than chasing a loose wire, does anyone else have any other ideas on what could be going on?
 

eddieleephd

Top Post Dawg
Joined
May 27, 2012
Location
Battle Ground, Wa
TDI
2002 jetta Wagon
Most of that runs through the T14A plug next to the battery in J/G next to ECM beetle. I would pull the battery and battery tray to inspect and run some continuity readings.
Try continuity between pins in the connector, should only be on ground.
Check for shorts in connector and up the wire tray where the T14a is likely to come in contact with the rest of the loom.

Could be right at the plugs, or along the way.

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k_pt

Veteran Member
Joined
Oct 13, 2017
Location
pt
TDI
VW MKIV TDI
Unplug the tach, check continuity beetween the tach plug t32 that gives you oil pressure sensor reading and the plug from the oil pressure.

Most of the times its just the wirings fault. If you get continuity always, even wiggling the cables, the tach is toasted.
 

eddieleephd

Top Post Dawg
Joined
May 27, 2012
Location
Battle Ground, Wa
TDI
2002 jetta Wagon
Hmm, more info on this please

Unplug the tach, check continuity beetween the tach plug t32 that gives you oil pressure sensor reading and the plug from the oil pressure.

Most of the times its just the wirings fault. If you get continuity always, even wiggling the cables, the tach is toasted.
Tach, or sensor would be toast? meaning would someone have to repair/replace the wiring and sensor, or wiring and cluster and sensor?
Mostly just curious myself.
 

k_pt

Veteran Member
Joined
Oct 13, 2017
Location
pt
TDI
VW MKIV TDI
Tach, or sensor would be toast? meaning would someone have to repair/replace the wiring and sensor, or wiring and cluster and sensor?
Mostly just curious myself.
Op said sensor was replaced, so I removed it from the equation.

Also Tach is having some other issue.
 

TDIFan1989

Veteran Member
Joined
Nov 28, 2013
Location
Toronto
TDI
2000 Mk4 Jetta TDI (Black)
Op said sensor was replaced, so I removed it from the equation.
Also Tach is having some other issue.

Riddle me this, I looked at the Alternator. According to my mechanic, there's a 2 prong plug that is responsible for the signal that the tachometer reads/sends to the computer. Is this how the car reads RPMs?

Now looking at that plug, it is cracked at the alternator connection. Assuming the wiring harness is bad, does anyone have a source for replacement besides the Stealership?

I am curious to know how that plug on the alternator works to tell the ECM what the RPMs are.
 

k_pt

Veteran Member
Joined
Oct 13, 2017
Location
pt
TDI
VW MKIV TDI
Riddle me this, I looked at the Alternator. According to my mechanic, there's a 2 prong plug that is responsible for the signal that the tachometer reads/sends to the computer. Is this how the car reads RPMs?
Now looking at that plug, it is cracked at the alternator connection. Assuming the wiring harness is bad, does anyone have a source for replacement besides the Stealership?
I am curious to know how that plug on the alternator works to tell the ECM what the RPMs are.
This?



Voltage regulator. Even unplugged, you get RPMs at the tach, I forgot to plug that wire once, rolled for 2 days without it.

I think RPM is read by crank or cam sensor.
 

Alchemist

Veteran Member
Joined
Nov 3, 2007
Location
Lethbridge, Alberta
TDI
'04 ALH Golf
One wire in that plug is used to operate the warning lamp that the alternator is not charging. The other wire reports how hard the alternator is working to the ECU.

Tach signal comes from the ECU based on the crank sensor.

I have encountered an erratic stepper motor in an instrument cluster before, but it was the speedometer. Very likely the same in your situation as all pointers are operated by the same type of motor. I was able to reuse one from another cluster, but that job requires skill with printed circuit desoldering/resolering.

If you have VCDS, you could do output tests to confirm it is the stepper motor. Any jerking during the test indicates a faulty motor.
 

ToxicDoc

Veteran Member
Joined
Mar 1, 2018
Location
Virginia, US
TDI
2001 Jetta, S7, .216
don't know VW well enough (we all know their designs are a little involved), but in general if you can get an electrical issue to alter by turning a different circuit on/off it's often a ground issue. I'd carefully check /clean all the grounds by the battery.
 

eddieleephd

Top Post Dawg
Joined
May 27, 2012
Location
Battle Ground, Wa
TDI
2002 jetta Wagon
don't know VW well enough (we all know their designs are a little involved), but in general if you can get an electrical issue to alter by turning a different circuit on/off it's often a ground issue. I'd carefully check /clean all the grounds by the battery.
Grounds, broken wires and sheathing are agreeably the most common. In many cases a fault in the wiring lead's to malfunction of one or more components, depending on the component it can cause failure as well.
I would run down the wiring between sensor and ECM and check stated grounds before moving on. A skinned wire, or fault in the sensor itself is a common issue that cannot be seen.
Crank sensor issues also tend to present initially at operating temperature and are intermittent.

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TDIFan1989

Veteran Member
Joined
Nov 28, 2013
Location
Toronto
TDI
2000 Mk4 Jetta TDI (Black)
Rpm is read by crank and it's between the transmission and oil filter

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Thanks very much for all the additional information everyone. I have a lot of things to investigate on.

I believe I can rule out the cluster item mentioned as I had already tried a cluster from a different vehicle and I got the same results of tach lost, oil light on, etc.

I will get to cleaning all those wiring connections and checking grounds or replacing the crank sensor when I can and report back.

Wish me luck.
 
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