91 L98 Tach Issue - Resistor Question

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Jun 16, 2015 | 09:50 PM

Thumper.DCS

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From: Cary NC

91 L98 Tach Issue - Resistor Question

I've read a couple threads on how to DIY fix the high rpm readings on the tach in my C4. Most talk about cutting specific pins off the chip and soldering 200K (preferred?) or 300K resistors across the pins. The problem is that both resistors come in different wattages and I don't know what wattage I should buy. Can someone please advise? I really appreciate any info you can give this rookie

Jun 17, 2015 | 12:02 AM

ToniJ1960

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From: Granite City Illinois

Quote:

Originally Posted by Thumper.DCS

if youre going to use 200K or 300k ohm resistors, most likely 1/4 watt will be fine.Unless theres some real high voltage there you wont get much current through that high of a resistance to generate much power.

Now Im not advising you to cut any pins off any chips.

Last edited by ToniJ1960; Jun 17, 2015 at 12:04 AM.

Jun 17, 2015 | 05:02 PM

Thumper.DCS

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From: Cary NC

Thanks for the advice.

Quote:

Originally Posted by ToniJ1960

I see you're a senior member, so definitely the pins will stay on. Should make soldering easier with higher probability of success. I think the lowest I've found so far are 1 watt resistors, so I'll go with 200K 5% tolerance 1 watt if I can't find 1/4 or 1/2 watt.

Thanks again ToniJ

Jun 17, 2015 | 05:09 PM

kg4fku

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From: Connecticut

Quote:

Originally Posted by Thumper.DCS

I don't know if you will have the room to put 1 watt resistors in there.

You will be better off with quarter watt

Jun 17, 2015 | 06:28 PM

ToniJ1960

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From: Granite City Illinois

Quote:

Originally Posted by kg4fku

I don't know if you will have the room to put 1 watt resistors in there.

You will be better off with quarter watt

I would expect so too plus 1/4 watt will sure be easier to find a source of.

Im only a senior member because Im senior lol never heard about this operation so have no experience with it and not sure Im in favour of this sort of `repair`.

I do have about 40 years of experience with electronics. In fact if you want I could send you some 220k or 330k 1/8 watt resistors.

Last edited by ToniJ1960; Jun 17, 2015 at 06:31 PM.

Jun 17, 2015 | 09:01 PM

WVZR-1

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I would think 1/4 watt 5% should be a fairly easy find.

Jun 17, 2015 | 10:14 PM

Thumper.DCS

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From: Cary NC

Quote:

Originally Posted by ToniJ1960

To be honest, I'm not 100% confident myself. I thought I found a good source to check out and repair the entire instrument cluster ($225 plus parts, 5-day turnaround), but they are no longer in business. So, I have to drop the steering column and take the cluster out no matter what route I take. At least my good bud has a real garage with all the tools and will rent space to me while i do this and other repairs. So yes, if you want to send me 4 220K resistors I'd be happy to take them off your hands. You can send me you PayPai or other info to thumper.dcs@gmail.com...the least I can do is reimburse you for the shipping. Then I'll send you my mailing address. Thanks again and wish me luck!

Dave S.

Jun 17, 2015 | 10:21 PM

Thumper.DCS

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From: Cary NC

Good point

Quote:

Originally Posted by kg4fku

I don't know if you will have the room to put 1 watt resistors in there.

You will be better off with quarter watt

Thanks KG4...good point on the space issue...the resistors in the photos are 200K and look to be pretty small. I did a lot of work on my cars in the late 60s and 70s, but this is a whole new ballgame to me. i appreciate the feedback and I'll learn form y'all out here!

Dave S.

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Jun 17, 2015 | 10:40 PM

twin540

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From: San Antonio TX

I did my 91 cluster years ago. I installed three 100k ohm resistors from radio shack. Ran them in series twisted togetherr then soldered the ends . Then the ends to the appropriate pins I forgot which ones..

Jun 17, 2015 | 11:00 PM

#10

ToniJ1960

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From: Granite City Illinois

Quote:

Originally Posted by Thumper.DCS

It cant take more than a stamp or two just post up pictures of the repair for us all to see and let us all know how its going and how it turns out,maybe post a link to where you got this idea from

Jun 18, 2015 | 12:26 AM

#11

Cliff Harris

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These threads discuss the problem and the fix, with plenty of pictures:

https://www.corvetteforum.com/forums...-problems.html
https://www.corvetteforum.com/forums...ch-repair.html

The basics of the fix are that the thick film resistors (the black rectangular areas) on the chip carrier need to be changed in value.

Last edited by Cliff Harris; Jun 18, 2015 at 12:37 AM.

Jun 18, 2015 | 02:58 AM

#12

ToniJ1960

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Quote:

Originally Posted by Cliff Harris

I was wondering if it might be a dip resistor in that case it is probably better to cut those 2 pins and solder a 300k resistor to the bottom of the board.

Jun 18, 2015 | 05:25 AM

#13

Joe C

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Quote:

Originally Posted by ToniJ1960

I was wondering if it might be a dip resistor in that case it is probably better to cut those 2 pins and solder a 300k resistor to the bottom of the board.

- from the pics, it looks to be a DIP resistor pack having two resistors, pins 1 and 2, and pins 4 and 10. I'm assuming the issue is, resistor #2 (pins 4-10), went belly up. there are a number of ways to repair this, but cutting those pins and soldering a 200K or 300K to the bottom should be a good. an 1/8W resistor is more than sufficient for this repair.

Jun 18, 2015 | 05:35 AM

#14

Joe C

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Quote:

Originally Posted by Thumper.DCS

I see you're a senior member, so definitely the pins will stay on. Should make soldering easier with higher probability of success. I think the lowest I've found so far are 1 watt resistors, so I'll go with 200K 5% tolerance 1 watt if I can't find 1/4 or 1/2 watt.

Thanks again ToniJ

thumper - FYI...

http://www.ebay.com/itm/25-pcs-200k-ohms-1-1-8W-metal-film-resistors-/151699856497?pt=LH_DefaultDomain_0&hash=item2352041c71
I know it's 25 pieces, and a quantity overkill, but 1%, metal film, shipped for $4 -

BTW, similar findings on 300K, 1/8W, 1%...

Last edited by Joe C; Jun 18, 2015 at 06:19 AM.

Jun 18, 2015 | 07:33 AM

#15

kg4fku

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From: Connecticut

Quote:

Originally Posted by Thumper.DCS

Have you looked at http://batee.com/

Jun 18, 2015 | 12:13 PM

#16

ToniJ1960

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From: Granite City Illinois

I sent an email waiting to hear if you need the 300k still

Jun 22, 2015 | 06:41 PM

#17

Thumper.DCS

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From: Cary NC

Thanks Cliff...

Quote:

Originally Posted by Cliff Harris

I'll definitely be looking at the links and pix. I appreciate all the suggestions I'm getting out here, for sure!

Dave S.

Jun 26, 2015 | 04:46 PM

#18

Thumper.DCS

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From: Cary NC

Got the goods!

Quote:

Originally Posted by ToniJ1960

Hi ToniJ.

Much obliged...I got the resistors in the mail yesterday (thanks for the spares). Man, I just forgot how tiny these things have to be! Good thing I have magnified modeling glasses with built-in LEDs. Later I'll post up the thread that gave me the ideas.

Dave S.

Jun 26, 2015 | 04:52 PM

#19

Thumper.DCS

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From: Cary NC

To ToniJ and everyone else out here. Below is the link (with pix) that was posted by Jim85IROC (Sr. Member) back in 2010.

https://www.corvetteforum.com/forums...y&p=1575165945

Jun 26, 2015 | 06:03 PM

#20

WVZR-1

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Quote:

Originally Posted by Thumper.DCS

To ToniJ and everyone else out here. Below is the link (with pix) that was posted by Jim85IROC (Sr. Member) back in 2010.

https://www.corvetteforum.com/forums...y&p=1575165945

Looks to be maybe a thread you left "OPEN"!!

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