Gen2n3

iTrader: (1)

Jeff,

I like your approach to tracing out the circuit. That was very cool! I will try that out on the next circuit board I examine.

Regarding the I/O signals for IC4, you venture into uncharted waters with dedicated bench testing. I like your methodical approach!

Thank you for correcting me on the different motor driver IC part numbers between the tach and speedo. At first glance, they looked the same; damn older eyes. However, the photos and parts list accurately reflect the proper part numbers that you so kindly verified.

Getting back to the 15V signal, it could be possible that TR8 amplifies a logic 1 (+5V) to the desired voltage. Most of the "TRx" transistors are actually Darlington Pair transistors. They typically offer tremendous signal amplification because they are 2 transistors combined into 1. I know it may seem odd to see voltages in excess of 12V (or 14 when alt is working) on a car but (to me) it is plausible to see that 15V pulse serve as a motor driver when a Darlington Paired transistor is used. I wish I could offer more info regarding stepper motors however, I would have to research more on that electrical component.

Cheers,
George

Gen2n3

iTrader: (1)

Jeff,

I need some clarification on your IC4 schematic diagram, specifically at some junctions.

Follow IC4 Pin 1 (OUT1) output. Is that a straight shot to the base of TR8? Or does that output have a branch that connects to the resistor network of R28 through R31 to the 5V line and also connect IC4 Pin 2 (In-), Pin 3 (In+), and Pin 4 (Gnd)?

Then take a look at IC4 Pin 8 (Vcc) input. Does the 5V signal only power it? Or does that same 5V line also connect to IC4 Pin 7 (OUT2) and Pin 5 (In2+) through the resistor network R21 through R24 (and impact the Speed Sensor branch)?

How does TR5 play into the 5V line?

Do you have pin locations for IC6 and the D751 (CPU) chips?

Lastly, what does "4P", "8P+" and "8P" represent on inputs 11, 10, and 9, respectively?

Thanks in advance for helping resolve any confusion!
George

ALIENR2

IC4 pin 1 connects to R30, R31, and TR8 only. My convention is: Where lines touch at 90 degree angles ("T" or "+") there is electrical connection; when lines cross without electrical connection I made a "C" shape so that the lines would not make a 90 degree angle.

IC4 pin 8 connects to R23, R21, C8, TR5, R30, and R28.

IC4 pin 7 connects to R23, D751, IC6 (pin 9, by the way), and R24.

Concerning TR5... I did some tracing of the speedo board power supply components but I don't have a complete diagram. IC2 (TA78DS05P) is a 5V voltage regulator chip, but the 5V output did not appear to me to connect directly to IC4. The 5V output does appear to connect directly to IC1, D751, C3 (1000 uF; known for the corroded leads problem), and TR6. According to my notes, TR6 connects to TR5. I'm not sure why IC4 would be powered differently than the other chips using 5V power.

I didn't determine the pin numbers for the D751. I did some web searching for D751 and AN8363UBK datasheets but didn't find anything.

"4P", "8P", and "8P+" are labels printed on the instrument console's flexible circuit 'board'. I guess that P might stand for Pulses (so maybe the D751 output to the PCME for code 6 "4P" has only one pulse for every two pulses input from the speed sensor's "8P+"?), but I'm not sure.

Thanks, for taking a good look!

Gen2n3

iTrader: (1)

Jeff,

I believe there are multiple errors in the schematic. Tracing the runs are difficult to do and by no means would I discredit your work. I hope that you do not take offense to my next series of questions. From an electrical perspective, some of these connections do not make sense.

1. Are you looking on both the top and bottom side of the board when tracing out each path for IC4?
2. The speedo board is dual layered so could there be a possibility of certain components passing over/under others without making contact?
3.a. How can IC4 Pin 8 & Pin 7 appear to share 1 resistor, R23? Pin 8 should be 5V power input and Pin 7 is an output to IC5 (D751xxx) and IC6.
3.b. If Pin 8 and Pin 7 share R23 then R24 would be a parallel path for Pin 8. Moreover, Pin 8 also shares R30 and R28 from Pin 1. How can IC4 share so many input and output components?
4.a. If Pin 1 shares R30 and R31 with TR8 then how could they also be shared with Pin 8?
4.b. If one side of R30 and R31 connect TR8 then where do the other sides connect?
5. Could we refer to the D751xxx chip as IC5? I have to tell myself that it isn't Diode #751.
6. Perhaps more research is necessary to establish how TR5 connects to this circuit?

To help reduce any confusion in other iterations of schematics, I recommend using bridges, some call them "C's", to indicate when a trace does not touch a crossing trace and a dot, ".", to indicate an electrical junction, or intersection, between two traces.

I wish I could see exactly what you are seeing so I could offer better guidance or work along side you. Presently, I do not want to unnecessarily yank out a working speedometer *knock on wood*. I think you are off to a good start in sketching out a schematic diagram. With added help, we could refine it to a proper reference point!

Cheers,
George

Gen2n3

iTrader: (1)

Jeff,

I started a new thread so the conversation can have a more detailed approach and allow this thread to continue to serve as the "extremely simple.." fix.

Please post your reply here: Troubleshooting the FD Speedometer-Odometer-Tachometer Circuit Board

Cheers,
George

ALIENR2

Oops... Posted to unintended thread.

bake3310

Not sure if anyone is going to reply to this old thread...

I have a non-functional Speedometer and Odometer but the tachometer seems to work just fine.

I've looked through this and other posts on the subject. I've had my circuit board expert look at the speedo board and the flexible board on the back and nothing jumped out at him.

So I have maybe a simple question, because I'm still learning how to read these automotive schematic drawings. Is there only the speed sensor that comes off the tranny, or is there another sensor for the tachometer? If there is only the one, it would seem like the issue is in either the speedo board or the flexible board.

Thanks for any help.

Gen2n3

iTrader: (1)

bake3310,

Your problem would be better suited in the Troubleshooting the FD Speedometer thread referenced in Post #55. To address the question about Vehicle Speed Sensors - the sensor used to determine vehicle speed originates at the transmission. This should not be confused with the wheel speed sensors used in the Anti-Lock Brake system.

In the meantime, would you take photos of your speedo board, if not previously done? Furthermore, would you please provide a better description of the problem with your speedometer and odometer? Does the speedometer work and is it accurate? Or is the problem centered on a blank odometer? You may post a reply in that (Troubleshooting) thread so we can jump down the rabbit hole. If your post restrictions cause a problem posting to that thread then shoot me a PM.

Nguyen Le

Anyone still on this thread? I tried this fix months ago and was very pleasantly surprised when the tach came back to life. Then I replaced the shattered top cover a few weeks ago and had to redo the wiring. The tach worked again, but there was a new, constant high pitched beeeeep. I just got two old clusters that are new to me. The tach didn't work so I tried the wiring jump again. The new/old tach still didn't work, but the beep is back. When I disconnect the jumped wires from the tach mounting screws, the beep good away. But of course the tach doesn't work.

Has anyone else had the constant beep alarm start when jumping the wires for the tach (ground, signal and wire)? Any recommendations for to get the tach working?

Gen2n3

iTrader: (1)

Nguyen Le,

The constant beeping could be the over-rev warning buzzer. If you try to jump the tach and it beeps then you should find a better repair. You can check out this thread: Over-rev Warning Alarm for some additional insight. I helped that member repair his speedo board because his over-rev buzzer kept going off at low RPMs. What was the original problem with your tach? Was it bouncing or didn't work?

Additionally, do a search for my screen name and the words "FD speedometer" or "FD speedo". I maintain a troubleshooting thread to catalog speedometer issues. BTW, your tach is controlled by the speedo circuit board, which is why I'm referencing it. You may post photos and/or discussions about your tach & speedo board there.

pietrino

iTrader: (1)

I solved the constant buzzer problem, here is the story:
I had problem with the Tachometer and i swapped with a used but in very well condition Mazdaspeed cluster.
I had the Buzzer sound continuously after a long research here on the forum I found no solution but a lot of information.
So whats trigger the buzzer to sound continuously? only 2 things i read:
1) Low coolant
2) Over rev rpm
Since the cluster was new and the old was working without the buzzer i was thinking that was the over rev rpm, but was much simpler to test the coolant sensor.

Here is the magic:
I tested the coolant sensor was working, but not good. You should have electrical continuity between the coolant sensor and the ground. There wasn't but it was close, I mean the differential voltage was V 11,85 instead of battery voltage of V 12,60.
This was working on the old cluster but not in this new (may be is more sensitive).
I also disconnect the coolant sensor and saw that the buzzer was finally OFF and the coolant warning light ON.
Then I connected the coolant cable to the ground instead of the sensor and everythings is now working fine, buzzer OFF coolant warning light OFF.

So even if Your coolant warning was not giving fault before, may be the new cluster has minor tolerance in Voltage and sound the buzzer alarm.

Hope to be helpfull.

Special thanks to Gen2n3 for all the support on many threads concerning Thaco and Speedometer

Gen2n3

iTrader: (1)

@pietrino,

Thank you for the compliment! I'm glad that those curated threads helped you diagnose and repair your coolant level sensor. Would you let us know when the part is installed and problem is finally fixed?

pietrino

iTrader: (1)

Yes, of course.
I hope to get the new sensor soon
Thanks again

dadadmj1

Hey Guys, Gals,
Try this first. Unhook battery and let it set for 10 mins. Reattach battery cables and see what you get. Worked for me. On a super hot day, I lost the even backlight of the odometer, but when interior of car cooled off it came back.

Dave

Gen2n3

iTrader: (1)

Dave,

Welcome to the club, new member!

I have good & bad news:

Good news:
That solution is mentioned in another thread. Specifically, disconnect the positive battery terminal for 10 mins, as you discovered. Then reconnect. The odometer may come back to life. This works if you recently noticed the odometer blank out. For example, yesterday it worked but today it is out. This works because you are manually discharging the electrolytic capacitors through the ground plane.

Bad news:
That solution for a blank odometer symptom is a temporary fix. It will return. Speaking from my experience with this approach: It worked for a short time, say several weeks. But then the odometer went out and stayed out.

Long term solution:
The most effective way to fix a blank odometer is to remove and inspect the the speedo board for leaking electrolytic capacitors. If an odometer is blank then the most likely cause of the problem is bad/leaked capacitors. If caught in time, the damage will be minimal. Removal and replacement of the capacitors would only be necessary. However, when left untouched for an extended time, the damage could be more severe to surrounding and less obtainable components.

You are more than welcome to search for my threads that cover the repair and troubleshooting of the speedo board. Post #60 above is a good start. Consequently, @pietrino, had a related problem with his speedo board that resulted in replacement of damaged capacitors. His posts above, Post #61 and Post #63 briefly touch upon it. However, he documented his repair effort in the speedo troubleshooting thread.

You may do as you like. However, IMHO, remove and inspect your speedo board for damaged capacitors soon. When allowed to sit for an extended period, the effect may be the same but the damage will only get worse.