deadhead

As far as the steam goes, I think this is right on, as there was extensive aftermarket wiring at some point, and I can see into the cab in a number of places from under the hood. Sealing the cab rewiring are on the list of things to get done this summer, for now I just need operational, so I am going to try to flush the hoses and water pump, as well as change the thermo this weekend, hopefully that takes care of it.... Dave

So what could be the most likely cause of overheating in a 1977 620 with a l20b motor? Here are a few more details on the issue.... The truck sat for around a year due to lack of funds to correct a carb issue, I finnally got that corrected, changed all the fluids, wires and other tune-up parts, and fired it up. Ran a little choppy at idle, but just fine under load, (i'm thinking it has vacuum/advance issues). Ran it for a couple of weeks with no problems, so I decided to try the highway. After about three days of using the highway on either end of my commute (in CO the limit is 75 mph, and I am running a stock 4 speed), the alt. took a dump. I got the altie fixed up, and went back to running her. Well, fast foreward a couple of two weeks or so, (with limited or no highway driving), to this morning. I had some stuff to do near the highway before work this morning, so I figured why not, and jumped on the highway to get to work. After about 10 miles of driving 60-75 mph I took my exit, and watched the cab start filling with steaming coolant. I pulled off the road, and waited for the temp to come down, then pulled the cap and filled the radiator. It got me the rest of the way to work without even budging the temp guage. (only about 3 more miles...) At the end of the day I checked to make sure that the radiator was still full, and tried to fire it up. It was real reluctant to start, but did anyways, and I headed home on the back roads. The temp needle climbed slowly to operating temp. and hung out there for most of the ride home. It started to edge up towards the hot side, so I stopped at the store, popped the hood and had a peek, all looked normal so I left the hood up and went in to grab some dinner stuff. When I got back out the needle was in the middle of the guage again, so I ran over to the post office to grab my mail. Left the hood down, and ran in for 2 seconds, (litterally), came back out and the needle was waaay up at the H again, and the truck tried real hard not to start. I got it fired up, and coasted to the house. Popped the hood, and checked to make sure that the fan was working, everything appeared to be working, and no fluid was coming out the overflow, but there were damp spots under the hood. Gonna give it a while to cool off, then I will re check the fluids.... Any thoughts????? Thanks, Dave

holy mother of dats! Thanks a ton guys, By using the info from GGZILLA and then folowing the reccomendation of banzai, I was at last able to figure out what was going on. After having the alt. tested at a number of shops and having it come up good on their machines, I just assumed it had to be a wiring or regulator issue, but after striking out on those as possible causes, I tried the short circuit test described by ggzilla, and then took banzais advice and just replaced the alt. with one from autozone with a lifetime warranty, and wouldn't you know it, the problem cleared right up, go figure. When returning the core to the parts store, I had them test the old one just to see what it would say, and sure enough, the one store I had not previously tested it in returned a fail result. Sure would have saved me a lot of time and frustration if I had just gone with my gut and swapped that out first and foremost! live and learn I guess, that is what I get for being a broke guy, afraid to buy parts that may not be the right ones... I still have the new VR I bought, which oddly enough was almost twice the cost of the alt., and what I am wondering is that I have heard such good things about the original, and now it is functioning properly with the oem VR in it, should I bother keeping the new one, and putting it on, or should the old one last? The guy at the parts store is my neighbor, so luckily for me, I can return it, and I am thinking this is what I should do, but since you guys have helped me this far, I just figured I would get your take on that too... So whatcha think? thanks again, Dave

I have a stock center console on my 77 620 kc, and there is a large flat tray in front of the shifter. I put a half a roll of duct tape on it a few weeks ago, and after a nice warm day, it seemed to like it's location, and was conveniantly the right size to hold cups from the fast food place, or cans of pop, or bottles of beer, or whatever, and I think i am into it for around 2.00 worth of tape I won't be using.... D

depending on the final price, I am in for a king cab.... D

yeah, gzilla, that is what I assumed, and after changing all the parts except the alt. I am still having the issue. A friend of mine who owned a 510 some years ago suggested that there may be something in the coil circuit that may be contributing to the issue, but that doesn't seem right to me... what do ya'all think? I am thinking that if I install an internally regulated alt. and bypass the wiring and VR loop in the circuit that may help alleviate the issue, but I am not quite sure exactly how to proceed with that, even after reading about so many others who have done just that, I am not entirely clear about which model of alternator would be the best fit. I am not the handiest when it comes to messing around with changing pulleys and fabricating brackets or drilling mounting holes in the block, so what would be a direct bolt on replacement alt. for this? thanks for the input.... Dave oh, and I did manage to get on olddatsuns.com and print off some wiring diagrams, so if I can't figure out what a direct fit after market alt. will work, I am going to start rewiring the entire charge circuit.....(a last resort, but one I am confidant will work....)

My original post was getting long winded, so I didn't get the chance to mention that My only computer at the moment is my work issued laptop, which has some quirky security settings, intended to keep me off the internet I think.... Most of the time, it doesn't cause many issues, however in a few instances, (old datsun.com for example), the AV program will not let me view some websites. I have tried to go there in search of good info, but since I can't see the site, haven't been able to search it. Maybe I will try to check out the library tomorrow to see what I can pull off of that site. In the mean time, what I feel makes this case unique is the fact that the CHG light will go out once a certain rpm level is reached. I have not been able to tell why or how, but I feel like that is central to the cause of the light being lit up in the first place... has anyone heard of a similar issue ever??? thanks. Dave

pulled, tested, and re-installed alternator this afternoon, belt tension is good... pulled and charged the battery this afternoon, cleaned all connections before re-installing... Removed, and re-mounted VR, twice... visually and physically inspected both terminal wires for signs of corrosion and voltage, both tested good... during testing, plugged VR in/off around two dozen times today, all connections bright... with engine running, battery tests at 12.7 volts...so, not charging... ????? I thought about swapping the alt. for an internal, but that is a last resort for me, as I am not really a "car guy", and being able to have any parts store look up "by the book" for me is a big plus, although if that fix eliminates the wire that may be the ultimate culprit it may be the way to go. The haynes book for 72-79 620 has been discontinued according to all my local parts stores, I did find one for more then I would like to spend on amazon, although all I really need is the wiring diagram, which seems to not be available in a free format online, although I can find plenty for the 510 model... Does anyone have a diagram for the 620 they would be willing to share??? thanks again.... Dave

well, I am new around here, and have been browsing as a guest for a while, and now I have a problem that it seems like the knowledgable folks around here may be able to help with, so here goes.... I had to let my 1977 620 (king cab, california model) sit for around a year due to a carberatuer issue that I was finally able to resolve, and then I drove it for a week with no issues, until yesterday... As I pulled off the highway on the way home from work, I noticed the CHG light come on on the dash board. I ran it home as fast as legally allowed, and popped the hood. While driving home, I noticed that the light appeared to flicker at random moments, so my first assumption was a loose or corroded wire, so I had my wife sit in the truck watching the light while I wiggled every wire I could possibly move, and could not replicate any flickering of the light. So I jumped in it, with the intention of taking it down to the parts store to test the alt. and bat. and I noticed that I could get the light to shut off when the engine was up over 4k rpm or so, (no tach, just a "educated guess" based on the sound of the engine/travel speed). Got to the auto parts store, and put it on the diagnostic equip. and got the results, battery is good, and nearly full charge, and "no voltage detected" coming back into the battery. Tried the test again while maintaining enough engine speed to have the CHG light go out, with no change in the test results. The guy at the parts store did some part look-ups to see what was available, and since the voltage regulator was closer then the alt. we went ahead and ordered one, with the intention of "trying it out" before I paid for it to make sure that that was the issue. Fast foreward a day and a half, during which time I operated the truck a few times, even using it to take the backroads in to work, being careful to maintain as high an engine speed as possible by undershifting a little bit, trying to keep the charge light off as much as possible. I got it back to the parts store to check on the VR, but the guy who had ordered it originally was not in, and the other employees would not let me try the new one on before purchase for fear that I would just burn it out, so instead I opted to have a more experinced guy re-run the diagnostic, only to return nearly identical results, (the only difference is that the battery was now reccomended for charging...) So, I pulled the battery out, and tossed it on the charger at the store, and in the mean time, pulled the alt. out to get it tested on the dyno in the store. While I had it out, I noticed it was in less then perfect condition, but did not seem to be too damaged, and the dyno confirmed that it was operating well within the proper range. I put it back on the truck, and when the battery was all charged up, I put that back in too, fired right up, but the light was still on, so I tried pulling the + cable, and the truck died. If I am not mistaken, that means to me, that the truck was indeed running solely on battery power, (at least at idle), and indicated to me that the VR was indeed the issue,(I had also visually inspected all of the wiring coming from the bat. and connecting in any way to the alternator while I had a little extra room in the engine compartment so I went back in and bought the new one. I took it out the the truck, mounted it up, and turned it on. Now the light was still lit, but would not turn off at 4k, instead, I would have to push it up to (guessing...) around 7k to get it to go out. I took it down the street to a friends shop, and we got out the voltometer and test light, and got to poking around. We found that the bat. voltage was good, there was power going in and coming out from the alternator, the output at the VR harness seemed to be correct, and the CHG light would still not go out. I'm stumped. So my question in a nutshell, would be... Why is the CHG light on at low rpm, if the alt., VR, and battery cables have been eliminated as possible causes?????? Thanks for any suggestions or help you may be able to offer.... DAve oh, and does anyone know where to find a free wiring diagram for the 620 engine with external regulator? My mechanic friend was pretty sure he could find the problem with access to one, although neither of us can find one.... D