glow plug resistor by-pass mod

I believe that cpf240 is correct.

I recently installed new glow plugs and changed plugs from 24 to 12 volt feed.
Experienced the same no start issue with a longer than normal WAIT light symptom.

Measured the voltage on the top of the GP relay -- 12.8 volts during the wait cycle.
Measured the voltage on the bottom of the relay --.238 volts during the wait cycle.

I am assuming that the glow plug relay was marginal and failed with the increased current draw.
Replaced relay and it starts fine now.

Karl

Why not spend 20 minutes diagnosing the whole system?

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I wouldn't spend time doing any of that. I would immediately go straight to new plugs, bypass mod, and then a functional check. I'm not sure why anyone would bother trying to use the GP's with questionable history when they more than likely are junk to begin with.

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So, you guys would agree that putting a voltmeter on the input and output terminals of the relay would be a good test? (I did)

After replacing your GP's as soon as you get a CUCV with AC60's. If you don't, it is the equivalent of trying to find an air leak on a dry rotted tire.

I coverted the truck back to 24v, simply so I can drive it until I can replace the GPs and redo the mod.

I tested all my GPs last night as well, for curiousity.

Two on the passenger side are dead as nails, as I suspected from the blowing smoke and skipping on cold starts. One of them looks to be a Wellman that someone put in there. The other 3 are stock. The two good ones are reading 3.0 ohms.

All four on the driver side are stock, and read 1.9-2.1 ohms.

I also cleaned up the contacts while I was doing the readings. This morning it started pretty **** quick in this cold weather.

So yeah, I know I already have bad glow plugs, but my next paycheck I'm ordering a set of AC60s. So I don't really care if the 24v can potentially ruin the rest - I'm replacing them anyway.

EDIT: After the AC60s get installed, I'm re-doing the mod, and then diagnosing the GP relay to see where it stands. It's pretty old too, so I'm pretty certain it will need to be replaced soon. I didn't diag it last night because I wanted to finish up and get out of the cold.

It's likely that your relay is gunked up and is presenting a higher resistance than normal to the circuit, resulting in lower current flow. With only 12v, the current drops so that it's not enough to heat the glow plugs.

24v is still pushing enough amperage to overcome this, so you are able to start with 24v and not with 12.

MarcusOReallyus said:

It's likely that your relay is gunked up and is presenting a higher resistance than normal to the circuit, resulting in lower current flow. With only 12v, the current drops so that it's not enough to heat the glow plugs.

24v is still pushing enough amperage to overcome this, so you are able to start with 24v and not with 12.

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That's what I was thinking. So the ST85 is a common replacement, from what I understand?

Right,sorted the chatter.Glow plug card out,WD 40 in,Glow plug card back in.job jobbed.

Interesting that a lot of members complain after the glow plug resister bypass. I liked the cold starting better before also so I think I am going to put it back to 24v through the resister. The light stays on longer now which is not a large issue, but even after cycling the key twice the truck definitely does not start nearly as quickly and frequently stalls once. I also had to replace the relay with a $63 NAPA relay after the bypass. I thought the price seemed way too high seeing the truck had an $18 relay on it prior to the conversion that was dated many many years prior. For the record, my right rear glow plug is open, but it has been since I've owned the truck.

Hopefully I don't get accused of highjacking again. I was going to start a new thread but figured I'd be advised to do a search and post on the existing thread.

I think I will let Doghead rebuttal your reply in more detail, but I will say that bypassing the resistor has absolutely nothing to do with your GP relay going out. The resistor when all GP are working supplies about 12 volts, bypassing it to the 12 volts side will always supply 12 volts regardless if you have 1 or 7 bad GPs. This is why people do this to avoid killing all the GP when there are bad ones.

So you are saying you do not have a GP in the block on one of your 8 GP holes? That would cause a compression leak and is not good. Is this because the treads are crossed?

EDIT: I have a 1990 GMC Suburban 6.2L and it has 12v system any normal civi 6.2l would have, I have never had any issues heating the GP on a cold morning and it starts right up. I do not see how bypassing the 24v resistor makes a CUCV start harder when all normal 6.2L diesel trucks are 12v GP systems.

K9Vic said:

I think I will let Doghead rebuttal your reply in more detail, but I will say that bypassing the resistor has absolutely nothing to do with your GP relay going out. The resistor when all GP are working supplies about 12 volts, bypassing it to the 12 volts side will always supply 12 volts regardless if you have 1 or 7 bad GPs. This is why people do this to avoid killing all the GP when there are bad ones.

So you are saying you do not have a GP in the block on one of your 8 GP holes? That would cause a compression leak and is not good. Is this because the treads are crossed?


EDIT: I have a 1990 GMC Suburban 6.2L and it has 12v system any normal civi 6.2l would have, I have never had any issues heating the GP on a cold morning and it starts right up. I do not see how bypassing the 24v resistor makes a CUCV start harder when all normal 6.2L diesel trucks are 12v GP systems.

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No no K9Vic. Sorry, you missed my points all the way. The mod did not cause my relay to go out, but the inner wear and poor contact inside the relay caused it not to be able to get a good connection once the voltage was lowered. This has been discussed here before and is a common problem after the mod I found out.

And by "open" on the glow plug, I mean electrically. It no worky. No, I am not driving around with an open hole on my rear cylinder. I'm wondering if that is making the difference now. I've learned that by design, in stock form the 24v system will deliver more voltage to the remaining glow plugs once they start failing and after the mod this is not the case. So in my instance the other 7 glow plugs are net getting the unatural spike in voltage as they were before, thereby not heating as much as they did.

Maybe I should just get around to replacing my glow plugs...

I don't think Doghead will be posting any time soon.

no comment

sschaefer3 said:

I don't think Doghead will be posting any time soon.

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Where did Doghead go?

I did not know that Doghead was banned at the time I posted that comment.
I do not need to know the reasons why, I feel this is not the place to discuss it.
I just wanted to note I did not know and I was just saying he would have had a better technical answer than I as to why I stated that.

K9Vic said:

I did not know that Doghead was banned at the time I posted that comment.

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I didn't know until now, but your right his profile says banned. The way I read it I thought something had happened to him.

1. It's probably temporary.
2. It's certainly best left without further discussion.

'Nuff said?