Gauge repair 101 for 40 series Crowns

[gianttomato]

I'm so excited I could shit.

Of course if you don't own a 40 series Crown you wouldn't know why.

40 series owners - sick of your temp gauge always showing H? What about your oil pressure gauge just always showing low oil pressure even though you know your 1UZ is pumping out a stonking 70 psi? What about the fuel gauge reading full or empty depending on which side of the street you park?

It's because of those stupid internally regulated senders with their gayass bimetallic strip that after 40 years decides it wants to die. Tried getting one from Toyota? Don't bother - they don't exist anymore but, of course, the part number has defaulted to the new senders. The new senders, which are not internally regulated, allow the gauge to see a full 12 volts (on the Dave good-bad continuum, bad) and give an erroneous reading, leading to disastrous consequences (for me a BHG - again, on the Dave good-bad continuum, bad).

Well today fellow 40 series owners, I have made a discovery that will solve all our troubles. I also discovered a, and possibly the only, good use for the TN Magna.

I will pictorialize this whole exciting adventure in the next 24 hours for you all to be amazed.

Anyway, I'm off to the toilet.

[gianttomato]

The problem lies with the internally regulated senders. The plan is to get rid of them using an external voltage regulator and use converntional VDO senders.

Now I'll confess - the idea isn't novel. In my research on how to solve this problem, I came across this page. http://www.imperialclub.com/Articles...pair/index.htm

So I went out and bought a 5V voltage reg chip and it just didn't allow enough movement of the needle (no matter how I adjusted the little tangs inside the gauge). Local Tandy or Dick Smith couldn't point me to a higher voltage regulator either.

However, today at the wreckers a TN Magna dash lay beside an RT142 - two shittier cars I couldn't imagine. It was in the way of the engine crane so I moved it and almost by accident I noticed some writing on the circuit board "+7V REG". And there it was, a 7V voltage regulator. Perfect.



The silver doover appears to be some sort of heat sink.

[myne]

Love the narrative.

[Mr DOHC]

they are very common to go in magnas, so be ready for the whole she-bang to fall apart

[gianttomato]

Thanks for the heads up. I'll see if there is a plug at one of the local electronics stores so I can just unplug them as they fail. That said, all four I have here work just fine. I find it surprising that in the depressed north west of Melbourne people had the cash to lash out on dash repair. Such a failure usually leads to the rego being cashed in, the car being joyridden then burnt to the ground.

OK. Here is a pic of the back of the dash showing the circuit board. You can see the +12V input and the sensor input for both the oil pressure and water temperature gauges. The gauges are grounded via the metal body of the dash cluster. You will note the Kanji Toyota symbol indicating this is a quality product designed to withstand another atomic assault on the mother land, not some late model inbuilt obsolescence engineered piece of rubbish (cue mental image XT130 Corona).


Opening up the dash reveals how the whole thing connects up. You can see these spring loaded copper tangs that contact directly with the gauge terminals. Interesting way to do things. Excuse the awesome auto focus by my digicam - after about 15 attempts, I gave up. I spray painted that filing cabinet 10 years ago with an Eager Beaver.


And here is the back of the gauge. The copper tangs make direct contact with the studs. The +12V input needs to become +7V so I'll need to isolate the 12V on the circuit board from the gauge terminals. I really hate cutting this old stuff so I'll bend the tengs out of the way and isolate them some more with some gasket paper. I'll also have to get +7V down to the terminals - and you can see I have started that by running the green wire directly on to the terminals.


Next step is to determine what senders to use. I'll need to check the resistances from minimum to maximum swing and pick some appropriate senders from the VDO catalogue. So that gauges make sense, it's reasonable to make them work best at values the engine might realistically see. Obviously water temp would be 40 - 120 degrees C (hopefully the latter not very often), and oil would be 0 - 80 psi.

[gianttomato]

I have been distracted by the V6 thing for a few days so I took my eye off the ball. Game on.

Well I bought a 0 -200 ohm variable resistor to get a rough idea of what resistances my gauges will need to get them to deflect from bottom to top with an input voltage of 7 volts.

The oil pressure gauge was quite easy - at the lowest end of the scale R = 110 ohm and at the top of the scale R = 9 ohm. A casual glance of the VDO catalogue revealed that sender 360.005 was a 500 kPa ( ~ 72 psi) sender that went from 73 ohm to 13 ohms. About as good as I'm going to get. Some very minor fine tuning of the little tangs in the gauge itself allows the gauge to have a reasonably normal swing.

The water temp was another kettle of fish altogether. At the lowest range, C, R = 22 ohm, and at the highest range, H, R = 68 ohm. There is absolutely nada in the way of easily available senders that increases resistance with temperature (apart from our gaylord, internally regulated, fragile as crystal senders).

Solution? Ingenious in its simplicity. Simply flip the little plastic doover on which the C and H are inscribed so they sit the other way! Only the most astute 40 series owners will notice - hopefully they're more intrigued by the 1UZ under the bonnet. Now the resistances work just dandy and another look at the VDO catalogue reveals that 320.003 is a 73 -13 ohm resistor. Some more tweaking ofthe little tangs and we're set.

I have decided that the dash needs a charge light and an oil warning light so I have decided to fit some high luminosity LEDs to the dash.

Pics coming.

[EldarO]

great guide, made me laugh too!

My water temp, oil pressure and fuel guage dont work, however i have an MA70 supra with digital dash!

Im too stupid to understand any of this stuff, so i cant fix it :/

Great read, love the filing cabinet comment!

Eldar.O.

[gianttomato]

See...hard to pick here.