Eockee: The following applies for an electronic speedometer and odometer unit which takes a signal from a VSS generator as in the nissan hardbody 2.7D single cab, 1998 model.
I have resolved my problem and hope the following will help you and any others with similar problems.
Tapping the dash definitely didnt help in my case.
In your case there is either a loose connection or a sticking indication mechanism. Does your mileage meter (odometer) work? I presume its a mechanical geared numbers type which is on the same unit with the speedometer. If it works then this rules out
any connection problems. Your problem is then on the speed indicator unit - probably a bearing problem, or much simpler a sticking pointer. If both the speedometer and odometer dont work then
you definitely have a loose connection. You will have to get to your instrument cluster, get to the speedometer and odometer unit and check for loose connections and dry joints. To get ideas
on accessing and carrying out some repairs see the following articles on this forum "How I Fixed My 1984 Vanagon Odometer" and "Classic RV pages". Note that I have excluded the possibility of dry joints on the printed circuit board - they aren't supposed to be common.
My problem was due to component failure. You got me re-thinking about why the speedometer always works with the bench test supply.
The test signals I now used where:
12Vdc supply from battery charger (up to 13.4V)
14Vac, 50Hz, tapped from the battery charger transformer before the rectifier.
2 Vac, 50Vac froma small transformer (1.5Vdc charger transformer).
(the VSS signal is an AC signal with voltage and frequency dependent
on vehicle speed - I have measured up to 12.3Vac)
I concluded there must be a switching problem. I drew the circuit schematic (a very tedious job in the case of my unit) for use in troubleshooting. Since the
unit sometimes works I ruled out an I.C. (The component that processes the VSS signal and drives the indicators) problem. I identified one of the inputs to the I.C. As a transistor. The transistor was controlled ( swicthing) by the
VSS signal. The only components in this part of the circuit are a tiny surface mounted capacitor and zener diode. I replaced the transistor and had the same problem. The components all passed two independent meter tests done by technicians who then gave up.
This is when I varied the bench test:
I applied the test VSS signal and measured voltages at each component. With 2Vac, the transistor would not conduct. With 14Vac, the transistor conducted, i.E switched on.
I replaced the capacitor and the transistor now conducted for both voltages. I fitted the unit onto the car and it only indicated for speeds greater than 15km/h. I was now cetain I had found the problem, i.E. I had been suffering a faulty capacitor. But I now had the wrong value. I took back the unit for bench testing. Using a potentiometer I confirmed that the unit only worked for VSS ouput voltages greater
than 1.5V.I put the next lower and higher value capacitors and saw a better response with a higher capacitance. I moved to the next higher capcitance
until I got the speedometer to indicate for the lowest voltage I could adjust to. I concluded that the original capacitor had failed
to a stable lower value hence why it had been passed as ok.
I am now certain that my speedometer will always work and I wont need to jump start it anymore.
From the testing I confirmed that the VSS signal voltage is irrelevant for speed indication. It is only the frequency that matters. However, a minimum output voltage is required and this is also determined by the value of the capacitor (& porpeties of transistor) in the swicthing circuit. Note that the VSS signal also goes directly into the I.C. Through a
seperate input. The circuit I have dwelt on probably provides reference pulses.
I wish the rest of you guys every success.
Tuesday, December 18th, 2007 AT 4:16 PM