Nissan S/CAB SPEEDOMETER

Itembo: I have a similar problem, perhaps we can compare notes.

You apparently have figured out how to bench test the speedometer. I'd like to know what you know. What does it need for a signal? Is it AC or DC? What voltage? What are you using for a transformer?

My 97 Nissan Pickup (Std Trans, 4WD, no tach, no Cruise Control, 94k miles) has a problem where the speedometer works intermittently. When it reads zero MPH and I'm going about 40, if I tap the dashboard several times, it starts to work. Have you tried this? I usually tap medium hard (not hard enough to damage the dash, just a bit harder than I would use to squash a fly).

The fact that tapping the dash works makes me think the problem is somewhere in the dash. But the fact that the check engine light came on once and the trouble code was for the speed sensor makes me less sure. Maybe the signal coming out of the speed sensor is compromised by a corroded connection or something and tapping the dash does something that allows the speedometer to start working even though it's getting a poor signal from the speed sensor? The check engine light hasn't come on again since I cleared that code and I've driven the truck on several short trips, so I'm not sure what to think.

Based on your information, my current suspicion for both your problem and mine is with the contact points where the wiring harness plugs into the back of the instrument cluster. The next time you are tempted to run your test with the transformer - don't even hook up the transformer. Instead, you might want to try just unplugging and reconnecting the wiring harness from the back of the instrument cluster and see if it's really that process, and not the process of connecting the transformer, that makes it start working again.

My secondary suspicion is with the internal wiring and/or mechanical points of the speedometer itself. My 97 has an analog (needle) display. Is yours also analog or is it digital? If it's analog, perhaps the mechanism that the needle pivots on is dirty and just needs to be cleaned or lubed with a drop of machine oil?

If all else fails, I just discovered guys that repair speedometers

I've e-mailed them for a quote to repair my speedometer and any info that they might offer. If I get this fixed through them or through any other means, I'll post back here.

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.