89 GP SE Instrument cluster repair 25122113

[89SE]

The analog tachometer of my 89 Grand Prix SE works, but indicates about 3 times the value it should. The cluster is 25122113. The 3 small gauges seem to be working fine, the speedometer is digital and just fine. I hear the analog gages are a common problem as on TGPs. I also read it is usually caused by a faulty resistor. The tacho does not have a calibration resistor on the Air Core motor itself, but there is one on the circuit board. I live in Europe, so getting a refurbished instrument panel is quite difficult/expensive for a gauge that is strictly speaking not necessary, so I am looking for a cheap and easy fix.

I am not 100% sure yet, but the culprit is most likely to be a calibrated 14 pin ceramic DIP resistor (the white and black on the picture). Since it is not a speedo, calibration is not the most important to me, so if I could get a replacement with about the right values it would be probably a good enough fix. Anyone here has the specs of the resistor?

I have found a replacement for a similar resistor, one for Corvettes: https://batee.com/collections/1990-1996-corvette-parts/products/90-96-analog-calibration-ic-adjustable-preset-for-6k-tach

So if there would be a source for the version used in the Pontiac GP, please post it here!

 

Edited June 2, 2023 by 89SE

[89SE]

It seems there are currently no solutions available for the resistor chip or the tacho circuit board for this specific instrument panel; other than bypassing pin 4 and 10 with a trimmer potentiometer in order to be able to obtain the correct resistance again. On all similar fixes, nobody seems to mention the other film resistor, in this case between pin 3 and 13. Eventually that one will also rise to infinite resistance, which should have some undesired effect, but I do not know it's purpose. If someone would have any schematic of instrument panel 25122113, feel free to post it!

[jiggity76]

I enjoyed reading your thread here but I must admit that I'm a moron when it comes to electronics.  You might as well be speaking Japanese to me.  It's not your fault of course, it's definitely mine.  

Typing in that number on Ebay, this cluster along with 2 others popped up.  This one though is a remanufactured unit.  Maybe they is a solution thru this place?

 

 

https://www.ebay.com/itm/124970672582?epid=26044439203&itmmeta=01HY9CCXBWVEKDBMQQ8P48BCGR&hash=item1d18d521c6:g:XMcAAOSwXvphdbou&itmprp=enc%3AAQAJAAAA4FvtXGhD3ynH%2FUR99gvrGyqpTrQlhnDyASk4eK4%2Fca4OOv6UKXx8PdOAyy%2FJXd3WMC0hivWON5tM5qLEs1dWmPx1EZ93EfcAB%2F%2BzJ5eZEvpxetPo6IIUMG9MT8OPMPQIcO%2BjpkzL%2FaQUte7afW4dnVEX7u%2B98Fy0mu75s5zL%2FRRVZAjq02LztCqfEyXJT09vgK%2BUnLtDo6NivgaICJxkRsB%2Fiy4y%2BwWsfIEQ4oIvDKd1qHxaifOZrp8N7VtlfiBc0bs8chKKpuww9TKZKQRFgUXj4X%2BTx%2FyCYrTQXudxf8%2B4|tkp%3ABk9SR_zVs6zyYw

[89SE]

@jiggity76, a remanufactured cluster would be a solution, after some odometer transplantation work to keep the mileage of course. I assume that a remanufactured one will also have just a resistor added in order to replace the faulty one between pin 4 and 10 on the infamous white resistor IC (I do not think any 100% accurate original ones still exist). I should maybe ask the Ebay seller if they indeed fix them like that, if it's not to much of a trade secret.

As said in my initial post, I live in Europe, so by the time all transport and taxes are paid, a replacement cluster is quite expensive, knowing that repairing my cluster is possible with some soldering work and a 1$ resistor. So that seems to be the best option in case a neat & quick solution is not available (like a replacement for the white resistor IC as Batee makes for Corvettes, or a circuit board as can be found for many other models on https://www.cajuntachshop.com/ ).

So I'll try to do that fix, many others on the internet did just that with success. But I remain curious about the effect of degradation of the other black field on the white resistor IC and any documentation would be welcome, I only have the service manual of the car.

[jiggity76]

4 minutes ago, 89SE said:

@jiggity76, a remanufactured cluster would be a solution, after some odometer transplantation work to keep the mileage of course. I assume that a remanufactured one will also have just a resistor added in order to replace the faulty one between pin 4 and 10 on the infamous white resistor IC (I do not think any 100% accurate original ones still exist). I should maybe ask the Ebay seller if they indeed fix them like that, if it's not to much of a trade secret.

As said in my initial post, I live in Europe, so by the time all transport and taxes are paid, a replacement cluster is quite expensive, knowing that repairing my cluster is possible with some soldering work and a 1$ resistor. So that seems to be the best option in case a neat & quick solution is not available (like a replacement for the white resistor IC as Batee makes for Corvettes, or a circuit board as can be found for many other models on https://www.cajuntachshop.com/ ).

So I'll try to do that fix, many others on the internet did just that with success. But I remain curious about the effect of degradation of the other black field on the white resistor IC and any documentation would be welcome, I only have the service manual of the car.

I was thinking that if this is a remanufactured unit, maybe they would be willing to repair yours at a reasonable price.  

[89SE]

Unfortunately, due to shipping costs and taxes it does not make much financial sense to get anything fixed across the Atlantic.

[jiggity76]

Gotcha.

[89SE]

In this post @RobertISaar can't remember what the other black pad is for on the white resistor IC, but at least he has not seen them causing issues.

 

[jiggity76]

1 hour ago, 89SE said:

In this post @RobertISaar can't remember what the other black pad is for on the white resistor IC, but at least he has not seen them causing issues.

 

In the meantime, I will pull spare clusters if you want when I do some junkyarding.  I go pretty often.  You'll just have to pay for the parts and shipping, it's up to you.  Parts for these cars at local yards aren't that bad really price wise.  Just let me know.

[89SE]

Thanks, I'll update you if I can get mine to work properly. I assume a spare cluster is normally not necessary, since I think there are only 2 common issues with these clusters (I hope): soldering going bad and the laser trimmed resistor going slowly to infinity. The first can be solved by re-soldering, the latter will be an issue with all clusters of this age, the laser trimmed resistor was just a bad design. Bad soldering started causing issues on mine as well by the way, resulting in warning lights for oil pressure and temperature turning on when not supposed to.

[89SE]

I repaired the tacho by installing a trimmer potentiometer of 500kOhm in parallel to pin 4 and 10 and cutting the connection of pin 4 of the existing resistor. That way, the old resistor is out of the circuit, which is necessary if it is not yet fully desintegrated: the resistance would otherwise still creep up and make the rev counter go too high again in the future. I do not have any equipment for a bench test, so I glued the potentiometer on the back of the tacho circuit board so the screw faces a vent hole on the top of the instrument cluster (see picture). This allowed me to install the cluster in the car and do live fine tuning of the resistance using a small screwdriver. Using an Actron CP9690 Elite, I am able to read out the exact rpm from the ALDL connector of the car. The potentiometer I used is a CT-94 eW504 (1 euro in the local electronics shop), the 18 turns of adjustment were enough to do a fine adjustment. I took the cluster apart again after adjustment in order to measure: It ended up at 210.9kOhm. For reference: the old one was above 700kOhm, resulting in the tacho showing about 3.5 times the actual rpm. You can by the way perfectly measure the resistance on the board, this part of the circuit is 'isolated' so you are not measuring any other resistance in parallel of the circuit between pin 4 and 10.

So the tacho is repaired, as long as my soldering will stand the test of time . Only issue remaining is the warning light of the water temp not briefly turning on during startup. It used to work a year ago (when I started this thread...). On the instrument cluster connector, C4 (the temperature input) effectively grounds to D8 when the key is turned to start. The gage works perfect, including going all the way up when the starter motor is running. Lamp and all circuits seem OK, apart from what seems a hair crack on a trace, but I measure a perfect connection. I should try to ground the sender to see if the lamp would at least work eventually with an overheated engine, but I am afraid to break the plastic connector of the sender cable. The few start-ups I found on youtube also don't show the the lamp flashing, although it should, so it is probably a common issue.

[89SE]

Update on the water temp lamp: it does turn on when grounding the sensor at it's connector. So the Instrument Cluster works just fine, exept during the lamp test when starting the car. According to the service manual, the temperature lamp test procedure just happens because the input of the sensor is connected to ground by turning the key in the start position, which is all happening outside of the instrument cluster. In my previous post I said the sensor effectively grounds when turning the key to start, but I actually measure an open loop between C4 (sensor) and D8 (supposed to be ground) of the connector that goes into the cluster, until the key goes to the start position: then I measure 54 Ohm. which is sufficient to make the gage go all the way up (60 ohm would correspond to 137°C or 279°F), but is maybe just not low enough for the lamp: the service manual states the indicator control unit switches the lamp on at ca. 60 ohm and below.  I will investigate this a bit more, it seems something is wrong here, logically, I should measure the resistance of the sensor in any key position except start, and less then 1 ohm when the key is in start.