Most definately check your main grounds, like the body to the chasis. I bought my brother's '84 a week or two ago and a couple of days later it quit starting. The temp and fuel gauges would go crazy before it quit and I just thought it was a shorted out instrument cluster. I swapped clusters from my wrecked '85 and still no dice. The weird thing is that the gauges would peg and every light in the cluster would come on when the speedo cable touched the back of the cluster. I took a jumper cable, hooked it from my EGR to my negative battery post and it started. I didn't have time to look for my main chasis-engine ground because I was leaving on vacation but when I get back I expect to find it in pretty bad shape. Also my other electrical items worked fine up until and during the period when it wouldn't start.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo